Friday, February 21, 2014

LIDAR with GPS (Sort of)

LIDAR images are highly detailed, but to be truly useful, there has to be a way to add and extract GPS points from the images.
     The LIDAR images shown on this blog were created using LIDAR data files downloaded from  These contain about 8 million datapoints, but so far, I have not been able to find any embedded GPS data. The LIDAR files come in a zipped folder, along with a small XML file containing information about the LIDAR data.  Luckily, this file contains 4 sets of longitude and latitude readings for the four corners of the area scanned for LIDAR data.  When I entered the coordinates for the Copicut area into my topo map program, I obtained the outline of the area.

This footprint corresponds to the one shown superimposed on satellite imagery at the earthexplorer download page.  This is 4950 feet on each side, or just less than 1 square mile.

    This is the entire  LIDAR ground surface for this Copicut area square.  The map color was so garish, I switched to grey.  The farm walls are in lower center.  Indian Town Road runs diagonally across the top.  There are three large pits directly north of the road.

     So, how to get GPS coordinates for points of interest? Eyeballing only works if there are prominent landmarks near a point of interest.  LIDAR images shown in this blog are actually created using rasters, which are data grids with 1500 rows and 1500 columns.  The raster has linear x,y coordinates which are shown at the lower left of the Grass GIS map viewer.  GPS coordinates are in degrees.    The GPS coordinates for the corners are known, so it is possible to write a spreadsheet function generating approximate GPS coordinates from the raster x,y coordinates.

The blue dots are prominent road intersections selected directly in the topo software.  The yellow dots are the same intersections selected in the LIDAR image, with the x,y coordinates converted to longitude and latitude with the spreadsheet function. The LIDAR coordinates are about 60 feet from the original GPS ones. Granted, this is not extremely accurate,  but one would be looking for larger features and areas, not small details.  GPS waypoints generated with LIDAR x,y coordinates for the house, building foundation, bridge and quarry seem to be in the right positions.

I look forward to trying out the LIDAR-GPS coordinates somewhere in RI when the weather improves.

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