api:dk11:delphi:gisviewshed.tgis_viewshed
Table of Contents
TGIS_Viewshed class
DK for Delphi | GisViewshed.TGIS_Viewshed | Classes | Constructors | Methods | Properties | Events
Viewshed, above-ground-level (AGL), and Fresnel zone clearance generator.
Syntax
// Delphi type TGIS_Viewshed = class( TGIS_BaseObjectDisposable ) end;
// C++ Builder class PASCALIMPLEMENTATION TGIS_Viewshed : public TGIS_BaseObjectDisposable { };
Inheritance
Constructors
Inherited
Overrides
Protected
Private
| Name | Visibility | Description | |
|---|---|---|---|
| Create | public | Creates an instance. | |
Methods
Inherited
Overrides
Protected
Private
| Name | Visibility | Description | |
|---|---|---|---|
| Generate | public | Generates viewshed (and, optionally, above-ground-level (AGL) raster) as a grid/Digital Elevation Model (DEM). | |
| Generate(TGIS_LayerPixel; TGIS_LayerVector; TGIS_LayerPixel; Single; String; Single) | public | Generates viewshed as a grid/Digital Elevation Model (DEM). | |
| Generate(TGIS_LayerPixel; TGIS_LayerVector; TGIS_LayerPixel; TGIS_LayerPixel; Single; String; Single) | public | Generates viewshed (and, optionally, above-ground-level (AGL) raster) as a grid/Digital Elevation Model (DEM). | |
| GenerateFresnel | public | Generates n-th Fresnel Zone clearance map as a grid/Digital Elevation Model (DEM). | |
| GenerateFresnel(TGIS_LayerPixel; TGIS_LayerVector; TGIS_LayerPixel; Integer; Single; String; Single) | public | Generates n-th Fresnel Zone clearance map as a grid/Digital Elevation Model (DEM). | |
Properties
Inherited
Protected
Private
| Name | Visibility | Description | |
|---|---|---|---|
| AboveGroundLevelLayer | public | Output layer containing values of how many meters should be added to a particular point to be visible from at least one observer; this layer must have the same extent, CS, and resolution as the OutputLayer; if NULL, this analysis will be skipped; layer is initially filled according to the FillWithZeros property. | |
| CurvedEarth | public | If True then the elevation of input data will be corrected for Earth curvature and refraction. | |
| FillWithZeros | public | If True then output is initially filled with zeros instead of NoData (default). | |
| Frequency | public | Radio frequency for Fresnel Zone analysis; default is 100 MHz. | |
| FresnelZoneNumber | public | Number of the Fresnel zone for clearance calculation. | |
| ObserverElevation | public | Specifies coordinate used as the elevation of the observers; default is DEM. | |
| ObserversLayer | public | Layer containing observer locations; layer is expected to contain one or more points; only first 50 points will be used. | |
| ObserversOffset | public | Fixed observers elevation offset in meters. | |
| ObserversOffsetField | public | Attribute field defining observers elevation offset in meters. | |
| OutputLayer | public | Output layer containing the viewshed analysis; values are set according to the ViewshedOutput property; the layer is initially filled according to the FillWithZeros property. | |
| Radius | public | Specifies the maximum distance (in meters) between the observer and the cell for the analysis to be performed; greater distance implies no visibility; default is 20 km. | |
| Refractivity | public | Specifies the refractivity coefficient of light; default is 0.13. | |
| TerrainLayer | public | Terrain model as a grid layer. | |
| TerrainOffset | public | Fixed terrain elevation offset in meters to for example add forest level. | |
| ViewshedOutput | public | Specifies output values of the viewshed analysis. | |
Events
Remarks
This class is OpenCL-enabled, so it can utilize OpenCL for increased performance. Use TGIS_OpenCLEngine.Enabled to enable OpenCL computation mode.
Currently OpenCL computation mode can be used to generate viewshed only. Above Ground Level and Fresnel zone clearance are always computed using CPU. For best computation performance (e.g. for continuous generation) set ViewshedOutput to Visibility and don't set AboveGroundLevelLayer.
General description
A viewshed is the area of visibility of an observed object(s), or, conversely, an area that can be seen from an observer location(s). For more information see this article on Wikipedia.
We provide a way to perform common analysis:
- Viewshed
Creates a pixel layer where pixel values represent the number of observers visible from a particular location. This analysis can be used in two common scenarios:- Visibility (binary) case - values greater than or equal to 1 mean that a location is visible by at least one observer.
- Frequency case - pixel value represents the number of observers visible from a particular location.
- Above-Ground-Level
Creates a pixel layer where pixel values indicate the minimum height that must be added to a non-visible cell to make it visible by at least one observer.
Sample results:
| Visibility | Frequency | Above-Ground-Level |
|---|---|---|
 |  |  |
| green - visible area | color ramp red→green where: red - one observer visible green - all observers visible | color ramp green→red where: green - 1m red - 1440m |
Advanced analyses
Polygonal areas
Analyses for polygonal areas (e.g., an exact city charter) can be done using the following steps:
- Uerform standard viewshed analysis for are which fully encompass a charter.
- Use TGIS_LayerPixel.Loop query for area covered by polygon on the result of the previous step. See how to do this in SimpleEdit sample.
Lines
Analyses for lines (e.g., how many times an observer is visible from a given segment of road) can be done using the following steps:
- Perform standard viewshed analysis for the area which encompassing the segment of road.
- Use TGIS_LayerPixel.Loop to query for terrain profile on the result of the previous step. See how to do this in SimpleEdit sample.
Vector results
Result of Generate method is a TGIS_LayerPixel. Polygonal shapes covering visibility areas can be generated from the TGIS_LayerPixel using TGIS_ContourGenerator.
References
2025/01/31 01:09