VisibilityIndexOperator.Rays Property
Gets or sets the number of sample rays per revolution.
Syntax
public AttributeVariable< System.Int64 > Rays { get; set; }Property Value
Default: 180
The number of sample rays per revolution.
Remarks
To get reasonable performance, the VisibilityIndexOperator does not compute the complete viewshed raster from each terrain cell in the observer polygon. Instead, the size of the viewshed is estimated statistically by tracing a limited number of sample rays from each terrain cell. A lower number will be faster but less accurate: low accuracy in the output is often recognizable as striped artifacts or as isolines that zig-zag too much.
When no explicit target area is used (in other words, when TargetInput is null), we recommended a number of sample rays per revolution in the range from 20 to 60. To learn how the accuracy can depend on the number of rays in this case, see section 6.5.1 of
W. R. Franklin, C. K. Ray and S. Mehta. Geometric algorithms for siting of air defense missile batteries.
Tech. report, Rensselaer Polytechnic Institute, Troy, New York, 1994.
However, when an explicit target area from TargetInput is used, it is less clear how many rays should be used. If the target area is very small compared to the MaxDistance, it is possible that all sample rays from some observer positions will miss the target area completely, if they are too few. So a suitable number should probably be in the range from 60 to 360 rays per revolution for an explicit target area, to ensure that enough of them will actually pass through the target area.
Regardless of the value of this property, the operator will never trace more rays from an observer position than a LineOfSightOperator with the same settings would do.
Platforms
Windows, Linux, Android