WO2002001257A1

WO2002001257A1 - Method for measuring water level

Info

Publication number: WO2002001257A1
Application number: PCT/SE2001/001328
Authority: WO
Inventors: Rune Axelsson; Rolf ENGSTRÖM; Andreas Axelsson
Original assignee: Saab Ab
Priority date: 2000-06-26
Filing date: 2001-06-11
Publication date: 2002-01-03
Also published as: SE515278C2; SE0002400D0; AU2001274730A1; SE515278C3; SE0002400L

Abstract

This invention concerns a method for calculating a normal water level at a measurement point (15) in an ocean, lake or waterway. The method is charcacterized in that a first water level (3) is recorded at a control point (10) relative to a reference ellopsoid (11), which control point has a precisely calculated normal water level (ho). In addition, a second water level (3) is recorded at the measurement point (15) relative to the reference ellipsoid (11). The two recorded measurements are then correlated with one another to calculate the normal water level at the measurement point (15) relative to the reference ellipsoid (11).

Description

METHODS FOR MEASURING WATER LEVEL

TECHNICAL SCOPE

This invention concerns methods for measuring water levels in the ocean, lakes and waterways.

STATE OF THE ART

Measurement sites where long-term average water surface levels are measured are present in navigated oceans, lakes, rivers and other waterways around the world. It is customary at such sites to measure the water surface level by recording water line observations by means of mareographs situated at the water's edge. Because the water level is measured over a long period of time, statistical processing of the measurement values provides a good idea of how the level of the water surface varies over time at the measurement site, and makes it possible to determine the average level of the water surface.

A number of factors affect the level of the water surface regionally, such as tides, atmospheric pressure, winds, ocean currents, water temperature, etc., while other factors affect the level locally, including bottom contours, islands, coastlines, etc. Because of the effects of one or more of these factors, it is difficult to draw conclusions about the areas between measurement sites on the basis of information about the level of the water surface at the measurement sites.

DESCRIPTION OF THE INVENTION

According to an embodiment of the invention, a method is provided for calculating a normal water level at a measurement point in an ocean, lake or waterway, wherein a first water level is recorded at a control point relative to a reference ellipsoid, which control point has a precisely calculated normal water level, wherein a second water level is recorded at the measurement point relative to the reference ellipsoid, and wherein the two recorded measurements are correlated with one another to calculate the normal water level at the measurement point relative to the reference ellipsoid. In this way the normal water level can be determined at a number of measurement points within an area at a distance from the control point such that the variations in the water level in the area do not directly agree with the water level variations at the control point. Given knowledge of the normal water levels within the area relative to the reference ellipsoid it is possible, by applying said knowledge, to correct water depth measurements obtained by means of, e.g. laser bathymetry.

In order to obtain a good idea of the water level at the measurement point relative to the level at the control point, the first and second water level recordings in a preferred embodiment comprise a series of more than one water level values, where the recordings are made in conjunction with one another.

In a simple embodiment, the recording of the second water level consists in that a bathymetry system measures a first distance between the water level and the system, and in that a position-finding. unit determines a second distance between the system and the reference ellipsoid, whereupon the first and the second distance are added to one another.

The recording of the first water level consists, e.g. in that a receiver unit is placed at the water's surface at the control point, which unit is arranged so as to receive information concerning its position relative to the reference ellipsoid, whereupon the receiver unit performs said water level recording. In an embodiment, the recording of the second water level occurs in the same way.

In an alternative embodiment, a bottom level is determined at the measurement point and/or control point relative to the reference ellipsoid, and the water depth is measured, whereupon the water level is recorded as the bottom level plus the water depth. The bottom level is measured by means of, e.g. a laser bathymetry system with a position-finding unit, and the depth is measured using, e.g. a pressure gage placed on the bottom. According to another embodiment of the invention, a method is provided to make water level measurements in an ocean, lake or waterway in which the water level is recorded at a control point relative to a reference ellipsoid, which control point has a precisely calculated normal water level, wherein the water level is recorded at at least one measurement point relative to the reference ellipsoid, and wherein the two recorded measurements are correlated with one another to calculate the water level at the measurement point relative to the water level, at the control point.

FIGURE DESCRIPTION

Figure 1 shows an example of a helicopter equipped with a laser bathymetry system.

Figure 2 shows a schematic view of the laser bathymetry system in Figure 1.

Figure 3 shows a pulse reflection to a receiver in the laser bathymetry system in Figure 2.

Figure 4 illustrates an example of a method for determining a normal water level.

Figure 5 shows an example of two measurement series taken in the method in Figure

4.

Figure 6 shows an example of a method for determining the water level along a stretch of water relative to the level at a control point..

PREFERRED EMBODIMENTS

In Figure 1 , reference number 1 indicates a helicopter with a helicopter-based laser bathymetry system 2 for measuring water depth in an ocean, lake, river or other waterway. Reference number 3 indicates the water surface, while reference number 4 indicates the bottom of the ocean, lake or waterway.

Figure 2 shows that the system 2 contains a laser 5. An aiming device (not shown) is placed in front of the laser 5 to aim the laser beam at the water surface at a selected angle. The aiming device consists of, e.g. mirrors that are rotatable in at least one direction placed in the radiation path of the laser. The laser emits pulsed radiation in the infrared wavelength range while simultaneously emitting pulsed radiation within the wavelength range of visible light, preferably green light. The infrared radiation is reflected from the water surface 3, while a substantial portion of the green light penetrates down into the water and is reflected back from the bottom 4. The system 2 includes a receiver 6 arranged to record the intensity of the reflected radiation. Figure 3 shows the thus recorded pulse reflection with two intensity peaks.. The first peak represents the reflection from the water surface, while the second peak represents the reflection from the bottom. A calculating unit 7 connected to the receiver 6 calculates the time interval between the two intensity peaks, whereupon the water depth is calculated as one-half of the time interval multiplied by the speed of light.

Using the laser bathymetry system 2, it is possible to make depth measurements over an area relatively quickly, since the helicopter flies at, e.g. 100 - 120 knots at an altitude of 200 - 500 meters above the water. The laser will then sweep an area of 100 - 250 meters transverse to the direction of flight. The system can be used for measurements within the depth range relevant to ocean travel, i.e. from very shallow water to depths of approximately 20 - 25 meters, depending on the water quality, type of bottom, etc. Such measurements may of course be made just as well from an airplane.

However, because the water depth at any single point varies over time, the measurements are encumbered by uncertainty with respect to the extent to which the depth at the time the measurement was made is representative of the depth at other times. Factors that affect depth and thus the level of the water surface regionally include tides, atmospheric pressure, winds, ocean currents, water temperature, etc., while bottom contours, islands, coastlines, etc. affect the level locally. To reduce this uncertainty, control measurement points are located worldwide in navigated oceans, lakes, rivers and other waterways. In figure 4, reference number 10 indicates such a control measurement site, where the level of the water surface is being measured over a long period of time. Because the water level is measured over a long period of time, statistical processing of the measurement values provides a good idea of how the level of the water surface varies over time at the measurement site 10, and makes it possible to determine the average level of the water surface. These control measurement sites 10 are located, e.g. along coastlines and the lakeshores. These control measurement sites are often so far apart that it is difficult or impossible to calculate the effects of at least the local factors on the water levels, and this may include the regional factors as well. Correcting the water depths measured using laser bathymetry by a value corresponding to the deviation of the water surface from an average value for the nearest control measurement site 10 would thus yield satisfactory results only for measurements in the immediate vicinity of the control measurement site 10.

A global coordinate system known as World Geodetic System 1984 (WGS 84), which is utilized by GPS, is used to obtain an improved value for use in correcting the measured depth. WGS 84 provides a global frame of reference that is fixed relative to the earth. This frame of reference is described by an reference ellipsoid 11, which comprises a mathematical surface that represents the general shape of the globe and is used to indicate positions in space, i.e. latitude, longitude and altitude. Altitude is thus expressed relative to the aforementioned reference ellipsoid 11. The reference ellipsoid used in WGS 84 is a global reference ellipsoid that attempts to optimally approximate the entire surface of the earth. There are other reference ellipsoids that attempt to optimally approximate specific parts of the earth's surface. For instance, Bessel's reference ellipsoid, which is considered to best approximate the surface of Sweden, is used in Sweden. It is possible to convert coordinates relative to a given reference ellipsoid into coordinates relative to a different reference ellipsoid, and the invention must not be considered to be limited to applicability solely to the coordinates given in WGS 84, even though WGS 84 is used exclusively in this description.

According to a method for obtaining said the improved value, the measured water level is determined at the control measurement site 10 relative to the reference ellipsoid 11. The level of the water surface at the control measurement site 10 is determined in the depicted embodiment with the help of a position-finding unit 12, such as a GPS receiver, which is arranged on a floating base 13. In an alternative embodiment, the level of the water surface is derived by means of, e.g. a device that is permanently mounted on a bridge and whose position relative to the reference ellipsoid has been precisely measured. The device is connected to a float, whose level is continuously recorded. Regardless of how the measurement values are obtained, a measurement comprising a series of multiple measured values is recorded for the control measurement site 10. Figure 5 shows a diagram (a) of an example of a measurement series for the control measurement site 10 where, as noted above, ho represents the long-term average level of the water surface relative to the reference ellipsoid, and the curve represents the measured water surface level 3 relative to the reference ellipsoid 11.

The level of the water surface at a measurement point 15 is measured relative to the reference ellipsoid simultaneous with the measurement made by the position-finding unit 12 at the control measurement site 10. In the embodiment shown in Figure 4, the former measurement is made using a helicopter 1. The helicopter 1 laser bathymetry system 2 in Figure 2 is equipped with a GPS receiver 8 for this purpose. The GPS receiver 8 receives information regarding its altitude relative to the reference ellipsoid 11 and is thus used to determine the distance between the system 2 and the reference ellipsoid 11. The laser 5, receiver 6 and calculating unit 7 are used to determine the distance between the system 2 and the water surface 3, e.g. by measuring the interval between the emission of a pulse from the laser 5 and the recording by the receiver 6; this time is divided by two and the result is then multiplied by the speed of light. The calculating unit 7 then adds the distance between the system 2 and the reference ellipsoid 11 to the distance between the system 2 and the water surface 3, thus yielding the distance between the reference ellipsoid 11 and the water surface 3 at the measurement point 15. Figure 5 provides a diagram (b) of a series of measured values at measurement point 15 representing the distance between the reference ellipsoid 11 and the water surface 3. In the embodiment in Figure 4, the position-finding unit 12 is connected to a radio transmitter 14 that transmits the position information from the unit 12 (measurement series (a)) to a processing unit 16. The system 2 also has a radio transmitter 9 (Figure 2) to transmit the received measurement series (b) to the processing unit 16. The measurement series (a) and (b) are correlated in the processing unit 16, and the average water surface level at the measurement point 15 is thus calculated relative to the reference ellipsoid 11. In one example, the difference between the water surface level at measurement point 15 and the level at the control measurement site 10 is calculated by correlation for each measurement case, whereupon an average level difference value is calculated along with the magnitude of the spread. In the subsequent calculation of the average water level at measurement point 15, this value is calculated as the average level ho measured relative to the reference ellipsoid 11 as corrected by the level difference obtained from said correlation.

In an alternative embodiment without transmitters 14 and 9, the measurement series for the measurement point 15 and the measurement control site 10 are stored in a memory or printed out on paper, whereupon the obtained measurement series are retrieved manually from the system 2. and the control measurement site 10 and input into the processing unit 16 to calculate the average surface level at the measurement point 15. Note that this is just an example; many solutions for transferring the measurement series to the processing unit 16 will be obvious to one skilled in the art.

If the effects of the local and regional factors entail only relatively long-term variations, such as variations depending on the tide, then it is not necessary to take measurement series a and b simultaneously, and it will instead suffice to carry out the measurements in conjunction with one another to such an extent that equivalent conditions prevail. In an example, the measurements are made in succession at the control measurement site 10 and at the measurement point 15 from the helicopter 1.

Using the aforedescribed method, we have now calculated an average value for the water level 3 relative to the reference ellipsoid 11 at the measurement point 15. Using laser bathymetry measurements made in the areas associated with this point, it is now possible to compensate for the obtained water depth. The water depth is measured in the usual manner, and the distance between the surface 3 and the ellipsoid 11 is determined. If the distance derived does not agree with the average water level, then the measured depth is corrected by the difference between the derived distance and the mean water level to yield an "average depth."

Simple methods exist to make repeated depth measurements at a point or within a limited area out at sea or on a lake, e.g. using a conventional pressure gage (not shown) placed on the bottom. In an alternative embodiment, the pressure gage is placed on the bottom 4 to measure the depth on a number of occasions. To obtain a reference to which to relate the measured depths, the level of the bottom 4 is measured relative to the reference ellipsoid using, e.g. the laser bathymetry system 2, whereupon a first distance between the system 2 and the reference ellipsoid 11 is recorded by the GPS receiver 8 and a second distance between the system 2 and the bottom 4 is determined using the laser 5, receiver 6 and calculating unit 7. The calculating unit 7 then adds the first distance to the second distance. Measurements of the bottom level relative to the reference ellipsoid 11 , which are relatively resource-intensive, thus need be performed only once, while the repeated depth measurements can be made using relatively simple equipment, such as pressure gages. Because the location of the bottom is known relative to the reference ellipsoid, the measurement series (b) in Figure 5 can be created by adding the measured depths to the bottom level. The measurement series (5) in Figure 5 is simultaneously recorded at the control measurement site 10 and correlated with the long-term average water line ho as described above. In recording these measurements, the water surface level is measured either directly by means of, e.g. the position-finding unit 12, or indirectly via water depth measurements made using a pressure gage and a measurement of the bottom level relative to the reference ellipsoid 11. The .; measurement series are then correlated with one another as described above, and the resulting level difference is used as a factor to correct ho so that the corrected h₀ , value indicates the average water surfaceTevel relative to the reference ellipsoid;! 1 within an area associated with the measurement point 15, within which area the:, effects of the local and regional factors may be assumed to be such that the level difference as determined above is consistent throughout the entire area.

The information about the location of the bottom 4 relative to the reference ellipsoid 11 is also useful for navigation. A vessel equipped with equipment, such as a GPS receiver, for determining the location of the vessel' s keel relative to the reference ellipsoid can use the information about the location of the bottom 4 expressed using the same frame of reference to determine the clearance between its keel and the bottom.

Figure 6 illustrates the use of the laser bathymetry system 2 to measure geographical tidal variations, e.g. along a stretch of coastline. In an example where factors that give rise to short-term variations in the water surface level can be ignored, the helicopter flies over the control measurement site 10 at which the surface level is recorded relative to the reference ellipsoid. The helicopter than flies along the stretch of coastline along a path 17 while at the same time measuring the water levels relative to the reference ellipsoid. In the figure, the measurement points are marked with checkmarks along the path 17. However, if short-term variations are suspected, the helicopter should fly along a path 18. Following path 18 and flying over the same points a number of times make it possible to set the calculating unit 7 to compensate for water level differences that are attributable to the short-term variations; in a simple embodiment, by averaging the values obtained at each point. After averaging, we obtain a measurement series in which each value corresponds to the water surface level at a specific point. The measurement points along path 18 are also marked with checkmarks. The obtained measurement series in the case involving path 17 and the case involving path 18 thus describe how the effects of tides vary geographically, assuming that the measurement values have been collected within a time interval during which the tidal water level may be assumed constant, i.e. characteristically within half an hour.

When the term "GPS receiver" is used in this description, it is assumed that the GPS receiver is equipped with another receiver for receiving correction signals, a so- called "DGPS". To further improve the accuracy of the measurements, one example cites the use of a reference point with known coordinates, which make it possible to achieve measurements that are accurate to within a matter of centimeters. However, this means of improving the accuracy of position fixes from a GPS receiver must be viewed as known to those skilled in the art.

Claims

1. A method for calculating a normal water level at a measurement point (15) in an ocean, lake or waterway, characterized in that a first water level (3) is recorded at a control point (10) relative to a reference ellipsoid (11), which control point has a precisely calculated normal water level (ho), and in that a second water level (3) is recorded at a measurement point (15) relative to the reference ellipsoid (11), and in that the two recorded measurements are correlated with one another to calculate the normal water level at the measurement point (15) relative to the reference ellipsoid (11).

2. A method according to Claim 1, characterized in that the first and second recorded water level measurements comprise respective series (a, b) of more " than one water level value.

3. A method according to Claim 2, characterized in that the measurements are recorded in conjunction with one another.

4. A method according to Claim 1, characterized in that the recording of the second water level entails that a laser bathymetry system (2) measures a first distance between the water surface (3) and the system (2), and in that a position-finding unit (8) determines a second distance between the system (2) and the reference ellipsoid (11), whereupon the first and second distances are added to one another. (11).

5. A method according to Claim 1, characterized in that a receiver unit (12) is arranged at the measurement point (15) and/or the control point (10), which unit is designed to receive its position relative to the reference ellipsoid (11), whereupon the receiver unit records the first and/or second water level.

6. A method according to Claim 1, characterized in that a bottom level (4) is determined at the measurement point (15) and/or the control point (10) relative to the reference ellipsoid (11), and in that the water depth is measured, whereupon the water level (3) is recorded as the bottom level (4) plus the water depth.

7. A method according to Claim 6, characterized in that the water depth is measured by measuring the pressure at the bottom.

8. A method according to Claim 6 or 7, characterized in that a laser bathymetry system (2) with a position-finding unit (8) is used to measure the bottom level.

9. A method for measuring water levels in an ocean, lake or waterway, characterized in that the water level is recorded at a control point (10) relative to a reference ellipsoid (11), which control point (10) has a precisely calculated normal water level, and in that the water level is recorded at at least one measurement point (15) relative to the reference ellipsoid, and in that the two recorded measurements are correlated with one another to calculate the water level at the measurement point (15) relative to the water level at the control point.