Archive for September, 2008

Viewsheds Compared

September 27, 2008 2 comments

By comparing line of sight DEM based viewsheds with a manual viewshed the different amount of error associated with different types of DEMs becomes evident.  I compared a manual viewshed against a 30 meter DEM from the Shuttle Radar Topography Mission (SRTM), a 30 meter DEM from the National Elevation Dataset (NED), 10 meter DEM from the NED and 3 meter DSM created from a .7 meter lidar cloud.  The same location and elevation was used in all the different viewsheds, the manual viewshed was both created using Google Streetview and I physically went to the location to verify the viewshed.  

To create the viewsheds I used Military Analyst for ArcGIS, this allowed the resulting viewshed to be limited by a range and the output was in vector format instead of raster format.  In order to calculate differences between the viewsheds the vector data was converted to a raster format using the same cell size and clipped to the same area to make sure each viewshed had the same number of pixels.

To quantify these findings I compared the difference between the pixels in the manual viewshed and the DEM viewshed.  I counted the differences where the manual viewshed’s pixel was visible and the dem viewshed was non-visible (V/NV), this was repeated for for visible/visible (V/V), non-visible/non-visible (NV/NV), and non-visible/visible (NV/V).  This method was used to compare the total number of pixels between viewsheds and total percentage of similar pixels.

These findings are far from complete but a step towards understanding the strengths and weaknesses in viewshed creation.  Further work would be to accurately account for the manual viewshed not taking tops of trees and buildings into consideration and to clean up the lidar DSM model by removing non-view obstructing artifacts such as power lines that show up as fence like objects.