DISTANCE ERROR CORRECTION METHOD

Description

[0001] The present invention relates to a system, method and computer program product according to the independent claims.

[0002] For staking out or measuring terrain points, surveying poles are often used in the surveying or construction industry. Surveying poles are typically used in combination with a primary sensor, usually embodied as a tachymeter or GNSS receiver. The position measurement of a point of interest on which a pole tip of the surveying pole is placed is usually not taken directly but rather by first determining position of a reflector or a GNSS receiver / antenna on the surveying pole, and then by inferring the position of the point of interest using a known spatial relationship between reflector / antenna and the pole tip.

[0003] Such an indirect measurement may require a free line of sight between the primary sensor and reflector / antenna. Additionally, the spatial relationship between a measured center of the reflector / antenna and the pole tip needs to be known, e.g US 2005 / 0077454 A1 discloses a method of determining a corrected distance between a primary sensor unit and a target based on the angle of incidence.

[0004] An operator using surveying poles for staking out or measuring terrain points is required to - in the absence of further sensors - a) detect the current pole length, b) detect the current pole pose and level the surveying pole for each measurement, and c) detect the current reflector pose and change the reflector pose so as to minimize measurement errors.

[0005] Manufacturers of surveying equipment have therefore started to develop additional secondary sensors which can be attached to the surveying pole. Such secondary sensors may be used as supplement or substitute to the primary sensor observation to measure points of interest.

[0006] Known solutions for attaching secondary sensors and a reflector to a surveying pole from the state of the art are often based on the stacking principle, e.g. realized by a screw thread interface or a quick release interface. With the screw thread interface, secondary sensors may be attached on top of a reflector (e.g. used in Topcon's RC-5 and in Leica's GS16) or in-between a reflector / antenna and the surveying pole (e.g. used in Trimble's AT360), and with the quick release interface, a reflector may be pushed from the top into a sensor (e.g. used in Trimble's V10). From the state of the art it is also known to attach sensors via clamps or clamping screws to a surveying pole.

[0007] Surveying poles from the state of the art to which both a reflector and a secondary sensor are attached often suffer from the problem that the presence of the secondary sensor changes the distance from the reflector to the pole tip. State of the art solutions furthermore often place the secondary sensor in such a way that possible incidence angles with which measurement light can impinge on the reflector are reduced as compared to a surveying pole without secondary sensor. In state of the art surveying protocols, the reflector is furthermore required to be oriented in a precise manner to the primary sensor so as to reduce distance measurement errors.

[0008] It is therefore an objective of the present invention to provide a method for numerically correcting distance measurement errors between the primary sensor and the reflector due to reflector orientation and position.

[0009] This objective is achieved by realizing the characterizing features of the independent claims. Features which further develop the invention in an alternative or advantageous manner are described in the dependent patent claims.

[0010] The invention relates to a method for distance error correction applied to an uncorrected distance measured between a primary sensor unit, in particular embodied as a tachymeter, and a reflector, in particular embodied as a 360° prism, in a primary sensor unit coordinate system using measurement light emitted by the primary sensor unit towards the reflector, the measurement light traveling along a line of sight to the reflector. The method is provided with an orientation and position of a reflector coordinate system with respect to the primary sensor unit coordinate system, and with a coordinate transform between the reflector coordinate system and the primary sensor unit coordinate system. The method is also provided with at least one angle of incidence of the measurement light used for obtaining the uncorrected distance in the reflector coordinate system, the at least one angle of incidence being determined based on the line of sight between the primary sensor unit and the reflector and on the reflector coordinate system. The method comprises the following steps: 1) determining a distance error in the reflector coordinate system using a calibrated distance error function with at least the at least one angle of incidence being provided as input to the calibrated distance error function, 2) determining a distance error in the primary sensor unit coordinate system using the coordinate transform and the distance error in the reflector coordinate system, and 3) correcting the uncorrected distance between the primary sensor unit and the reflector using the distance error in the primary sensor unit coordinate system.

[0011] The term distance is to be understood as referring to coordinate values of coordinates of a coordinate system. In case of a Cartesian coordinate system, for example, distance would be evaluated in terms of the usual x, y and z axes, while for a spherical coordinate system, a coordinate may refer to an angle. The term distance error is also related to coordinates of a specific coordinate system, and may express errors in a coordinate relating to an angle, e.g. in a spherical coordinate system, or in a coordinate relating to an actual spatial distance, e.g. in a Cartesian coordinate system. A distance error may also be determined only for individual coordinates.

[0012] The term uncorrected distance is to be understood as uncorrected with respect to the correction step carried out by the method according to the invention. Other correction steps may have been applied to the uncorrected distance beforehand.

[0013] In an embodiment of the method for distance error correction according to the invention, the calibrated distance error function additionally uses parameters describing the primary sensor unit as input, in particular parameters relating to a laser of a tachymeter.

[0014] In a further embodiment of the method for distance error correction according to the invention, three angles of incidence are provided to the calibrated distance error function as input, or only those angles of incidence are provided as input which relate to possible construction-related orientations of the reflector coordinate system.

[0015] The invention also relates to a computer program product according to claim 4.

[0016] The invention also relates to a system according to claim 5.

[0017] The surveying pole need not be placed orthogonally onto the ground at a terrain point to be measured. The computer program product according to the invention is configured to correct distance measurement errors due to suboptimal surveying pole placement and orientation.

[0018] The inventive system is described below in more detail purely by way of example with the aid of concrete exemplary embodiments illustrated schematically in the drawings, further advantages of the invention also being examined. Identical elements are labelled with the same reference numerals in the figures. In detail: Figure 1shows an embodiment of a surveying pole and of a secondary sensor unit; Figure 2shows an embodiment of a surveying pole and of a secondary sensor unit, with a reflector attached to the surveying pole; Figure 3shows a closer view of an embodiment of a surveying pole and of a secondary sensor unit; Figure 4shows an embodiment of a surveying pole and of a secondary sensor unit, and a tachymeter; Figure 5shows an embodiment of a surveying pole and of a secondary sensor unit, with a rotational locking mechanism; Figure 6shows an embodiment of a surveying pole, with a rotational locking mechanism for a reflector; Figure 7shows an illustrative depiction of the impact of reflector orientation and position on distance measurement accuracy with respect to a tachymeter, and a surveying pole, secondary sensor unit and reflector arrangement for correcting measurement errors due to reflector orientation and position; and Figure 8shows an illustrative depiction of the method according to the invention - as well as steps preceding the method according to the invention - for correcting distance measurement errors.

[0019] Figure 1 shows an embodiment of a surveying pole 1 and of a secondary sensor unit 2.

[0020] The secondary sensor unit 2 comprises a secondary sensor housing (shown in Figure 1) and a secondary sensor (not shown). The secondary sensor is arranged within the secondary sensor housing. The secondary sensor may e.g. comprise an inertial measurement unit (IMU), and / or a camera, and / or a laser unit configured for distance and position estimation. The secondary sensor unit 2 is configured to be attached to the surveying pole 1; it may be used for determining orientation of the surveying pole 1 to which it is attached with respect to some coordinate system (not shown). The secondary sensor housing corresponds topologically substantially to a torus, i.e. the secondary sensor housing can substantially be continuously deformed to have a donut shape. The secondary sensor housing comprises a central hole 7 around which the secondary sensor housing lies. In a view of the secondary sensor unit 2 from above 2a, the central hole 7 is clearly visible. The central hole 7 pierces through the entire secondary sensor housing.

[0021] The surveying pole 1 comprises a first segment 8 and a second segment 9. The second segment 9 comprises a shoulder 3 and a notch 4 at one of its ends. To attach the secondary sensor unit 2 to the surveying pole 1, the surveying pole 1 is moved 5 - starting with the first segment 8 and proceeding with the second segment 9 - through the central hole 7 of the secondary sensor housing. A part of the secondary sensor housing is configured to interact with the shoulder 3 and the notch 4. Specifically, when moving 5 the surveying pole 1 through the central hole 7, a part of the secondary sensor housing is configured to come to rest on the shoulder 3 of the second segment 9 of the surveying pole 1. The interaction of shoulder 3 and secondary sensor housing is therefore configured to limit the possible motion range of the secondary sensor unit 2 along the surveying pole 1, i.e. once the part of the secondary sensor housing configured to interact with the shoulder 3 comes to rest on the shoulder 3, no further movement of the second segment 9 of the surveying pole 1 through the secondary sensor unit 2 is possible. The secondary sensor housing is configured in such a way so as to provide a locking mechanism of the secondary sensor unit 2 to the surveying pole 1 through the interplay of the secondary sensor housing with the notch 4, e.g. by automatically snapping into the notch 4 once the secondary sensor housing comes to rest on the shoulder 3. A release mechanism, e.g. started by pressing a release button 6 of the secondary sensor unit 2, is configured to release the secondary sensor unit 2 from being locked to the surveying pole 1. After releasing, the secondary sensor unit 2 can be removed from the surveying pole 1.

[0022] Figure 2 shows an embodiment of a surveying pole 1 and of a secondary sensor unit 2, with a reflector 10 attached to the surveying pole 1. Instead of the reflector 10, a GNSS receiver (not shown) may be attached to the surveying pole 1. The term reflector may be replaced by the term GNSS receiver. The secondary sensor unit 2 is also attached to the surveying pole 1. The secondary sensor unit 2 is configured as described in the embodiment of Figure 1. After attaching the secondary sensor unit 2 to the surveying pole 1 as described in the embodiment of Figure 1, the first segment 8 of the surveying pole 1 protrudes from the secondary sensor unit 2. The reflector 10 is attached to the first segment 8 of the surveying pole 1. In Figure 2, the reflector 10 is embodied as a 360° prism. For the surveying pole 1, the reflector 10 can be attached with or without the secondary sensor unit 2 to the surveying pole 1. If both the secondary sensor unit 2 and the reflector 10 are attached to the surveying pole 1, a known distance between the secondary sensor unit 2 and the reflector 10 exists.

[0023] Figure 3 shows a closer view of an embodiment of a surveying pole 1 and of a secondary sensor unit 2. The surveying pole 1 and the secondary sensor unit 1 are configured as described in the embodiment of Figure 1 and are configured to be attached to one another as described in the embodiment of Figure 1. In Figure 3, the surveying pole 1 and the secondary sensor unit 2 are juxtaposed, wherein the surveying pole 1 and the secondary sensor unit 2 are positioned in height relative to one another in such a way as if the secondary sensor unit 2 were attached to the surveying pole 1. The release button 6 is on the same height as the notch 4 which is used for locking the secondary sensor unit 2 to the surveying pole 1. A part 11 of the secondary sensor housing around the central hole 7 comes to rest on the shoulder 3, limiting further downward motion of the secondary sensor unit 3 along the surveying pole 1.

[0024] Figure 4 shows an embodiment of a surveying pole 1 and of a secondary sensor unit 2, and a tachymeter 14. A reflector 10, in Figure 4 embodied as a 360° prism, is attached to the surveying pole 1. The surveying pole 1 comprises a pole tip 12 which is placed on a terrain point to be measured. Specifically, if the distance between the reflector 10 and the pole tip 12 is known and if a distance between the reflector 10 and the tachymeter 14 has been determined, a position of the pole tip 12 can be determined, provided the attitude of the surveying pole 1 can be determined or is known. The secondary sensor unit 2 may also be configured to be able to determine a distance and orientation to an outside object 13. It may also, e.g. using an IMU, determine the orientation of the surveying pole 1 to which it is attached. If both secondary sensor unit 2 and reflector 10 are attached to the surveying pole 1, a distance between them is known and fixed. The distance between the reflector 10 and the pole tip 12 is independent of whether or not the secondary sensor unit 2 is attached to the surveying pole.

[0025] Figure 5 shows an embodiment of the surveying pole 1 and the secondary sensor unit 2, with an additional rotational locking mechanism for fixing an orientation between the surveying pole 1 and the secondary sensor unit 2. Compared to the embodiment of Figure 1, the surveying pole 1 comprises additional single notches 16. The single notches 16 can be spaced around the surveying pole 1, preferentially being placed so as to allow only one possible orientation which the secondary sensor unit 2 can snap into. The secondary sensor unit 2 may comprise pins which snap into the single notches, for example. Locking occurs by rotating 15 the secondary sensor unit 2 around a longitudinal axis of the surveying pole 1 until e.g. the pins of the second sensor unit 2 snap into the single notches 16 of the surveying pole 1.

[0026] Figure 6 shows a 360° prism used as reflector 10 and a part of a surveying pole 1. The reflector 10 can be attached by moving 17 it along the first segment of the surveying pole 1. A further rotational locking mechanism may be provided, e.g. by a notch 19 on the first segment. After rotational locking, e.g. by achieved by rotating 18 the reflector 10 around the first segment 8, a fixed orientation between reflector 10 and surveying pole 1 can be achieved. Such a fixed orientation may also allow for a fixed orientation between the reflector 10 and a secondary sensor unit 2 (not shown), facilitating further measurements.

[0027] Figure 7 shows an illustrative depiction of the impact of reflector orientation and position on distance measurement accuracy with respect to a tachymeter 14, and a surveying pole 1, secondary sensor unit 2 and reflector 10c arrangement for correcting measurement errors due to reflector orientation and position. The reflector 10a,10b,10c is embodied as a 360° prism in Figure 7. The orientation of reflector 10b with respect to the tachymeter is worse than the orientation of reflector 10a. In general, reflector orientation and position impacts the accuracy of distance estimation between the tachymeter 14 and a reflector 10a,10b,10c. By using a surveying pole 1 with a reflector 10c and secondary sensor unit 2, measurement errors may be numerically corrected.

[0028] Figure 8 shows an illustrative depiction of the method according to the invention - as well as steps preceding the method according to the invention - for correcting distance measurement errors occurring in distance measurements between e.g. a tachymeter and a reflector.

[0029] Both the tachymeter and the reflector can each be associated to a coordinate system, a primary sensor unit coordinate system and a reflector coordinate system. Using e.g. a secondary sensor unit 2 as in the embodiment of Figure 1, which secondary sensor unit 2 is rigidly attached to a surveying pole 1, the secondary sensor unit 2 can be used for tracking orientation and position of the surveying pole, e.g. by using an inertial measurement unit, and using a known orientation between the reflector and the surveying pole 1, the orientation and position of the reflector can be tracked as in a step 20 of Figure 8. From the tracked reflector coordinate system, a coordinate transformation can be determined, wherein a coordinate transformation maps the reflector coordinate system onto the primary sensor unit coordinate system or vice versa.

[0030] In another step 21, using a known line of sight between the tachymeter and the reflector, incidence angles of measurement light, the measurement light emitted by the tachymeter, impinging on the reflector can be determined in the reflector coordinate system.

[0031] In a first step 22 of the method according to the invention, a predetermined calibrated distance error function can be used for determining a measurement distance error caused by the orientation of the reflector with respect to impinging measurement light. The predetermined calibrated distance error function can be specifically tailored to a single reflector, or to a production batch, or to a reflector model type etc. The calibrated distance error function can take the incidence angles as input, providing a distance error expressed in the reflector coordinate system. Besides incidence angles, other types of input can be provided to the calibrated distance error function as well, e.g. inputs relating to the tachymeter.

[0032] In a second step 23 of the method according to the invention, the distance error expressed in the reflector coordinate system is transformed to a distance error expressed in the primary sensor unit coordinate system.

[0033] The distance error expressed in the primary sensor unit coordinate system is used in a third step 24 of the method according to the invention for numerically correcting an uncorrected distance determined between the tachymeter and the reflector.

Claims

1. Method for distance error correction applied to an uncorrected distance measured between a primary sensor unit (14), in particular embodied as a tachymeter, and a reflector (10), in particular embodied as a 360° prism, in a primary sensor unit coordinate system using measurement light emitted by the primary sensor unit (14) towards the reflector (10), the measurement light traveling along a line of sight to the reflector, wherein the method comprises: • providing (20) an orientation and position of a reflector coordinate system with respect to the primary sensor unit coordinate system, and providing (20) a coordinate transform between the reflector coordinate system and the primary sensor unit coordinate system, and • providing (21) at least one angle of incidence of the measurement light used for obtaining the uncorrected distance in the reflector coordinate system, the at least one angle of incidence being determined based on the line of sight between the primary sensor unit and the reflector and on the reflector coordinate system, characterized by • determining (22) a distance error in the reflector coordinate system using a calibrated distance error function with at least the at least one angle of incidence being provided as input to the calibrated distance error function, • determining (23) a distance error in the primary sensor unit coordinate system using the coordinate transform and the distance error in the reflector coordinate system, and • correcting (24) the uncorrected distance between the primary sensor unit and the reflector using the distance error in the primary sensor unit coordinate system.

2. Method for distance error correction according to claim 1, characterized in that the calibrated distance error function additionally uses parameters describing the primary sensor unit as input, in particular, when the primary sensor is embodied as a tachymeter, parameters relating to a laser of a tachymeter (14).

3. Method for distance error correction according to one of claims 1 or 2, characterized in that three angles of incidence are provided to the calibrated distance error function as input, or only those angles of incidence are provided as input which relate to possible construction-related orientations of the reflector coordinate system.

4. Computer program product comprising program code when executed by a computing unit, configured to provide the computational steps of the distance error correction as in the method according to one of claims 1, 2 or 3.

5. Tachymeter (14) for determining distance to a reflector (10) on a surveying pole (1), and surveying pole (1) to which a secondary sensor unit (2) is attached, the tachymeter (14) and / or secondary sensor unit (2) comprising a computing unit comprising program code that when executed by the computing unit provides the computational steps of the distance error correction as in the method according to one of claims 1, 2 or 3, wherein the surveying pole (1) is configured to be placed onto a terrain point with an arbitrary orientation.