MICRODEM Viewshed Algorithms

Method Description Advantages Disadvantages
Radials, full coverage (MICRODEM default) The fan is drawn on another bitmap, and then merged onto the map. The map will show through. The program draws a number of radial lines, the number of radials being selected so that they overlap at the edges of the map and achieve complete coverage at the map scale. Points near the observer are calculated extremely close together, but the extra effort is worth it because the alternative is much slower.
  • Default spacing along profiles computed at 1/2 DEM spacing (suggested by Nyquist limit) or 1/2 the map pixel size, whichever is less; you can set these two scaling factors and override the default settings. The max number of points on a radial is currently 25,000
  • Enough radials for complete coverage at the edges of the fan.
  • Two saved fans allowed computation of mixed pixel coverage.
  • Radial spacing from screen resolution, so might not get complete coverage this way in terms of DEM points.
  • At less than 1:1 resolution you will  have points that are not shown on the map.
Radial Lines, discrete Draws series of radial lines away from the observer. Select ray spacing in degrees, and spacing of points along each profile to compute. At each point the intervisibility is computed, and drawn on screen. If the spacing for computing exceeds the pixel size you will see voids, and if the spacing is less than the pixel size, several computed points will plot on the same pixel. The fan is drawn over the map, and the only way to see the map is through the radials. This algorithm is the fastest of the three, but probably only if you do not go for full coverage.
  • Very fast
  • Control of ray spacing, and points along the ray (further speed control)
  • Voids between radials
Point to point Fan is drawn on another bitmap, and then merged onto the map. This algorithm computes point to point intervisibility for every point on the screen and the observer. If screen pixel size is less than DEM spacing it will miss points, and for best accuracy you should blow up the map to 1:1 resolution. For large fans this can be very slow.  
  • Can be very slow
  • Computed at screen resolution

Mixed pixels on fans.

Viewshed Validation

Last revision 8/24/2017