METHOD FOR UNDERWATER SCANNING OF AN OBJECT AND TARGET FOR UNDERWATER SCANNING OF AN OBJECT

Abstract

A method and corresponding arrangement for improving positioning accuracy of scanning of an underwater object includes equipping the object with at least one floating target having a part above the surface and a part below the surface, determining position data of the target from the part above the surface, scanning the object from under the surface to create measurement observations, detecting the target from the measurement observations under the surface, aligning the position data of the target from the measurement observations with the detected target in order to improve the positioning accuracy of the scanning, determining the target's attitude data time-dependently during the scanning in order to determine the position data of the part of the target below the surface on the basis of the part of the target above the surface, and correcting the alignment of the initial position data of the part of the target below the surface with the measurement observations on the basis of the attitude data.

Claims

1-14. (canceled)

15. Method for improving positioning accuracy of scanning of an underwater object, in which method equipping the object to be scanned with at least one floating target, which includes both a part above surface of water and a part below the surface, determining position data of the target on basis of the part of the target above the surface of the water, determining target's attitude data time-dependently during the scanning, determining initial position data of the part of the target below the surface of the water on the basis of the said position data of the target, scanning the object to be scanned from under the surface of the water to create measurement observations, detecting the target from said measurement observations under the surface of the water, aligning the position data of the target from the measurement observations with identified target to improve the positioning accuracy of the scanning, correcting alignment of the initial position data of the part of the target below the water surface with the measurement observations on the basis of the said attitude data.

16. Method according to claim 15, wherein target's position data is determined with aid of satellite positioning, using the part of the target above the water surface.

17. Method according to claim 15, wherein target's position data is determined optically with a tachymeter using the part of the target above the water surface.

18. Method according to claim 17, wherein a scanning unit is used for scanning, which includes echo-sounding means for performing the scanning, and determining the position data of the scanning unit with satellite positioning.

19. Method according to claim 15, wherein angle of tilt is determined as the said attitude data.

20. Method according to claim 19, wherein additionally direction of the tilt is determined as the said attitude data.

21. Method according to claim 15, wherein the attitude data is determined at a frequency that is 30-400 Hz

22. Method according to claim 15, wherein the attitude data is determined at a frequency that is 60-200 Hz.

23. Arrangement for the scanning of an object taking place from under surface of water, which arrangement comprises a floating target, positioning means for determining the target's position data and attitude-detection means for determining the target's attitude data time-dependently, the target comprising: a body comprising a part under the water surface for distinguishing the target on basis of acoustic measurement and a part above the water surface for determining the target's precise position data, securing means joined to the said body for securing the target in place in desired location in vicinity of the object.

24. Arrangement according to claim 23, wherein the said attitude-detection means are situated in the target, to detect the target's attitude, and memory means to attach the momentary attitude data to the time data.

25. Arrangement according to claim 23, wherein said body is arranged to float.

26. Arrangement according to claim 23, wherein the securing means is an anchor.

27. Arrangement according to claim 23, wherein the target includes a target mark below the water surface fitted to the part of the target below the water surface, to distinguish the target.

28. Arrangement according to claim 24, wherein the target includes a target mark above the water surface fitted to the part of the target above the water surface, to determine the target's precise position data,

29. Arrangement according to claim 28, wherein the said target mark below the water surface comprises shaped target-surface shapes to facilitate detection of the target mark below the water surface.

Description

[0062] In the following, the invention is examined in detail with reference to the accompanying drawings depicting some embodiments of the invention, in which

[0063] FIG. 1a shows a schematic side view of a first embodiment of the method according to the invention,

[0064] FIG. 1b shows a schematic side view of a second embodiment of the method according to the invention,

[0065] FIG. 1c shows a schematic side view of a third embodiment of the method according to the invention,

[0066] FIG. 1d shows a schematic side view of the second embodiment of the method according to the invention, when the target is tilted,

[0067] FIG. 2a shows a schematic top view of the embodiment of FIG. 1a,

[0068] FIG. 2b shows a schematic top view of the embodiment of FIG. 1b,

[0069] FIG. 3a shows a schematic side view of a first embodiment of the target of an arrangement according to the invention,

[0070] FIG. 3b shows a schematic side view of a second embodiment of the target of an arrangement according to the invention,

[0071] FIG. 4 shows an axonometric view of a three-dimensional point cloud of the part under water of an object being scanned

[0072] FIG. 5a shows a schematic view of a first embodiment of the underwater target mark of the target,

[0073] FIG. 5b shows a schematic view of a second embodiment of the underwater target mark of the target.

[0074] FIGS. 1a-1c show three embodiments of an apparatus suitable for implementing the method and arrangement according to the invention, in each of which embodiment the apparatus includes echo-sounding means 36 arranged in a scanning unit 50, for example, a boat, for scanning an object 10 from several different directions under the surface 12 of the water, and at least one target 32 situated in connection with the object 10 being scanned. In addition, the apparatus includes positioning means 38 for combining the position data of the target 32 and preferably also the direction of the echo-sounding means 36 with each measurement observation created using the echo-sounding means 36, as well as a computer 40 comprising software means 42. In this connection, the object 10 being scanned can be, for example, a support pillar of a bridge according to FIGS. 1a-2b, which has a part 10 above the water surface 12 and a part 10 under the water surface 12. The computer used can be, for example, a normal laptop computer. The target 32 preferably includes a part 33 above the water surface 12 and a part 33.1 below the water surface 12.

[0075] The echo-sounding means 36 can be, in turn, attached to the boat acting as the scanning unit 50 or to some other base moving under the water surface 12, from which the echo-sounding means 36 used as the scanning means have preferably a direct and unobstructed connection to the object being scanned. Alternatively, the echo-sounding means can also be situated at one point, so that only their orientation is changed during scanning. In addition to a boat, other moving bases can be aircraft, helicopters, water scooters, ships, and boats. The term echo-sounding means refers to measurement devices based on the progression of sound, for example, ultrasound devices. The echo-sounding means can be preferably aimed at the object being scanned. In connection with the description of the figures, echo-sounding means act as the scanning means, but it should be understood that the scanning means can also be laser-scanning means.

[0076] The positioning means 38 used in the method for positioning the target can be, for example, a GPS positioning device receiving it position from a satellite 39, as in FIGS. 1a and 1b, or an optical measurement device 16, such as the tachymeter of FIG. 1c, in which position definition takes place, for example, by GPS positioning. Instead of GPS positioning, satellite positioning such as GNNS positioning can generally be used.

[0077] In the method according to the invention, scanning takes place in such a way that initially the object 10 being scanned is equipped with at least one floating target 32, which can be attached to the object to be scanned according to the embodiments of FIGS. 1a and 1c, or which is preferably attached in the vicinity of the object 10 to be scanned according to the embodiment of FIG. 1b. After this, in the embodiment of the figures, with the aid of the echo-sounding means 36 attached to the boat acting as the scanning unit 50, the object 10 selected is scanned by going round the object 10 along the routes 30 shown in FIGS. 2a and 2b. In principle, the method can even be used to align an individual measurement observation, but preferably there are several scanning runs and measurement observations obtained from the scanning runs and the object being scanned is scanned at least once, preferably two or three times, or driving parallel to the object (for example, along the jetty line). The echo-sounding means 36 measure the echo returning from the object at a preselected frequency, which depends on the speed of movement of the echo-sounding means relative to the object 10 being scanned. The speed of the boat or other scanning base can be 0.1-2.5, preferably 0.8-1.5 m/s, which is a sufficiently low speed for scanning the object with sufficient precision using existing echo-sounding technology. In the future, the speed of the scanning base can possibly be raised, if the frequency and operating speed used in echo-sounding technique increase.

[0078] Using existing apparatus, the scanning frequency of the echo-sounding means can be a maximum of 60 Hz and the scanning frequency is preferably 10-60 Hz. In this connection, the term scanning frequency is also referred to as the ping rate, which tells how many measurement observations are collected each second. The operating frequency of the echo-sounding means can be, for example, 400-700 kHz. The scanning frequency should be chosen in such a way that the distance of measurement observations from each other in the running direction is at most 30 cm, preferably less than 5 cm. The running speed and scanning frequency must be chosen so that a sufficient number of measurement observations are obtained from the target and object being scanned. If in the future the echo-sounding means scanning frequency can be raised, the running speed can also be raised. The precision demanded from the measurement in the end determines the scanning frequency required.

[0079] The determining of the position and attitude of the target is preferably performed simultaneously with the scanning. The determining of the position data should take place in such a way that the position data of the target are known at the same moment as the measurement observation is obtained. The target's position data need not necessarily be determined as frequently as measurement observations are obtained. For example, 60 measurement observations can be obtained per second and the target's position data can be determined 1-10 times per second. According to FIGS. 1a and 1b, the target 32 can include a GPS receiver, to which the satellite 39 sends time and position data, which can be later transferred to the apparatus's computer 40. Alternatively, an optical measurement device, for example, a tachymeter 41, can be used, according to FIG. 1c, which optically determines the distance and direction of the target 32 from the tachymeter 41 and can transmit, by a transmitter, data on its own position and the time in question by radio to the computer. Alternatively, the data can be recorded in the target and the final determining of the positioning data can be performed by post-calculation. The target's 32 position data are linked to each measurement observation taken at the same moment in time. The method according to the invention can also be implemented in such a way that a target floating in the vicinity of a floating object is positioned with the aid of a tachymeter, unlike in FIG. 1c. The attachment means of the floating target can be an anchor, by which the target is locked to the bottom of the waterway.

[0080] The arrangement also includes attitude-detection means 68, which are preferably an attitude sensor attached to the target 32 and, if necessary a gyro (compass). The attitude sensor preferably includes an acceleration sensor and an angle sensor. With the aid of the attitude-detection means, the target's tilt and its direction are advantageously ascertained. The target's underwater position data can be calculated exploiting information on the distance between the target's part above the water surface and that below it, the tilt of the target and the position data of the part above the water surface. Thus, for example, a momentary tilt of the target due to waves can be taken into account at each measurement observation. A gyro is not necessarily needed, but can act as a standby source, when the same data are obtained from several sources. In this way the mutual accuracy of the different sources can be compared and, in possible disturbances, another one can be used.

[0081] According to one embodiment, the acceleration sensor used as the attitude sensor can be a 3-axis acceleration sensor, which can be situated anywhere on the target, assuming that internal deformations do not take place in the target. In addition to an acceleration sensor, the attitude sensor can also include an angle sensor. With the aid of a 3-axis acceleration sensor the direction of the acceleration can be determined, whereas the magnitude of tilt is obtained with the aid of the angle sensor. An angle sensor will detect a change in angle of a magnitude of even 0.01. The attitude can be determined at a frequency of 20-400 Hz, preferably 50-200 Hz. The frequency of determining the attitude defines how many times a second the attitude data of the target is determined. If the target moves much during measurement, for example in a rough sea, the attitude determining frequency should be great, because the position of the target changes continuously. The acceleration sensor mainly measures the attitude of the target, but in exceptional situations, for example, during breaks in GPS or similar, the attitude and position can be determined using parameters.

[0082] If the attitude of the target is determined optically with the aid of a tachymeter, determining can be performed in the following stages. First orientation takes place relative to the set of co-ordinates to be used in the measurement, with the aid of a tachymeter or similar optical device. The term set of coordinates of the measurement refers to a direction selected at the start of the measurement, which is defined relative to the x, y, and z axes. The co-ordinate and elevation system used in the satellite positioning of the tachymeter is selected from several existing systems. What is important is that the echo-sounding means and the target make their observations in the same order. Next the tachymeter or tachymeters to be used are selected and the distance between the target and each tachymeter is measured. As a partner for the tachymeter at least two prisms should be attached to the target, which the tachymeter monitors. The tachymeters' positioning data in the set of co-ordinates is defined with the aid of positioning. The anchor points of the set of co-ordinates are obtained from the tachymeter's positioning data. With the aid of the distances measured between the tachymeter and the prisms, the direction of the target (tilt/rotation) can be determined and with the aid of the positioning means the position data of the tachymeter in the set of co-ordinates and the orientation of the tachymeters is known. The target's attitude data obtained as a result of the measurement is time-stamped, preferably with the aid of a clock belonging to the attitude sensor or GPS-positioning device, simultaneously with the measurement.

[0083] The floating target 32 according to FIG. 1d tilts according to the waves and the difference between the positioning data of the part 33 of the target 32 above the water surface 12 and the part 33.3 below is d. Therefore the position data p1 determined on the basis of the part 33 of the target 32 above the water surface 12 do not correspond to the position data p2 of the part 33.1 below the water surface 12. Due to this, the position data p1 should be corrected using the attitude data, when the position data p2 can be determined precisely and correctly on the basis of the position data p1.

[0084] The attitude data can be determined either in real time or by post-calculation. Preferably, however, the determining takes place in real time. In real-time determining, the position data of the target are preferably determined at the moment of measurement, for example, with the aid of satellite positioning and the position data of the target obtained from the target, for example with the aid of echo-sounding means, are automatically compared with the position data measured at the same moment with the aid of satellite positioning. If everything takes place by post-calculation, it will then be sufficient for the position data of the echo-sounding means and target to be recorded, for example in the relevant intermediate memory, and retrieved and processed later.

[0085] The data from the echo-sounding means 36 can be transferred immediately wirelessly or along conductors to the computer 40, where at least one target 32 position datum, which is obtained from the positioning means 38 to which the computer 40 is connected, is attached to each measurement observation. Alternatively, the data transfer need not necessarily be in real time, instead it can be stored in the apparatuses and the final determining of the position data can take place by post-calculation. The position data contains at least one coordinate of the target, but preferably also attitude data and time data. In addition, the position of the echo-sounding means 36 at the moment of performing the measurement, and the orientation and rotational angle of the echo-sounding means 36 can also be attached to the measurement observation. The orientation of the echo-sounding means 36 at the moment of scanning can be determined on the basis of the route 30 of FIG. 2. In other words, in connection with scanning, the measurement observations can receive not only the target's position data but also an approximate location (co-ordinates+orientation), when they can be taken to the software means 42 situated in the computer 40.

[0086] The software means 42 can be arranged to combine the consecutive underwater measurement observations on the basis of the target's position data, in order to form the three-dimensional point cloud 20 of FIG. 4. The 3D point cloud can for formed by measuring the boat's sailing line and attitude at each moment in time. The individual measurement observations are placed along the sailing route and the angles are corrected using values according to the attitude sensors, from which the point cloud is formed. The target being measured appears in this point cloud. Even at this time, the entire sailing line and position of the target can be slightly incorrect. The definition of the position of the target, for example from the shore, will tell the real position of the target. One alternative is to move the measured sailing line, in which the target appears in such a way that the centre point of the target is at a more precisely defined point than previously. Another alternative is to change the boat's sailing line and in that way define a new position for the observation group, i.e. the point cloud collected during the entire sailing line. All of this can be done either in real time or later with the aid of post-calculation. The software means detect from the images the underwater part of the target and, on the basis of its positioning data the measurement observations are placed in the set of co-ordinates. Alternatively, the software means can place the measurement observations to form a point cloud in the set of co-ordinates also on the basis of the position calculated on the basis of the original position data of the scanning base, but then the measurement observations should be finally corrected by utilizing the target's position data and attitude data.

[0087] FIGS. 3a and 3b show two different embodiments of the target 32 of the arrangement according to the invention. The target 32 of the arrangement according to the invention includes, in all embodiments, a body 60, attachment means 62 joined to the body 60 for attaching the target 32 in place to be at least partly floating in the desired location in the vicinity of the object and target marks 64 and 66 preferably attached to the body 60. The term partly floating refers to the fact that the target is free to move in at least two directions. More specifically, the target 32 can include a target mark 64 below the water surface, for distinguishing the target 32 on the basis of acoustic measurement, and a target mark 66 above the water surface attached in connection with the body 60, for determining the precise position of the target 32 by means of optical measurement.

[0088] According to FIGS. 1a, 1c, and 2a, the target 32 can be attached to the object 10 to be scanned with the aid of the attachment means 62 of FIG. 3a. The target thus remains very firmly in place and can be implemented without an anchor. In this case, the attachment means 62 can be, for example, a shoe or similar support iron for attachment, by means of which the target is attached to the object in a pivoted manner. However, such an embodiment has the drawback of the attachment being formed between the target and the object to be scanned, which demands drilling or similar mechanical work to ensure the attachment. In addition there may be the problem of the target hiding part of the object to be scanned from view, in which case a hole the size of the target will remain in the measurement observations.

[0089] According to FIGS. 1b and 2b, in a second preferred embodiment the target 32 can be secured in the vicinity of the object 10 to be scanned, when the entire object to be scanned can be mapped with the aid of overlapping measurement observations. In such an embodiment according to FIG. 3b, the target's 32 attachment means 62 can be, for example, a cable 70 secured to the target 32 at one end, to the other end of which a weight 72 acting as an anchor is attached. With the aid of the attachment means 62, the target 32 can be made to remain more or less in place, which will prevent the target 32 from escaping outside the area being scanned.

[0090] The target mark 66 above the water surface can be, for example, in projection an A4-sized object, which can be easily detected by a tachymeter. The part 33 of the target 32 above the water surface 12 can be in length, for example 1-2 m, when it will be clearly distinguished above the water surface and can be easily lowered into the water from a boat, the shore, or a jetty 45 (FIG. 1c). The target's 32 target mark 64 under the water surface can, in turn, have a diameter of 0.1-1.0 m, preferably 0.4-0.7 m. The target mark 66 below the water surface is preferably shaped in such a way that it includes shaped target surface shapes 14, which facilitate the detection of the target from measurement observations according to FIGS. 5a and 5b. The surface of the target mark 64 below the water surface preferably comprises target surface shapes, which can be a corrugated surface or otherwise a shape that reflects a sufficient echo back to the echo-sounding means. The surface can be, for example, a pitted surface like that of a golf ball, as in FIG. 5a. Alternatively, the target mark 66 below the water surface can consist of several differently-sized target surface shapes 14, which vary in width, depth, and height, according to FIG. 5b. According to one embodiment, the target mark can be formed by the body of the target 32. The target mark of a tachymeter can also be a mirror prism, which can have a diameter of, for example, 0.5-5.0 cm.

[0091] According to one embodiment, the target mark can include separate orienting means that facilitate the determining of the orientation, which can be, for example, shaped target-surface shapes. The target-surface shapes can have a certain preselected orientation, on the basis of which the orientation of the target can be deduced.

[0092] According to one embodiment, the target's attitude-detection means can be optical measurement means, such as, for example, a tachymeter, by means of which the position of two or more overlapping identifiers attached to the part of the target above water can be measured continuously.

[0093] The method according to the invention can be used, for example, for scanning the bottoms of lakes, rivers, and seas, for inspecting the condition of bridge foundations, harbours, and jetties, and for many other suitable purposes. The method and target according to the invention can also be used for the underwater positioning of cave objects.

[0094] In this connection, it should be understood that, according to one embodiment the method according to the invention can also be used in such way that the position of a scanning unit above the water surface is determined with the aid of the target. In practice, this can be used, for example, with objects, in which definition of the position data produced by the scanning unit's satellite antennae is prevented. Such places can be, for instance, shadow areas, such as bridges, canyons, and caves. In such an embodiment, there can be, for example, a tachymeter and positioning means in the target, and a prism in the scanning unit. Thus, the scanning unit's position data can be defined with the aid of the target.

[0095] According to one embodiment, the arrangement according to the invention can also be used to ensure the accuracy of the scanning of an underwater object, in stages in which [0096] the object to be scanned is equipped with at least one target, which includes both a part above the water surface and a part below the water surface and positioning means, [0097] A floating scanning unit is equipped with echo-sounding means and with second positioning means, [0098] the position data of the target's part above the water surface is determined, [0099] the target's attitude data is determined time-dependently during the scanning, [0100] the object is scanned from below the water surface using the echo-sounding technique, in order to create measurement observations, [0101] the target is detected from the underwater measurement observations, [0102] the position data of the part of the target below the water surface are determined from the echo-sounding's measurement observations, [0103] the reference position data of the underwater part of the target are determined on the basis of the attitude data and the position data of the part of the target above the water surface, and [0104] the reference position data and the position data of the underwater part are compared with each other to ensure accuracy.

[0105] By means of such a method, the accuracy of the scanning measurement observations and the positioning of the target can be ensured advantageously in real time.