Friday, January 24, 2014

LIDAR Resolution

     All these images were created using LAS files downloaded from  These are massive files of LIDAR datapoints, with no accompanying geo-referenced  images.  I created all the  images  by sorting out the ground surface points using Fusion software, which was designed for analyzing forests.

Here is the Fusion interface, running  a project of the Copicut area east of Fall River, MA.  Fusion can generate its own reference images from the LIDAR data.  The red area to the right is Copicut reservoir, and is colored red due to low datapoint density.  The topo lines were created from groundpoint data. The white area denotes the area being sampled for imaging.  It helps to have a real topo map or satellite imagery open in another window for reference while working.

The commands for processing data are entered  at the C-prompt.  I hadn't done this since the 1980s, and found that it is not as hard as it looks. The screen shows part of the shoreline of South Watuppa Pond. 
      While working with data from different sites, I noted differences in image quality. This was explained in the Fusion user's manual as follows:

" In open, flat areas, ground contours can be recorded from an aircraft flying overhead providing accuracy within 6 inches of actual elevation. In steep, forested areas accuracy is typically in the range of 1 to 2 feet and depends on many factors, including density of canopy cover and the spacing of laser shots."

     The type of forest also seems to affect resolution in LIDAR.  This is a deciduous second-growth forest along a lake in RI. The false color indicates tree height.

The ground surface image of the same area shows boulders along the shore, a farm road, walls, and a cellar hole with an outcrop directly north.

     There are low cairns at this site, but they are not visible.  Presumably the resolution is too low to detect structures only 1 or 2 feet tall. LIDAR can detect taller cairns, such as those at Parker Woodland.

      LIDAR imaging of areas in dense pine forests produces impressive results. Here's the satellite imagery of part of the Copicut area with a house at lower right, and a field at bottom center. There is nothing visible through the dense pine forest.

This is the same area in LIDAR data.  Taller trees are yellow or red, and the field is deep blue.

Here is the ground surface image of the same area, with the house at lower right. The walls of an old farmstead form a neat grid.  There is a farm road that  runs through the fields at the top of the image, and exits at the upper right corner.  I have never been to this area, and had no idea what was there.

Here is another view further north in the same site, with a road and houses in the upper left corner.

The conical objects  are too large to be  boulders. They may be artifacts from very dense stands of  pine trees. They don't appear in images from deciduous forests, even if there are numerous large boulders present. There is a Filter command to remove such artifacts.
     All these images were taken from Fusion's viewer (LDV), which is only practical when working with smaller areas.  Fusion can export surface image files as JPEG files.  Depending on file size and image quality, it may be possible to create LIDAR atlases of forested areas.  Of course, nothing replaces actual visits to a site, but  LIDAR images can help in selection of sites.

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