GB2366381A

GB2366381A - Retroreflective device for use with an EDM total station

Info

Publication number: GB2366381A
Application number: GB0020827A
Authority: GB
Inventors: James Edward Taylor
Original assignee: OIS PLC
Current assignee: OIS PLC
Priority date: 2000-08-23
Filing date: 2000-08-23
Publication date: 2002-03-06
Also published as: GB0020827D0

Abstract

A retroreflector 1 for use as an EDM Total Station target comprises a plurality of retroreflective elements 4 disposed in an arc of constant radius. The retroreflector may have releasable attachment means (7 and 8, Fig 2) such as a magnet for attaching it to a structure. The retroreflector may also have a semi-disc shaped body 2 with the retroreflective elements, which may be in the form of a retroreflective sheet material, mounted to the curved face 3 of the body. A sighting element (16, Fig 3) encompassed in a transparent layer (15, Fig 3) on the body may project axially from the body at the centre of the arc's curvature. A method of surveying ferromagnetic posts (12, Fig 5) either side of a permanent way (11, Fig 5) using the magnet mounted retroreflector is also disclosed.

Description

<Desc/Clms Page number 1> Surveying Device and Method of Surveying Description The present invention relates to a surveying device and to a method of surveying.

A survey of an area of land is an essential requirement before any construction work is undertaken. It is also important to conduct regular repeat surveys in order to monitor subsequent movement of the land and structures constructed thereon. In the case of railways with overhead electrification, for example, it is necessary to periodically survey the support structures of the catenary and power cables. Conventionally, the surveying of structures such an overhead power cable supports has been carried out in the following manner. A reference framework is determined by obtaining x, y and z co-ordinates of at least two fixed reference points from which all future measurements are subsequently derived. In the United Kingdom, the Ordinance Survey National Grid co-ordinates of these two points are obtained using a GPS (Global Positioning System) terminal. Since the likely movements of the structures being surveyed are very small, the accuracy of the determination of the co-ordinates must be very high and consequently GPS apparatus, which can correct for the systemically introduced timing errors, is used. Once the co-ordinates have been established, permanent ground marks (PGMs) in the form of pins or bolts set into the ground are installed so that future mapping can be conducted based on measurements taken from exactly the same points. An Electric Distance Measurement Total Station (EDM) and a retroreflector, placed at respective PGMs, can then be used to calibrate the EDM, in particular its orientation with respect to north.

Once calibrated, and with the EDM remaining at the first PGM, the retroreflector is taken in turn to each structure, e.g. overhead cable support posts, that is visible with the EDM and the bearing and distance to each structure is measured.

When measurements from the first PGM are complete, the EDM is moved to the second PGM and re-calibrated using a retroreflector at a third PGM. A further series of structures can then be surveyed.

The retroreflector typically includes a tribach (a centring prism and prism at which the telescope is aimed) mounted on a tripod incorporating levelling equipment. The retroreflector must be precisely positioned and levelled above the PGMs to ensure that accurate, repeatable measurements are taken.

A problem with the above described technique is that the PGM's are often dislodged or removed by mistake, by vandals or by employees who are unaware of their purpose. If they are removed, a surveyor must re-establish the reference points using GPS before any further readings can be taken and this is time consuming and laborious. If the PGM has been moved, the surveyor might take inaccurate measurements based on the incorrectly positioned PGM without realising until much later, meaning that all his measurements will be worthless and the survey must be repeated after re-positioning the PGM at the correct co-ordinates.

In addition to the foregoing, the accurate positioruing of the retroreflector target is a complex procedure and it is a cumbersome and heavy piece of equipment to carry to and around the survey site.

According to the present invention, there is provided a retroreflector for use as a target for an EDM, the retroreflector comprising a plurality of retroreflective elements disposed in an arc of constant radius. The arc is preferably 180 degrees. If the arc is less than 180 degrees some reference structure may be required to ensure that the retroreflectors are equidistant from the EDM aiming point. The retroreflective elements may be disposed in arcs in more than one plane and maybe disposed over a hemispherical surface. However, it is often sufficient for the retroreflective elements to be disposed over a semi-cylindrical surface.

Preferably, releaseable attachment means are provided for attaching the retroreflector to a structure. More preferably, the attachment means comprises one or more magnets.

Preferably, the retroreflective elements comprise elements of retroreflective sheet material.

Preferably, a retroreflector according to the present invention includes a semi-disc- shaped body, which may be Solid, with the retroreflective elements mounted to its curved face.

Preferably, there is provided a sighting element projecting axially from the body at centre of curvature of said arc. More preferably, the sighting element is encompassed in a transparent layer on said body.

Preferably, a level indication, such as a spirit level, is incorporated in the retroreflector to assist in levelling it.

According to the present invention, there is also provided a method of carrying out a repeat survey of a plurality of permanent structures on an area of land, the method comprising the steps of:- (a) locating an EDM at an arbitrary position from which said structures are surveyable; (b) determining the bearings and distances to a subset of said structures using the EDM at said arbitrary position; (c) determining the translation and rotation required to map a subset of the coordinates, relative to the EDM, obtained in step (b) onto the corresponding coordinates previously obtained for the same structures according to another reference frame; (d) applying said translation and rotation to origin coordinates to obtain the EDM's coordinates in said other reference frame; and

(e) determining the bearing and distance of the structures from the EDM and determining the positions of said structures relative to said other reference frame therefrom.

Conveniently, the subset of coordinates may be exclude coordinates of structures in said subset of structures that have moved significantly relative to the other members of the subset of structures.

According to the present invention, there is further provided a method of surveying including placing a retroreflector according to the present invention against a structure to be surveyed at a predetermined position thereon.

According to the present invention, there is yet further provided a method of surveying ferromagnetic posts on either side of a permanent way, the method comprising the steps of:- (a) magnetically attaching a retroreflector according to the present invention on a first plurality of posts on one side of the permanent way in a first direction; (b) magnetically attaching a retroreflector according to the present invention on a second plurality of posts opposite the first plurality of posts on the other side of the permanent way in a second opposite direction; (c) removing the retroreflectors attached in step (a); and (d) removing the retroreflectors attached in step (b), wherein bearing and distance measurements to each attached retroreflector are made using an EDM.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 shows a first retroreflector according to the present invention; Figure 2 is a rear view of the retroreflector of Figure 1; Figure 3 shows a second retroreflector according to the present invention; Figure 4 is a rear view of the second retroreflector of Figure 3;

Figure 5 is a plan view of a section of permanent way with overhead power lines to show how the theodolite is initially calibrated using two fixed reference points; and Figure 6 is the same plan view of Figure 5 to show how the arbitrary position of the theodolite is determined when a repeat survey is performed.

Referring to Figure 1, a retroreflector 1 for use in surveying comprises a semi-disc- shaped plastic body 2. The curved face 3 of the body 2 is covered with a layer of a retroreflective sheet material 4 (e.g. LGP Diamond grade obtainable from 3M). A bull's eye bubble level 5 is located in one of the semi-circular major faces 6 of the body 2. The device may be fixed to non-magnetic structures by bolting into a prefixed thread.

Referring to Figure 2, first and second magnets 7, 8 are located towards opposite ends of the rectangular, back side 9 of the body 2. The magnets 7, 8 enable the retroreflector 1 to be temporarily mounted to ferrornagnetic structures. A centre line 10 extends across the major face 6 from the middle of the rectangular back side 9 of the body 2 to the curved face 3.

In a modified embodiment of the retroreflector of the invention, shown in Figures 3 and 4, the thickness of the semi-disc shaped plastic body is increased by the addition of a transparent plastics layer 15 extending over a major face 6. A brightly coloured finger element 16 upstands from the major face 6 from the centre of the rectangular back face 9 through the transparent plastics layer 15. The reason for this will become apparent.

Referring now to Figure 5, there is shown an aerial view of an area of land over which a survey is to be carried out and on which a permanent way 11 has been laid. A number of posts 12, supporting overhead power lines for powering trains running along the permanent way 11, are positioned at regular intervals alongside the track. The posts 12 are generally made from I-section steel beams which are securely bolted to respective deep concrete bases (not shown) to prevent movement.

Referring again to Figure 5, the co-ordinates (x, y, z) of two reference points 14, 15 are established using a high-precision GPS terminal. As explained above with reference to the prior art, an EDM can then be located at the first reference point 14 and a conventional retroreflector at the second to enable the EDM to be calibrated with respect to north.

Next, a permanent mark (not shown) is made at a predetermined elevation on each post 12 on a side visible to the EDM which is maintained on the first reference point 14. The mark can be made with a centre punch or by any other means that ensures its permanency. Ideally, the initial mark at a precise measured elevation is made with a centre punch and two lines are then scribed on the surface of the post 12 that intersect on the punch mark. This makes the centre punch mark easier to locate when repeat survey measurements are to be taken.

The rectangular back side 6 of a retroreflector 1, described with reference to Figures 1 and 2 or 3 and 4 above, is then placed against all the posts 2 visible to the EDM and is positioned so that the centre line 10 on the upper major face 6 is in precise alignment with the centre punch mark on the post 12 and is level. Levelling is achieved using the bull's eye bubble level 5 located in the major face 6. The retroreflector 1 is easily positonable and mountable on the post 12 due to the magnetic attraction between the magnets 7, 8 in the back side 9 of the body 2 and the post 12.

The EDM is aimed directly at the mark on a first post 12 behind the retroreflector 1 and a measurement of the bearing and distance between the post 12 and the EDM is obtained. The distance from the EDM to the mark can be obtained in a conventional manner using a laser beam emitted from the EDM which is directly reflected back from the retroreflective sheet material 4 on the curved face 3 on the retroreflector 1 to give a range measurement. As the curved face 3 is formed on a semi-circle, the distance from the point of impact of the laser on the retroreflector 1 to the mark on the post 12 is always accurately known because it is equal to the radius of the semi-circle irrespective of where the laser hits the retroreflective sheet

material 4. This measurement can be added to the range measured by the EDM to give the exact distance from the EDM to the mark on the post 12.

When the modified retroreflector 1 described above and shown in Figures 3 and 4 is used, the finger element 16 indicates the exact location of the centre of the back face and therefore the position of the permanent mark on the structure on which the retroreflector 1 is placed. This ensures that the EDM is aimed exactly on the mark by pointing it at the finger 15 visible through the transparent plastics layer 15. The finger 15 is embedded in the transparent layer 15 to prevent it snagging when the retroreflector 1 is carried in a bag or a pocket.

The above described procedure is repeated so that the positions of all of the posts 12 visible to the EDM are recorded. It will be appreciated that data can be taken quickly if a retroreflector 1 is positioned on all the posts 12 rather than moving one retroreflector I from post to post before each reading is taken. This is made possible because the retroreflectors I of the present invention are small and light, meaning that one man can carry a relatively large number of them in a bag and walk up and down the track placing one on each post 12 as he does so.

Conveniently, a retroreflector placer first walks up one side (direction A) of the permanent way 11 placing a retroreflector 1 on each post 12. The surveyor takes measurements with respect to the retroreflectors I as they are deposited. When the placer has reached a position where the posts 12 are no longer visible to the EDM, the placer returns (direction B) along the opposite side of the permanent way 11, depositing retroreflectors on the posts 12 on that side. Again the surveyor takes measurements with respect to the retroreflectors 1 as they are deposited. On reaching his starting position, the placer retraces his steps in direction A removing the retroreflectors 1 from the posts 12 on that side of the permanent way 11 as he goes. The retroreflectors 1 on the posts 11 on the other side of the permanent way 11 are recovered by the surveyor as he moves the EDM to its next position.

Using the data obtained in the previous step and the Ordinance Survey National Grid co-ordinates of the reference point on which the EDM is placed, the National

Grid co-ordinates of each of the marks on all the posts 12 visible to the EDM are determined.

The steps to be taken in conducting a repeat survey of the posts 12 will now be explained with reference to Figure 6. The surveyor places an EDM in a position 16 in which the permanent marks on a relatively large number or set of posts 12 are visible to it. It should be noted that the exact position at which the EDM is placed is unimportant and need not be at the original reference point 14. A retroreflector 1 is then placed on each post 12 in exactly the same positions they were in when the first survey was conducted, the permanent mark on each post 12 indicating the correct position.

In the next step, the position in which the EDM is placed is determined in the following manner. The surveyor chooses a subset of four posts 12a, 12b, 12c, 12d to provide him with a reference framework. Ideally, the four posts 12a, 12b, 12c, 12d chosen are those positioned closest to the EDM. The EDM is then aimed at the mark on each post 12a, 12b, 12c, 12d in turn, using the retroreflector 1 on each post 12a, 12b, 12c, 12d as described above, the data recorded for each post 12a, 12b, 12c, 12d thereby establishing the positions of the posts 12a, 12b, 12c, 12 d relative to the EDM (the EDM is deemed to be at (0,0,0)).

Co-ordinates representing the positions of the permanent marks on each of the four posts 12a, 12b, 12c, 12d, relative to the EDM, are then mapped onto the National Grid co-ordinates of the same posts 12a, 12b, 12c, 12d recorded at the first survey by translating and rotating the new set of co-ordinates until the best fit with the originally determined co-ordinates is achieved. If, for example, one of the posts seems to have moved then it is ignored and the remaining three co-ordinates 12b, 12c, 12d are mapped onto the National Grid co-ordinates of the same three posts 12a, 12b, 12c recorded previously. If the new co-ordinates of the remaining three posts 12b, 12c, 12d precisely map onto the previous co-ordinates, then this indicates to the surveyor that none of these posts 12b, 12c, 12d have moved and so can be used as a reference framework to determine the new position of the EDM. If the new co-ordinates for two of the posts 12a, 12b do not map onto the same co-

ordinates taken previously, the surveyor may decide that this set of four posts is not the optimum set to use as a reference framework. He can then obtain data for a different set of four posts 12 to find a set in which the new co-ordinates for at least three of the four posts map onto the co-ordinates recorded previously for the same posts 12.

If the new co-ordinates for one or two posts 12a, 12b do not map onto the previous co-ordinates recorded for the same posts 12a, 12b, this suggests to the surveyor that post movement due to damage or maintenance has occurred since the last survey was conducted. The surveyor is therefore prompted to check these posts 12a, 12b for signs of damage to see if any maintenance or repositioning is required.

Once the new co-ordinates for the three or four posts 12a, 12b, 12c, 12d have been mapped onto the old co-ordinates for the same posts 12a, 12b, 12c, 12d, and the best fit obtained, the determined translation and rotation can be applied to the EDM's position (0,0,0) to obtain its National Grid co-ordinates.

Once the new position of the EDM has been determined, the surveyor can now survey other posts 12e, 12f visible to the EDM and to which a retroreflector 1 has been attached by aiming the EDM at each mark to record the bearing and distance data for each post 12e, 12f. The new data representing the positions of each post 12e, 12f can then be compared with the data originally recorded, or those taken at a previous survey, to determine if movement of any of the posts 12e, 12f has occurred in the intervening period. Once data for all the posts 12 visible to the EDM have been recorded, the EDM is moved to a new position and its position re- determined by taking a the bearing and distance from one of the posts 12 whose position has just been determined.

Once the initial position of the EDM has been established, the retroreflectors 1 are preferably placed in the manner described above with reference to the initial survey. It will be appreciated that there is no need to permanently mark each fixed reference point using a PGM to ensure that future measurements are taken from

exactly the same place when repeat survey measurements are taken, or avoid the need to re-establish the reference points using GPS each time, as the marks on three or four posts 12a, 12b, 12c, 12d are used as a reference to determine the position of the EDM when a future survey is conducted.

Although the reference framework to establish the new random position of the EDM is based on the posts 12a, 12b, 12c, 12d forming the subject of the survey, by obtaining the new co-ordinates for four posts 12a, 12b, 12c, 12d and by mapping them on the co-ordinates for the same posts 12 obtained previously, the occurrence of any movement in one or more of the four posts 12 can be realised quickly by the surveyor and the data obtained for that/those posts discounted for the basis of forming the reference framework.

It will be appreciated that there is now no need to have a cumbersome tripod mounted target which must be moved from one target location to another and accurately set up and levelled before measurement data can be taken.

Although it is the positions of the posts 12 which are generally the subject of the survey, it will be apparent that other permanent environmental structures can also be surveyed using the same technique.

Furthermore, if the survey subjects, in the aforementioned case the posts 12, are all prone to movement, the initial position of the EDM can be establised with reference to some more stable structures.

Claims

Claims 1. A retroreflector for use as a target for an Electronic Distance Measurement Total Station, the retroreflector comprising a plurality of retroreflective elements disposed in an arc of constant radius.
2. A retroreflector according to claim 1, including releasable attachment means for attaching the retroreflector to a structure.
3. A retroreflector according to claim 2, wherein the attachment means comprises a magnet.
4. A retroreflector according to claim 1, 2 or 3, wherein the plurality of retroreflective elements are arranged in a semi-circle.
5. A retroreflector according to any preceding claim, wherein the retroreflective elements comprise elements of retroreflective sheet material.
6. A retroreflector according to any preceding claim, including a semi-disc- shaped body, wherein the retroreflective elements are mounted to the curved face of the body.
7. A retroreflector according to claim 6, including a sighting element projecting axially from the body at centre of curvature of said arc.
8. A retroreflector according to claim 7, wherein the sighting element is encompassed in a transparent layer on said body.
9. A method of carrying out a repeat survey of a plurality of permanent structures on an area of land, the method comprising the steps of:- (a) locating an EDM at an arbitrary position from which said structures are surveyable;

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(b) determining the bearings and distances to a subset of said structures using the theodolite at said arbitrary position; (c) determining the translation and rotation required to map a subset of the coordinates, relative to the EDM' obtained in step (b) onto the corresponding coordinates previously obtained for the same structures according to another reference frame; (d) applying said translation and rotation to origin coordinates to obtain the EDM's coordinates in said other reference frame; and (e) determining the bearing and distance of the structures from the EDM and determining the positions of said structures relative to said other reference frame therefrom.
10. A method according to claim 9, wherein the subset of coordinates excludes coordinates of structures in said subset of structures that have moved significantly relative to the other members of the subset of structures.
11. A method of surveying including placing a retroreflector according to any one of claims 1 to 8 against a structure to be surveyed at a predetermined position thereon.
12. A method of surveying ferromagnetic posts on either side of a permanent way, the method comprising the steps of:- (a) magnetically attaching a retroreflector according to claim 3 on a first plurality of posts on one side of the permanent way in a first direction; (b) magnetically attaching a retroreflector according to claim 3 on a second plurality of posts opposite the first plurality of posts on the other side of the permanent way in a second opposite direction; (c) removing the retroreflectors attached in step (a); and (d) removing the retroreflectors attached in step (b), wherein bearing and distance measurements to each attached retroreflector are made using an EDM.

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13. A retroreflector substantially as hereinbefore described with reference to Figures 1 and 2 or 3 and 4 of the accompanying drawings.
14. A method of surveying substantially as hereinbefore described.