WO2012007070A2

WO2012007070A2 - Generation and maintenance of digital terrain models

Info

Publication number: WO2012007070A2
Application number: PCT/EP2010/068239
Authority: WO
Inventors: Jacobus Johannes Hamming
Original assignee: Jacobus Johannes Hamming
Priority date: 2010-07-13
Filing date: 2010-11-25
Publication date: 2012-01-19
Also published as: WO2012007070A3

Abstract

Methods and systems for building and modifying at least one digital terrain model are described. One method comprises: selecting a predetermined number of surveys for building a digital terrain model, each survey comprising at least one survey hull, said survey hull defining a geographical area in which geo-located data, preferably bathymetric data, associated with said survey are located; ranking said survey hulls on the basis of a priority rule; determining one or more areas of overlap between at least two overlapping survey hulls; and, generating survey hull segments on the basis of said ranked survey hulls, said survey hull segments defining a non-overlapping continuous geographical area associated with said digital terrain model, wherein said one or more areas of overlap are removed by modifying at least one survey hull of said at least two overlapping survey hulls on the basis of said ranking.

Description

Generation and maintenance of digital terrain models

Field of the invention The invention generally relates to the generation and maintenance of digital terrain models and, in particular, though not necessarily, to methods and systems for building and modifying one or more digital terrain models, a data structure for use in such system and a computer product program using such method.

Background of the invention

Currently digital terrain model producing

organisations face major challenges in keeping up with the ever-growing amount of data that are produced by modern sensor techniques such as laser scanning (LIDAR) . For example, digital nautical charts are generated on the basis of

bathymetric data comprising depth information relative to the ocean' s surface measured at multiple locations within a predetermined geographical area. These data are collected by a digital filing system on board of a survey ship and sent as a very large (order of terabytes or more) survey file, to a hydrographic institute where these surveys are stored in a large survey archive.

Typically a digital terrain model (DTM) , e.g. a digital chart or map, covers a geographical area, which is much larger than the area covered by a single survey.

Therefore data of multiple surveys are used in order to generate a DTM. This process is complex as surveys often overlap and may comprise conflicting data (e.g. different depths at the same location) . Hence certain techniques are used to remove these overlaps and to produce a seamless continuous chart on the basis of the surveys. These techniques involve dividing the geographical area of a desired DTM and the surveys into a grid and copying data from gridded surveys into the gridded DTM. One example of a conventional technique for generating a DTM is described in US 6,721,694. Moreover, apart from DTM for save navigation there is an increasing demand for other DTM products. For example, navigational DTMs show a simplified model of the seabed emphasizing shallows. In contrast, for off-shore activities (e.g. the construction of a wind farm or laying a pipe line) a much more accurate depiction of the seabed is require, showing not only the shallows but also the deeper parts of the seabed. Yet other data requirements are needed for use in other applications e.g. morphology, dredging or coastal management. DTM producing organisations thus generate and maintain

different digital charts of different content, scale and size wherein each digital chart is generated on the basis of a predetermined number of surveys according to certain rules. A number of these DTMs may overlap. Hence, with increasing survey data volumes, increasing number of survey data updates and increasing demand for different types of DTMs, the

conventional approach used by DTM producing organisations cause a number of problems.

A first problem relates to inefficient data processing as during the generation of a DTM survey

information is duplicated and managed at the level of the DTM. Moreover, because liability issues and scientific research require not only the storage of the most recent DTMs but also all historical data this problem becomes even more prominent.

A second problem relates to the maintenance of existing DTMs. Different DTMs may party of fully overlap.

Hence, a new survey may imply updating many different DTMs. As updating a DTM using conventional techniques is time consuming and inefficient in terms of data processing, it often lead to situations wherein only the DTMs that are about to be

published are updated.

Hence, there is a need in the art for methods and systems efficient generation of DTMs on the basis of survey information. In particular, there is a need in the art for methods and systems for generation and maintenance of DTMs, which provide efficient use of survey data and simple

generation of different DTMs on the basis of survey data. Summary of the invention

It is an object of the invention to reduce or eliminate at least one of the drawbacks known in the prior art. In a first aspect the invention may relate to a computer- implemented method for building at least one digital terrain model comprising: selecting a predetermined number of surveys for building a digital terrain model, each survey comprising at least one survey hull, said survey hull defining a

geographical area in which geo-located data, preferably point- cloud data, more preferably bathymetric data, associated with said survey are located; ranking said survey hulls on the basis of a priority rule; determining one or more areas of overlap between at least two overlapping survey hulls; and, generating survey hull segments on the basis of said ranked survey hulls, said survey hull segments defining a non- conflicting continuous geographical area associated with said digital terrain model, wherein said one or more areas of overlap are removed by modifying at least one survey hull of said at least two overlapping survey hulls on the basis of said ranking.

The invention uses hulls to efficiently select the correct part of the point-cloud data from the total set of point-cloud data associated with a survey (IM) without the need of processing data in overlapping areas. Moreover the invention eliminates the need to store all point-cloud data associated with one DTM in a DTM database. Instead, a VCM defining a particular DTM product on the basis of IM hull segments, ie. a set of polygons, may be stored for later use, thereby providing a considerable improvement in terms of data management and storage.

In one embodiment said method may further comprise generating a survey hull segment by subtracting said area of overlap from at least one of said overlapping survey hulls. In another embodiment said method may further comprise:

determining said on or more areas of overlap by calculating one or more intersections between said survey hulls. In a further embodiment said survey hulls and/or survey hull segments are determined on the basis of one or more concave hull algorithms. Conflicting overlapping areas may be resolved and eliminated by processing IM hulls and by using efficient polygon algorithms to determine intersection between IM hulls.

In yet another embodiment said may further comprise the step of determining geo-located data associated with said survey hull segments; generating a digital terrain model on the basis of said determined geo-located data.

In one variant said geo-located data associated with a survey hull segment may be determined by the geo-located data, which lie within the geographical area determined by said survey hull segment.

In another variant said method may comprise storing said one or more survey hull segments associated with a digital terrain model in a segment table, preferably said segment table further comprising survey identifiers for identifying a survey associated with a survey hull segment. The survey hull segments form the parts of a virtual

continuous model associated with a particular DTM.

In a further variant said at least one priority rule may be used for determining in an area of overlap associated with two or more overlapping survey hulls which of said overlapping survey hulls has the highest priority, preferably said priority rule being based on survey metadata information. Processing the survey hulls on the basis of one or more rules provides an efficient and intuitive way of defining complex DTM products.

In yet a further variant said survey metadata information may include at least one survey parameter from the list including: time of generation of the geo-located data, sensor system used for measuring the geo-located data,

In another aspect, the invention may relate to a computer-implemented method for modifying at least one digital terrain model comprising: identifying of one or more survey hull segments and at least one priority rule, said survey hull segments and said rule being associated with said digital terrain model, said survey hull segments defining a non- conflicting continuous geographical area associated with said digital terrain model and each of said survey hull segments being associated with geo-located survey data; receiving at least one further survey hull; ranking said survey hulls segments and said further survey hull on the basis of said priority rule; determining one or more areas of overlap between said further survey hull and said one or more survey hull segments; generating a further survey hull segment on the basis of said one or more areas of overlap and said ranking; and, storing said one or more survey hull segments and said at least one further survey hull in a segment table associated with said digital terrain model.

In yet another aspect, the invention may relate to a computer-implemented method for modifying at least one digital terrain model comprising: identifying of one or more survey hull segments and at least one priority rule associated with said digital terrain model, said survey hull segments defining a non-conflicting continuous geographical area associated with said digital terrain model and each survey hull segment defining a hull of a geographical area in which said geo- located data are located; removing at least one survey hull segment from the said identified survey hull segments; ranking said survey hull segments on the basis of a priority rule; determining one or more areas of overlap between said survey hull segments; generating survey hull segments on the basis of said ranked survey hulls, said survey hull segments defining a ^"non-overlapping continuous geographical area associated with said digital terrain model, wherein said one or more areas of overlap are removed by modifying at least one survey hull of said at least two overlapping survey hulls on the basis of said ranking; and, storing said one or more survey hull segments and said at least one further survey hull in a segment table associated with said digital terrain model.

In a further aspect, the invention may relate to a system for generating at least one digital terrain model comprising: a survey database comprising one or more surveys comprising geo-located data, preferably bathymetric data, and one or more survey hulls, each survey hull defining a hull of a geographical area in which said geo-located data are located; a computer system configured for accessing said survey database, said computer system further being configured for selecting a predetermined number of surveys for building a digital terrain model; ranking said survey hulls on the basis of a priority rule; determining one or more areas of overlap between at least two overlapping survey hulls; and,

generating survey hull segments on the basis of said ranked survey hulls, said survey hull segments defining a non- overlapping continuous geographical area associated with said digital terrain model, wherein said one or more areas of overlap are removed by modifying at least one survey hull of said at least two overlapping survey hulls on the basis of said ranking.

In one embodiment said system may further comprise: a database for storing one or more segment tables, each being associated with a digital terrain model and each comprising at least one or more non-overlapping survey hull segments. In yet a further embodiment said one or more segment tables may be associated with one or more priority rules.

In a further aspect the invention may relate to a data structure for use in a system as described above, wherein said data structure may comprise one or more survey hull segments and a survey identifier associated with each of said survey hull segments, wherein said survey hull segments define a non-overlapping continuous geographical area associated with a digital terrain model, and wherein each survey hull segment defining a hull of a geographical area in which geo-located data, preferably bathymetric data, associated with said survey are located.

The invention may also relate to a computer program product, wherein the computer program product may comprise software code portions configured for, when run a computer, executing to any method as described above.

The invention will be further illustrated with reference to the attached drawings, which schematically show embodiments according to the invention. It will be understood that the invention is not in any way restricted to these specific embodiments. Brief description of the drawings

Fig. 1 depicts a schematic of a conventional DTM processing system.

Fig. 2 depicts a conventional process for generating a digital terrain model.

Fig. 3 depicts a schematic of a DTM processing system according to one embodiment of the invention.

Fig. 4 illustrates at least part of a process for building a DTM according to one embodiment of the invention.

Fig. 5 depicts a process for generating a DTM according to one embodiment of the invention.

Fig. 6 depicts a process of updating a virtual continuous model according to one embodiment of the invention.

Fig. 7 depicts a process of updating a virtual continuous model according to another embodiment of the invention . Detailed description

Fig. 1 depicts a schematic of a conventional system 100 for generating a DTM. The system typically comprises a computer system 102 connected to a survey storage system 104 (i.e. a digital survey archive) and a DTM database 122. The computer system may comprise an Operating System (OS) 108 for managing computer resources e.g. one or more Central

Processing Units (CPUs) 110, a memory 112 for storing program instructions and data and an I/O terminal 114 for allowing an operator to control the DTM generating process.

The survey archive may comprise a predetermined number of different digital surveys II6₁-II6₄. In the context of the invention the term survey relates to any individual data source containing depth or height information with optionally additional parameters (e.g. quality information). Non-limiting examples of such data sources are (bathymetric) survey files containing an irregular distributed point cloud and pre-processed models containing regular gridded data. The surveys form the basis of the archive. A survey may comprise depth or height information at multiple locations within a predetermined geographical area. The survey data typically cover a geographical area of irregular shape. When surveys are stored in the archive together with the associated metadata 118 they may be referred to as individual models or IMs .

On the basis of metadata 118 associated with a survey, a DTM program 119 executed in the memory of the computer system may be configured to select and retrieve one or more IMs from the archive for generating a DTM associated with a predetermined geographical area. Survey metadata may comprise a survey identifier, geo-location information on the surveyed geographical area, the date the data were generated, the method used for measuring the data, the accuracy of the data (e.g. standard deviation for a given set of data), etc. The DTM program may be configured to generate a DTM on the basis of one or more IMs from the survey achieve using a number of process steps and store the generated DTM 120 in a DTM database 122.

Fig . 2 schematically depicts a conventional process

200 for generating a DTM associated with a predetermined geographical area 202. In this example the DTM is generated on the basis of four IMs 204i-204₄, each being associated with a predetermined geographical area. As illustrated in Fig . 2, the IMs may relate to geo-located point-cloud data covering an irregular shaped geographical areas. These geographical areas may overlap each other and/or may contain data, which falls partly outside the geographical area associated with the DTM.

The process is illustrated for the first three IMs 204i-204₃. The DTM program may start the process by reserving a memory block in the computer system representing the "empty" DTM covering the selected geographical area 202. Thereafter, the geographical area of the DTM is divided into a grid 206 comprising cells, wherein each cell is associated with certain geographical area. An IM 204i-204₃ is subsequently retrieved from the archive in order to generate the desired DTM on the basis of the bathymetric data associated with an IM. In order to correctly process the selected IMs, the geographical area associated with an IM is divided in a grid of cells in a similar way as described above with respect to the DTM. Then, bathymetric data associated with a cell of a gridded IM area 2O8₁-2O8₃ is copied into the geographically corresponding cell of the DTM (as indicated by arrows 210i-210₃) . This process is repeated for each cell in each selected IM.

Hence, in a conventional DTM generating process, a memory block is filled with bathymetric data by geographically associating a DTM cell with an IM cell and - if a match is found - by filling the DTM cell with point-cloud data from the survey cell. If in the process a later IM area overlaps an earlier IM area, the DTM cells in the overlapping area 212-218 may be processed according to certain rules. A simple rule would be for example that an IM of a more recent date is given priority so that point-cloud data in a DTM cell associated with the earlier IM is replaced with (or statistically

modified by) point-cloud data associated with the later IM. When all IMs are processed, the DTM program is finished and the resulting DTM data file 120 representing a continuous, non-overlapping geographical area is stored in the DTM

database 122.

Using the process depicted in Fig. 2 many DTMs of different size, scale, accuracy, etc. may be produced. In such process, an IM may be used in many different DTMs so that if such IM is updated (e.g. replaced by a more recent one) in principle all DTMs based on that IM need to be updated using the method described above. Hence, the conventional DTM generating systems and methods as depicted in Fig. 1 and 2 are inefficient in terms of data management as they are based on schemes wherein large amounts of identical and/or similar data (i.e. redundant data) are copied, processed and stored. Such schemes are thus not very good scalable to systems for

processing increasingly large amounts of survey data.

Fig. 3 depicts a schematic of a system 300 for generating a DTM according to one embodiment of the invention. In this system, IMs are stored in a survey database 302, e.g. a relational database, which may be accessed and manipulated by a computer system 304 comprising an OS 306, one or more CPUs 108, a memory 110 and an I/O terminal 112 similar to the computer system as described with reference to Fig. 1.

Each IM may comprise data 314a, i.e. a set of geo- located point-cloud data as depicted by inset 316. Further, each IM in the survey database may comprise survey metadata 314b including similar type of information as described with reference to Fig. 1. These metadata may be based on an

international metadata standard such as ISO 19155 and/or the OGC CS standard. In addition, the metadata may comprise survey hull information 314c, i.e. information associated with the geographical boundaries of a survey. In one embodiment, the survey hull information may comprise a polygon delimiting the geographical area that is occupied by the geo-located point-cloud data of the IM. In one embodiment the survey hull may be a polygon determined on the basis of a (two- dimensional) concave hull algorithm so that also hulls of irregularly shaped survey areas may be accurately determined. An IM hull may define an outer boundary and, optionally, an inner boundary. In that case, an IM hull may comprise more than one polygon. For example, an IM hull may be donut shaped defining an outer hull and an inner hull, wherein the area between these hulls define the geographical area associated with the geo-located point-cloud data. In case of nautical applications such one or more "holes" in an IM hull may relate for example to one or more islands or shallow areas where no bathymetric data can be obtained.

Inset 318 depicts an exemplary concave IM hull 320, wherein the vertexes 322 of the polygon are defined in terms of location coordinates of a common geodetic reference system, e.g. latitudes and longitudes.

The survey database may be accessed by the computer system using SQL queries. The survey database may be

continuously updated with surveys of new geographical

locations and/or survey updates associated with the same or similar geographic location. A survey update may relate to point-cloud data of the same or similar geographical area measured at a more recent point in time, measured on the basis of a different sensor system, measured using a different sensor system settings, etc.

A processor in the computer system may execute a DTM program 324 , which is configured to generate a DTM of a desired geographical area (e.g. a continuous model of a terrain such as a seabed) on the basis of one or more IMs and IM metadata. To that end, the DTM program may identify the IMs for building a DTM and process the IM hulls associated with the identified IMs on the basis of one or more priority rules, which are used by the DTM program in order to eliminate overlapping areas between surveys. In these overlapping areas point-cloud data may conflict, e.g. having different depths at the same positions. The priority rules are used to eliminate these conflicting areas. An operator of the DTM system may provide one or more priority rules to the DTM program using the I/O terminal.

The IM hulls may be processed in accordance with one or more priority rules resulting in a so-called IM hull segment. During the processing of the IM hulls, the DTM program may use a polygon intersection algorithm to determine the overlap between different IM hulls and to modify an IM hull on the basis of the determined overlap. The processed IM hulls, the IM hull segments, each cover a geographical area of the DTM wherein the areas of overlap between the IM hulls may be removed or at least processed in accordance with the priority rules. Hence, the IMs form the basis for the

generation of continuous and seamless digital terrain models comprising one or more IM segments. Such continuous and seamless digital terrain model may also be referred to as a Continuous Model (CM) .

The set of IM hull segments form part of a so-called virtual continuous model (VCM) , which is stored in the VCM database 326 . The structure and content of a VCM and the advantages associated with the use of such VCM for processing geo-located data will be described hereunder in more detail.

Inset 328 illustrates an IM hull segment 330 stored as part of a VCM 332 in the VCM database. In this example, the IM hull segment is the result of processing IM hull 320 on the basis of one or more priority rules. During processing an overlapping area 334 with another IM hull (not shown) was determined. Thereafter, the IM hull 320 was modified in accordance with a priority rule such that the overlapping area is eliminated, e.g. subtracted, from it thereby resulting in IM hull segment 330.

By repeating this process for all IM hulls used for building the DTM, a set of IM hull segments is generated associated with a continuous, non-overlapping geographical area. Hence, a VCM may comprise the IM hull segments

participating in a DTM and IM metadata, including IM

identifiers (e.g. in the form of a SQL query for accessing the survey database) and the priority rules used for generating IM hull segments in the VCM.

In one embodiment a VCM may further comprise an overlap function. Such overlap function may be used by the DTM to determine how data in an overlapping area between two or more IM hulls are processed in a particular case. Such overlap function may be configured to remove, i.e. subtract, the area of overlap from one of the two or more overlapping IM hulls. In further variants, such overlap function may be configured to process data in the area of overlap when two IM hulls have the same priority under a priority rule. Examples of such overlap functions will be described hereunder in more detail.

Hence, in contrast with the conventional system as described with reference to Fig. 1, a DTM is not generated directly by processing the data associated with the IMs (i.e. "filing" cells of a gridded geographical area with data from different IMs) . Instead, a VCM is build on the basis of IM hulls, one or more priority rules and an overlap function. The VCM, a set of pre-processed IMs, may be used by the DTM program to build or update a DTM. The processing of IMs hulls in accordance with the priority rules, the generation and modification of a VCM and the generation of a DTM on the basis of a VCM are described in more detail with reference to Fig. 4-6.

Fig. 4 schematically illustrates a process 400 for generating a VCM according to one embodiment of the invention. The process executed by the DTM program may be started by receiving the information required for generating the desired DTM. This information may be provided in a batch file or alternatively by an operator of the DTM system, e.g. in the form of a priority rule. For example, an operator may select IMs which may be identified by an IM identifier and which are used in the process for generating the desired DTM. Further, in one embodiment, the operator may also select a geographical area for which the DTM should be generated. The DTM program may determine a DTM hull defining the geographical area covered by a DTM. A DTM hull may be defined by a simply polygon such as a rectangle. Further, the operator may provide the DTM program with one or more priority rules for

determining the stacking order of the selected IM hulls.

The geographical coverage of an exemplary selection of IMs 406-412 within a DTM hull 404 is schematically depicted in 402a. A priority rule is not yet applied so that the geographical area of one IM hull may extend outside the area covered by the DTM hull and so that IMs hulls may comprise one or more overlapping areas.

After identifying the IMs, the DTM program may use the IM identifiers to retrieve associated IM hulls defined by one or more polygons from the survey database. In one

embodiment an IM hull may be determined by a concave hull algorithm as described with reference to Fig. 3. On the basis of one or more priority rules, the IM hulls may be ranked in a predetermined order. A simple (default) priority rule may give the highest priority to the most recent survey (e.g. survey 406) and the lowest to the oldest one (e.g. survey 412).

More complex priority rules may also be used.

Priority rules may be conditional and include selection criteria. In one example a particular DTM a rule may defined as follows: Select all IM hulls of sensor type A and rank them based on quality criteria B. In case of similar B, use the source date instead (more recent IMs takes precedent over older surveys) . In another example the rule may be defined as:

Select all IM hulls within the VCM coverage with source date A. If 100% coverage cannot be achieved based on IMs with source date A use a source data that is as close as possible to A (either before or after) . Continue until the whole coverage is complete. Hence, complex rules may be formulated in order to process the IM hulls. As the hulls are defined as polygons, simple and efficient algorithms may be used to process the polygons .

The DTM program may execute a process wherein the DTM hull and the IM hulls are processed in order to produce a VCM comprising a set of IM hull segments, i.e. IM hulls processed on the basis of a priority rule and an overlap function. In this example, (default) overlap function may be selected, which is configured to subtract the overlapping area between two IM hulls from the IM hull of lowest priority.

To that end, the IM hulls may be ranked in accordance with the selected one or more priority rules and stored in an IM priority list. The DTM program may thereafter start by selecting the DTM hull and a first IM hull of the highest priority 406 in the IM priority list and determining whether the first IM hull lies within the geographical area determined by the DTM hull 404. This may be determined on the basis of the presence of an intersection of the polygons defining both hulls.

In the example of 402b, the DTM program may determine an intersection (the overlapping area) between these two hulls, wherein the overlapping may be defined by a polygon enclosing the gray area 414. This polygon, which coincides with part of the polygon defining the first IM hull, is stored by the program as a first IM hull segment 416 in an IM hull segment table (IM-HST) associated with the VCM.

In the next step (depicted in inset 402c) , the second ranked IM hull 408 is selected. Similar to the first IM hull, the DTM program checks whether it is located within the DTM hull and whether it overlaps the first IM hull segment. If this is the case, the overlapping area 418 may be determined and the second IM hull may be modified by subtracting the overlapping area from the second IM hull so that a second IM hull segment 420 associated with the VCM is formed.

In the following steps (depicted in insets 402d and 402e) , the process is repeated for the third and fourth IM hull 410,412 so that on the basis of overlapping areas

422,426,428 third and fourth IM hull segments 424,430

respectively are determined. As depicted in inset 403e, a survey hull may be modified on the basis of more than one overlaps between different IM hull segments and the IM hull.

During these process depicted in Fig. 4, the IM hull segments are stored in the IM hull segment table. This table may form a data structure comprising a DTM identifier for identifying the DTM, one or more IM identifiers for

identifying IMs and an IM hull segment associated with each IM identifier .

One example for building an IM hull segment table is provided by the following pseudo code listing:

# Build IM list with IM' s that are part of VCM im_list is

select im id

, im_geom

from im_tab

where <im selection criteria>

order by im_priority desc

# Loop through IM list for im_id, im_geom in im_list

loop

# Determine overlap between VCM geometry and IM geometry im__segment_geom = overlap ( vcm_geom, im_geom )

# Store IM segment geometry insert into im_segment_tab ( im_id

, vcm id , im_segment_geom

)

# Subtract IM segment geometry from VCM geometry vcm_geom = vcm geom minus im segment geom

# Check exit conditions

exit when vcm_geom is null

exit when end of im_list

# Go to next IM in IM list end loop

Here IM hulls in an IM list identified by an

identifier are processed by determining the overlap between IM hulls, determining a IM hull segment by subtracting the overlap from a IM hull and storing the IM hull segment in a table in a similar way as illustrated by Fig. 4.

The result of the process is depicted in inset 402f , illustrating the IM hull segments which are the result of processing the IM hulls on the basis of the priority rule. The resulting set of IM hull segments are part of a VCM, which is used by the DTM program to define a seamless continuous geographical area on the basis of which a DTM may be

generated .

It is submitted that Fig. 4 merely provides a non- limiting example for illustrating the invention. Other ways of processing the IM hulls as depicted are also possible without departing from the scope of the invention. For example, in one embodiment a DTM is build without defining its graphical area. In that case, the steps related to inset 402b can neglected.

Furthermore, other ways of forming IM hull segments may be applied. For example, on the basis of the area of overlap of an IM hull segment and an IM hull three IM hull segments may be defined. For example, in the situation

depicted in inset 402c the overlap 418 between IM hull segment 416 and IM hull 408 may define a separate IM hull segment, which may be used to modify IM hull segment 416 and IM hull 408 by subtraction. This way more IM hull segments may be formed wherein segments associated with overlapping areas may require data processing in according to a predetermined overlap function.

For example, it may be possible that on the basis of the IM priority rule two or more IMs hulls receive the same priority in the IM priority list. For example this may be the case when a priority rule is based on the year when an IM hull was generated. If a first and second IM hull are generated in the same year, these hulls may receive the same priority. In that case, the overlap function may determine the area of overlap as at least one separate IM hull segment wherein the overlap function may determine which data in the area of overlap and how these data should be processed.

In one embodiment, the overlap function may relate to a binary overlap function configured to combine or add the data points located within an area of overlap of two or more IM hull segments. In another embodiment, the overlap function may relate to an overlap function configured to select data points within the overlap area from the first occurrence of the overlapping IM hulls in the IM priority list.

In yet another embodiment, an overlap function may be configured to evaluate data points, e.g. each individual data points or groups of data points, in the overlapping IM hull segments. Evaluation of said data points by the overlap function may generate a collection of data points in the area of overlap having an attributed measurement uncertainty below a predetermined threshold value.

The VCM comprises the information required for efficiently generating the desired DTM. Fig. 5 depicts such process for generating a DTM 500 in more detail. When an operator instructs the DTM program to generate a DTM on the basis of the VCM in the VCM database, the DTM program

retrieves in a first step 502 the IM hull segments associated with the VCM. Each IM hull segment will comprises an IM

identifier, which is subsequently used by the DTM program to access point-cloud data in an IM stored in the survey

database. Then in step 504 the DTM program may send a request comprising the IM identifiers and the IM hull segments to the survey database. On the basis of the request, the database may retrieve for each IM identifier and associated IM hull

segment, the point-cloud data that are geographically located within the IM hull segment (step 506) . These data are

subsequently sent to the DTM program, which may geographically insert the bathymetric data within the geographical area of the DTM hull (step 508) . The DTM program may display the resulting DTM on a display.

Hence, the invention uses IM hulls to efficiently select the correct part of the point-cloud data from the total set of point-cloud data associated with an IM without the need of processing data in overlapping survey areas. Conflicting overlapping survey areas may be resolved by processing survey hulls and by using efficient polygon algorithms to eliminate areas of overlap.

As a DTM may be generated on the basis of a VCM on the fly, there is no need to store all point-cloud data associated with one DTM in a DTM database. Instead, a VCM defining a particular DTM product on the basis of IM hull segments, ie. a set of polygons, may be stored for later use, thereby providing a considerable improvement in terms of data management and storage. Moreover, as will be described

hereunder in more detail, DTM products may be easily updated by modifying the VCM without requiring large amounts of data processing.

Fig. 6 schematically depicts the process of updating a VCM. In this example, the DTM should be generated on the basis of the IMs identified in a VCM database and on one or more new additional IMs.

The process depicted in Fig. 6 may start with the DTM program retrieving the IM hull segments and the one or more priority rules from the VCM stored in the VCM database (step 620) . Further, the program may receive update information, e.g. in the form of IM identifiers, associated with one or new IMs (hereafter referred to as an update IM) for use in the updating process. On the basis of the update information, the DTM process may retrieve the update IM hulls from the survey database (step 622). Using the priority rules, the position of the update IM hulls in the IM priority list is determined (step 624) . Thereafter, the one or more overlaps between an update IM hull and the IM hull segments of lower and higher priority with respect to the update IM hull are processed (step 628) .

An IM hull segment of lower priority is modified on the basis of the overlap between that IM hull and the update IM hull by subtracting the area of overlap (defined as a polygon) from the IM hull segment. Similarly, an update IM hull is modified on the basis of the overlap between the update IM hull and an IM hull segment of higher priority by subtracting the area of overlap (defined as a polygon) from the update IM hull.

Insets 602a and 602b schematically illustrate an example of the update process. Inset 602a depicts the IM hull segments 604-610 of the VCM as described with Fig. 4 and a new (update) IM hull 612 which should be added to the VCM. The new IM hull has two geographically overlapping areas 614,616. The DTM program determines that the position of the update IM in the IM priority list is in between the IM associated with IM hull segment 608 (of lower priority) and IM hull segment 606 (of higher priority) . Thereafter, IM hull segment 608 is modified on the basis of its overlap 614 with update IM hull 612 resulting in IM hull segment 618 and update IM hull 612 is modified on the basis of its overlap 616 with IM hull segment 614 resulting in IM hull segment 620. The updated VCM is subsequently stored in the VCM database (either a new VCM or as a replacement of the earlier VCM) . The updated VCM is schematically illustrated in inset 602b.

An example of pseudo-code for updating an exemplary

VCM is provided hereunder:

# Build list if IM segments that are part of CM and overlap with geometry new IM im_list is

select im_id

, im_segment_geom

from im_segment_tab

where <im selection criteria>

and im_geometry in new_im_geom order by im_priority desc

# Set IM segment priority to higher than new IM lower_priority = false

# Loop through IM list for im_id, im_segment_geom in im_list

loop

# Compare priority of new IM with IM segment if lower_priority = false

then if im_id = new_im_id

then

# Set IM segment priority to lower than new IM lower_priority = true

else

# IM segment has higher priority than new IM

# Subtract geometry IM segment from geometry new new_im_geom = new_im_geom minus im_segment_geom endif endif

# IM segment has lower priority than new IM if lower priority = true

then

Store geometry new IM segment in VCM if im_id = new_im

then insert into im_segment tab ( new_im_id

, vcm id

new_im_geom else

# IM segment has lower priority than new # Substract geometry new IM from geometry IM segment im_segment_geom_updated = im_segment_geom minus new_im__geom

# Update IM segment geometry update im_segment_tab

set im_segment_geom = im_segment_geom_updated where im_id = im_id end if end if

# Check exit conditions

exit when new_im_geom is null

exit when end of im_list

# Go to next IM in IM list end loop

This algorithm thus allows simple updates of a VCM without actually processing the survey data as such. The VCM may be stored next to the earlier VCMs so that the history of a DTM associated with these VCM may be efficiently recorded.

In addition adding one or more IM hulls to a VCM, the DTM program may also allow removal of one or more IM hull segments from a VCM. This is schematically illustrated in Fig. 7. Inset 702a depicts an example similar to the VCM as

described with Fig. 4, wherein IM hull segments 708 is removed from the VCM comprising hull segments 704-710.

So after having received the IM hull segments

associated with a DTM (step 720) and the removal of at least one of the survey hull segments (step 722) , the DTM program may first determine on the basis of the priority rule (step

724) a new IM priority list from which the one or more removed IMs are excluded. Thereafter, on the basis of the new IM priority list, new IM hull segments are determined on the basis of the priority rule (steps 726 and 728) in a similar way as described with reference to Fig. 4. For example, in the example depicted in inset 702a, IM hull segment 708 is removed from the VCM. Hence, when building IM hull segments on the basis of the new IM priority list, the IM hull associated with the (earlier) IM hull segment 710 is now only modified on the basis of the overlap with IM hull segment 706. The updated VCM is subsequently stored in the VCM database (either a new VCM or as a replacement of the earlier VCM) . The updated VCM is schematically illustrated in inset 702b.

The pseudo-code associated with an algorithm for removing a IM hull segment from the VCM may look as follows:

# Delete IM segment from IM segment table

Delete from im_segment_tab where im_id = deleted_im_id

# Build IM list with IM' s that that are part of CM and overlap with geometry of deleted IM segment select im_id

, im__geom

from imjtab

where <im selection criteria>

and im___geom in deleted__im__geom

order by imjpriority desc

# Loop through IM list for im_id, im_geom in im__list

loop

# Determine overlap between deleted IM geometry and IM geometry

im_segment_geom_add = overlap (deleted__im_geom, im_geom )

# Select geometry IM segment for IM select im_segment_geom

from im_segment_tab

where im_id = im_id

# Union im_segment_geom and im_segment_geom__add for IM im_segment_geom__updated = union ( im_segment_geom,

im_segment_geom_add)

# Update IM segment geometry update im__segment_tab

set im_segment_geom = im_segment_geom_updated where im_id

# Subtract IM segment geometry from CM geometry deleted_im_geom = deleted_im_geom minus

im_segment_geom_add

# Check exit conditions exit when im_segment_geom_add is null

exit when end of im_list

# Go to next IM in IM list

end loop Hence, on the basis of a virtual continuous model comprising pre-processed IM hull segments it is possible to produce a DTM on the fly. A DTM may be updated by simply updating and storing its associated VCM, thereby providing substantial advantages in terms of flexibility and data management when compared with the conventional way of

producing a DTM.

It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. One embodiment of the invention may be

implemented as a program product for use with a computer system. The program (s) of the program product define functions of the embodiments (including the methods described herein) and can be contained on a variety of computer-readable storage media. Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, flash memory, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive or any type of solid-state random-access semiconductor memory) on which alterable information is stored. Moreover, the invention is not limited to the embodiments described above, which may be varied within the scope of the accompanying claims without departing from the scope of the invention.

Claims

1. Computer-implemented method for building at least one digital terrain model comprising:

selecting a predetermined number of surveys for building a digital terrain model, each survey comprising at least one survey hull, said survey hull defining a

geographical area in which geo-located data, preferably geo- located point-cloud data, more preferably bathymetric data, associated with said survey are located;

ranking said survey hulls on the basis of at least one priority rule;

determining one or more areas of overlap between at least two overlapping survey hulls; and,

generating survey hull segments on the basis of said ranked survey hulls, said survey hull segments defining a non- conflicting continuous geographical area associated with said digital terrain model, said one or more areas of overlap are removed by modifying at least one survey hull of said at least two overlapping survey hulls on the basis of said ranking.

2. Method according to claim 1, said method further comprising :

generating a survey hull segment by subtracting said area of overlap from at least one of said overlapping survey hulls.

3. Method according to claims 1 or 2, said method further comprising:

determining said on or more areas of overlap by calculating one or more intersections between said survey hulls.

4. Method according to any of claims 1-3, wherein said survey hulls and/or survey hull segments are determined on the basis of one or more concave hull algorithms.

5. Method according to any of claims 1-4, further comprising the step of:

determining geo-located data associated with said survey hull segments;

generating a digital terrain model on the basis of said determined geo-located data.

6. Method according to claim 5, wherein geo-located data associated with a survey hull segment are determined by the geo-located data which lie within the geographical area determined by said survey hull segment.

7. Method according to any of claims 1-6 comprising: storing said one or more survey hull segments associated with a digital terrain model in a segment table, preferably said segment table further comprising survey identifiers for identifying a survey associated with a survey hull segment.

8. Method according to any of claims 1-7 wherein said at least one priority rule is used for determining in an area of overlap associated with two or more overlapping survey hulls which of said overlapping survey hulls has the highest priority, preferably said priority rule being based on survey metadata information.

9. Method according to claim 8, wherein said survey metadata information including at least one survey parameter from the list including: time of generation of the geo-located data, sensor system used for measuring the geo-located data,

10. Computer-implemented method for modifying at least one digital terrain model comprising:

identifying of one or more survey hull segments and at least one priority rule, said survey hull segments and said rule being associated with said digital terrain model, said survey hull segments defining a non-conflicting continuous geographical area associated with said digital terrain model and each of said survey hull segments being associated with geo-located survey data, preferably geo-located point-cloud data, more preferably bathymetric data;

receiving at least one further survey hull; ranking said survey hulls segments and said further survey hull on the basis of said priority rule;

determining one or more areas of overlap between said further survey hull and said one or more survey hull segments;

generating a further survey hull segment on the basis of said one or more areas of overlap and said ranking; and, storing said one or more survey hull segments and said at least one further survey hull in a segment table associated with said digital terrain model.

11. Computer-implemented method for modifying at least one digital terrain model comprising:

removing at least one survey hull segment from the said identified survey hull segments;

ranking said survey hull segments on the basis of at least one priority rule;

determining one or more areas of overlap between said survey hull segments;

generating survey hull segments on the basis of said ranked survey hulls, said survey hull segments defining a non- overlapping continuous geographical area associated with said digital terrain model, wherein said one or more areas of overlap are removed by modifying at least one survey hull of said at least two overlapping survey hulls on the basis of said ranking; and, storing said one or more survey hull segments and said at least one further survey hull in a segment table associated with said digital terrain model.

12. System for generating at least one digital terrain model comprising:

a survey database comprising one or more surveys comprising geo-located data, preferably geo-located point- cloud data, more preferably bathymetric data, preferably bathymetric data, and one or more survey hulls, each survey hull defining a hull of a geographical area in which said geo- located data are located;

a computer system configured for accessing said survey database, said computer system further being configured for selecting a predetermined number of surveys for building a digital terrain model; ranking said survey hulls on the basis of at least one priority rule; determining one or more areas of overlap between at least two overlapping survey hulls; and, generating survey hull segments on the basis of said ranked survey hulls, said survey hull segments defining a non- overlapping continuous geographical area associated with said digital terrain model, wherein said one or more areas of overlap are removed by modifying at least one survey hull of said at least two overlapping survey hulls on the basis of said ranking.

13. System according to claim 12 further comprising: a database for storing one or more segment tables, each being associated with a digital terrain model and each comprising at least one or more non-overlapping survey hull segments .

14. A data structure for use in a system according to claims 12 or 13, said data structure comprising one or more survey hull segments and a survey identifier associated with each of said survey hull segments, wherein said survey hull segments define a non-overlapping continuous geographical area associated with a digital terrain model, and wherein each survey hull segment defining a hull of a geographical area in which geo-located data, preferably bathymetric data,

associated with said survey are located.

15. Δ computer program product, the computer program product comprising software code portions configured for, when run a computer, executing a method according to any of claims 1-11.