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Mar 15 2011

"Simple" Pointcloud Georeferencing

I'm aware that we have multiple transform optimization algorithms of varying completeness in the pipeline, but I decided to try and figure out the simplest means of georeferencing a pointcloud last night.  This is a crude, error-prone method that is only usable when flat ground can be identified.  It performs better when the flat area is large in both X and Y dimensions.  A warning - ArcGIS took tens of seconds to display and classify my pointcloud, and minutes to spatially adjust it.  Shut it down, and you may lose previous work, not just what you're currently doing.  Plan on this taking a while, and multitask.

1. Place readily identifiable markers in your sample area.

2. Take GPS points of those markers using an accurate, WAAS-corrected signal.

3. Take photos.

4. Synth photos.

5. Denoise synth.

6. Convert those GPS points to shapefile format.

7. In Meshlab's Render menu, select the bounding box and labelled axes.  Use the Normals, Curvatures & Orientation -> Transform: Rotate tool in Meshlab with the 'barycenter' option selected to rotate the synth until the flat ground is coplanar with at the X-Y plane.

8. Export the pointcloud as a .ply with non-binary(plaintext) encoding.

9. Rename the .ply to .txt extension.

10. Open the .txt file in Notepad.

11. Replace the header information with space-delimited 'x y z r g b alpha'  and save.

12. Open the .txt file in Excel as a space-delimited spreadsheet.

14. Save as a .csv file.

15. Open the .csv in your planning document in ArcMap, where you already have the GPS points open with UTM coordinate system.

16. Use 'Add XY data' and use the X and Y columns.

17. Right click on the new 'Events' layer and export it as a new shapefile.  Add it to your map.

18. Begin editing that new shapefile.

19. Symbolize the points by color or color ratios using R, G, B columns and cross-reference manually with Meshlab in order to locate your markers.

20. In column alpha (which should have the default value 255), set the marker points to 1.  Symbolize by alpha, unique categories, to make the markers stand out.  Save your edits.

21. Write down the X and Y coordinates of each marker after finding them using 'select by attributes'.

22. Enable the 'Spatial Adjustment' extension, put a check next to the similarity feature, and set adjust data to all features in your pointcloud layer.

23. Place a new displacement link for each marker with the first end at your marker in the pointcloud, and the other end at the corresponding GPS marker in ArcGIS.

24. Hit the 'Adjust' button. 

25. Save your edits and stop editing.

26. Optional: Use SQRT(X^2+Y^2) to determine the distance between two markers in your original coordinate system.  Use the ruler to detemine the distance between them in UTM.  Using the field calculator, multiply the Z factor by the ratio of UTM distance to pointcloud distance.