Description of field acquisition of LIDAR point clouds and photos. CyberMapping Lab UT-Dallas

Transcription

1 Description of field acquisition of LIDAR point clouds and photos CyberMapping Lab UT-Dallas

2 Description of field acquisition of LIDAR point clouds and photos Objective: Introduce the basic steps that are taken in the field to capture the data needed to create a photorealistic model Provide guidelines on critical issues in setting up the field project

3 Field Equipment Laptop Controls to align all the scanning data together Ground Laser Scanner (LPM 800)

4 Field Equipment Camera Tripod Topcon Totalstation Imagine System RTK GPS

5 Manmade VS natural models Manmade Fuzzy point cloud with smooth surfaces. Easy to define control points. Highly reflective features will have misplaced surfaces Edges are jagged Model errors are apparent Natural Rough surfaces = good scans. Difficult to find control points. Geology hides a lot of errors in the scan and the model

6 Manmade VS natural models Wall from the side Building from the top

7 Issues with vegetation Vegetation can cause problems with the models. Vegetation moves, and is not a continuous surface resulting in poor construction of the TIN mesh.

8 LIDAR site selection (multiple locations, selection of point density vs time) It is necessary to scan an outcrop from at least two oblique directions in order to minimize occluded parts of the outcrop. Three scans are best, left/center/right. Point density is inversely dependent upon distance to the outcrop. If the distance has a wide range of values, the time to scan the outcrop can be optimized by selecting a finer angular resolution for the more distant parts of the outcrop compared to the closer parts of the outcrop. Scan time is inversely dependent upon the square of the scan angluar resolution. Increasing the scan step angle by 2X reduces the scan time by 4X. Partition the outcrop scans to maintain a nearly uniform linear stepping distance at the outcrop surface.

9 Scan Positions overhang Choose scan positions to minimized occluded (shadowed or hidden) geometries. Scanner blue will not image beneath the overhang or the right side of the overhang. Scanner red will image underneath the overhang and will image the right side of the overhang.

10 Moab Utah-Google Earth Screen Capture

11 Multiple Scan Positions Moab Utah

12 Scan Partition as a Function of Range

13 Scan Partition as a Function of Angle of Incidence

14 Scan Partitioning Avoids Unnecessary Scan Time

15 Scan Partitioning Scan of the Pyramid at Slaughter Canyon, Carlsbad Caverns National Monument, New Mexico Scanner was on a 200m high hill. Scan ranges were 50m to 800m

16 Scan Partitioning Scanning of the total outcrop at the scan step angle needed for the longest scan would have dramatically increased the scan time. Scanning the outcrop in a single scan which covered the entire outcrop would Result in a large amount of empty data.

17 This program help to calculate the input accuracy in degree of LPM scanner based on distance and the resolution the user wants to get Resolution Calculation

18 Types of controls

19 Placement and Survey of the Controls Use of scanned control reflectors improves the accuracy of the model and greatly reduces the effort to align the scans in Polyworks In order to align two scans, an absolute minimum of three control points are needed. It is best to have at least five. This allows for errors. If multiple scan sites are used, it is not necessary to have all of the control reflectors visible to all of the scan sites. However, it is necessary that each scan site be able to see at least three reflectors that have been correlated with the other scan sites The control reflectors should cover a wide area, do not place all of them in a linear fashion or group them in a tight bunch. The spacing of the reflectors optimally approximates or exceeds the distances in the scan region. However, this may not be practical. It is not necessary to have reflectors on the outcrop, although it is desirable to do so if practical and is aesthetically acceptable.

20 Placement and Survey of the Controls

21 Scan Reflectors before Scanning Outcrop It is best to scan the reflectors before scanning the outcrop. If you do not have the controls with the scan data, you may not be able to use the scans If something happens to disorient the scanner or there is a power or software crash during the subsequent scans, the work up to that point can still be used For double protection, rescan at least some of the reflectors after completing the outcrop scan. If the scanner has lost alignment, the final reflector scan will identify the problem. When using the LPM with the telescopic sight, the scan window must be larger than expected. There is parallax between the scanner and the telescope. This is a much larger problem at close range than at long range.

22 Photography (coaxially mounted cameras) Riegl LMS-Z620 Riegl LMP-321 Optech Ilris Mount and Rotation transformation matrices are provided with each camera image. We are still in the development of using these parameters to drape photos on the TIN models.

23 Photography (coaxially mounted cameras) The Riegl LPM scanners Physically rotate (vertically/horizontally) the laser source and detector. Maximum flexibility in the camera lens focal length Scanner will first scan the outcrop, then position the camera as required to take the photographs. The number of images and positioning of the camera is automatically done by the scanner software. Scanners with rotating mirrors (Riegl LMS series, Optech Ilris, etc) Camera focal length is selected to capture the entire scan scene Riegl LMS series (+/- 40 deg vertical) mirror rotation => 14 mm lens on a Nikon D300 camera (UTD 620 has a 20 mm lens) Optech Ilris (+/- 20 deg vertical/horizontal) mirror rotation => 20 mm lens on a Nikon D300 camera (this is hypothetical, we do not know the sensor size) Short focal length of lens limits the spatial resolution of the image Nikon D300 camera with 20 mm lens at 500 m will have a spatial resolution of 14 cm per pixel. At 1000 m it will be approximately 0.3 m per pixel Use of a longer focal length lens reduces the scan field that can be photographed with auto-alignment of the photo to the scan

24 Photography (coaxially mounted cameras) The Riegl LPM series scanners do not use rotating mirrors to traverse the scan beam. The housing is rotated vertically and horizontally. The side mounted camera can have long focal length lenses attached. The UTD LPM-800 has an 85 mm lens at 500m spatial resolution of 3 cm with a Nikon D300, 6 cm at 1000m at 500m spatial resolution of 3.8 cm with a Canon 350d, 7.6cm at 1000m (UTD s LPM uses the Canon camera) The LPM series has a beam divergence of 0.8 mradian. The LMS 620 has a beam divergence of 0.07 mradian. The LPM can have great spatial resolution for the photographs but has poor spatial resolution for the scan. The LMS has great spatial resolution for the scan but poor spatial resolution for the photograph.

25 Off Axis Cameras Cameras that are not aligned to the coordinate axes of the scanners must be registered to the scan using control tiepoints between the model and the photograph. Off axis cameras provide maximum flexibility in camera used and the focal length of the lens. We have used zoom lenses with a magnification of 12X (spatial resolution of 1 cm at 500m) Off axis cameras provide maximum flexibility in camera angle to the outcrop. Proper selection of camera location can minimize smearing of the image on the outcrop model. The quality of the draped model with off axis camera shots is controlled by the quality of the control methods that are used to tie the photograph to the model.

26 Pixel Smearing Single Photo Oblique Angle Smearing Two Photo with Optimum Angles to Outcrop Outcrop Overhang, Single Photo Smearing on Underside of Overhang

27 Pixel Smearing Pixel Elongation 10 Elongation Factor Angle to Surface The smaller the angle that is made between the optical axis of the lens and the outcrop face the greater the distortion that is contained in the image. This results in noticeable smearing of the image if the angle is smaller than about 45 o (1.4X elongation factor). The optimum angle for the photograph is 90 o.

28 Use of the IS to Register Photographs to the Model

29 Use of Topcon IS for Tiepoint Assignment Topcon IS - remotely controlled, reflectorless, imaging total station Demonstrated range of >800 meters on sandstone outcrops Use Dell ATG laptop for control (wireless LAN)

30 Log File for Photo to Model Transformation ARI01Ga04c_log.txt Input data for parameter generation - first line ARI01Ga04c.JPG 50.00mm mm mm Control point data Total number of solutions = 92 Number of unique solutions = 5 Residuals are in pixels Parameter line generated "ARI01Ga04c.JPG " Mean Squared Ordered solution set Residuals (meters) e e e e e-004 GHVM

31 Guidelines for Using the Topcon IS The total station (Topcon IS) should be close the point where the photograph was taken. The image in the photograph is very similar to the image in the IS camera and locating tiepoints between the photograph and the IS camera image is simplified. Put the IS into Wide Field mode to navigate to the region where the tiepoint has been identified in the photograph. Overlapping photographs can share common tiepoints. Pick them on only one of the photographs. Plan on 3 to 5 minutes per tiepoint Pick 6 or more tiepoints per photo, 4 is the minimum but some may not be as good as others. Errors happen. Be sure to have a laptop with a high contrast screen in full sunlight. The Dell ATG or Dell XFR are the best that we have seen to date. Use a roller-ball mouse. A conventional mouse works poorly. You will go crazy trying to use the navigation pad on the laptop. Get comfortable: camp stool and field desk.

32 Use of RTK GPS GPS is essential to georeference the model RTK quickly provides centimeter level accuracy for the model Georeferencing options: 2D or 3D (2D assumes that scanner was level) Two points (2D): scanner & one control or two controls, calculation provides no measure of error Multiple points(2d): more than two control points, calculation is a least squares solution and provides residuals for error assessment Multiple points(3d): three or more points, calculation is a least squares solution and provides residuals for error assessment Procedure Establish base station (run for at least 2 hours for OPUS-S and 15 min to 4 hours for OPUS-RS) Scan control reflectors Take GPS readings with the rover for the control reflectors and scanner (use a count of 10 for the averaging) Create the model, use the control reflector coordinates from the model, use the GeoDimensional Tools program to make the desired correction to the model coordinates.

33 Scan, Control, Photo Numbering System A very, very large number of files will be generated during a complex project. Even a simple project can have over 100 files. Keeping this straight is a major task. Variables affecting the number of files Days of the project Number of scan sites Number of photograph camera positions Number of controls One Riegl Scan creates 14 folders with multiple files in each Riscan control export files and export text files Each Photo creates 3 distorted photo files, 2 undistorted photo files, 6 ArcMap files, 5 draping files Polyworks files, GPS control files, Confusion and loss of data integrity are high probability The workflow provides a naming and number convention and software support to minimize this very complex data problem.

34 Numbering System

35 Place where you highlight the row Show all tables Add or delete record tab Description of numbering order List of Tables To generate number: Select the table from the list Highlight the row that you want from all the available tables Click generate Click copy then past the name in the folder or file name.

36 To add new record: Select the name of the target table Input the record in description and short key boxes Click insert then the new data will be inserted in the new row in the table To delete record: Select the table name Highlight the row that you need to delete Click delete Table selection

37 Action Sequence in the Field First, decide on the scan locations and ensure that they completely cover the target area. Second, decide on the location for the controls Third, review the naming and number conventions to be used Make sure that the site name in the software and the folder and site abbreviation in the camera set is correctly set (can be done night before) Set up controls and locate them with GPS Set up first scan site and decide on camera sites Scan controls before scanning the outcrop The photo team with the Topcon IS needs to be working in parallel with the scan team. One can get ahead of the other, but the jobs need to proceed in parallel. It takes a lot of time. Review the progress with each other Double check the work Save all work Review the data in the field if possible Start model construction as soon as possible in order to correct errors or fill in unintentional holes in the data

38 Demonstrations Demonstration of number system program Demonstration of the Riegl LPM or the 620 Demonstration of the Topcon IS

39 Field Work Capture an outcrop in the field for use in subsequent classes.