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TerrainAzimuthElevationMaskCreateDelayedMask Method

Creates an azimuth-elevation mask which can compute its values dynamically from the specified terrain provider. The values will be computed and stored when requested rather than computed all at once initially. This can increase performance and decrease memory requirements in cases where a large number of masks must be computed.

Namespace:  AGI.Foundation.Terrain
Assembly:  AGI.Foundation.TerrainAnalysis (in AGI.Foundation.TerrainAnalysis.dll) Version: 19.5.402.0 (19.5.402.0)
Syntax
public static DelayedTerrainAzimuthElevationMask CreateDelayedMask(
	TerrainProvider provider,
	Cartographic observerPosition,
	int numberOfAzimuthSteps,
	double stepSize,
	double maximumSearchAngle
)

Parameters

provider
Type: AGI.Foundation.TerrainTerrainProvider
The source of terrain from which to compute the mask.
observerPosition
Type: AGI.Foundation.CoordinatesCartographic
The planetodetic position of the stationary observer around which the mask is computed.
numberOfAzimuthSteps
Type: SystemInt32
The number of azimuth steps to use to compute the mask. This will define the resolution of the mask around the horizon.
stepSize
Type: SystemDouble
The size of the step along each azimuth ray, in radians.
maximumSearchAngle
Type: SystemDouble
The maximum angle, in radians, to move along each azimuth ray in constructing the mask.

Return Value

Type: DelayedTerrainAzimuthElevationMask
The azimuth-elevation mask.
Exceptions
ExceptionCondition
ArgumentOutOfRangeExceptionThrown if numberOfAzimuthSteps is less than or equal to zero.
Remarks

This method works by casting a ray out from the observerPosition in a number of directions to determine the minimum elevation angle that is visible above the terrain in that direction. More specifically, the various distances at which the minimum elevation angle increases in that direction are determined.

Selecting values for numberOfAzimuthSteps, stepSize, and maximumSearchAngle requires making a trade-off between the accuracy of the result and the time to compute it. The ideal values to use depend on the specifics of the problem being solved and the nature of the terrain data employed.

numberOfAzimuthSteps is the number of rays to cast out from the observerPosition in computing the mask. A value of 360 casts a ray for each degree, and is a reasonable value to use. Notice that when a value of 360 is used, the count of elevation masks will be 361 because it contains the elevation for both 0 and 360. The elevation data for 360 copies data from 0 with the exception of the azimuth for efficiency.

stepSize is the distance between steps along the ray, in radians. A reasonable value for this parameter would sample approximately three times per terrain grid cell, or 1/3 of the smallest of DeltaLongitude and DeltaLatitude.

maximumSearchAngle is the maximum angle, in radians, to move along each ray. In other words, this parameter indicates the stopping point for sampling along the ray. A reasonable value will take into account the difference between the maximum and minimum heights of terrain provided by provider and potentially how close together the minimum and maximum heights are located.

Examples
The following example shows one way this value might be chosen:
C#
double approximateHeightOfMtEverest = 8850.0;
double semimajorAxisLength = WorldGeodeticSystem1984.Shape.SemimajorAxisLength;
double maximumSearchAngle = Math.Acos(semimajorAxisLength / (semimajorAxisLength + approximateHeightOfMtEverest));

See Also