The Viewshed analysis tool identifies the visible and invisible regions from a observer's location. The analysis uses the elevation value of each cell of the digital elevation model (DEM) to determine visibility to or from a particular cell. This tool produces raster having details of visible and invisible cells from the observer's location. For example, you can use Viewshed to analyze the spatial coverage by a series of telecommunication or repeater towers, or to estimate the position of wildfire from the observation platforms.
In the example below, the observer's location is marked with a green triangle. The output displays the viewshed region in blue and red color from the observer's location. The visible regions are indicated by the red color. The regions in the blue color indicate the invisible regions.
DEM Raster |
Output Viewshed |
For viewsheds from multiple locations, you will need a MapInfo TAB file that contains multiple observer's locations.
In the example below, the viewshed from multiple locations are calculated from a TAB file. Notice how the viewshed coverage has increased after adding two more towers.
DEM Raster |
Viewshed of Points |
Viewshed Analysis Options
You can analyze the viewshed of a point or multiple points with the help of the following options:
- View Point Height - This indicates the height of the observation or transmission tower from where you will calculate the viewshed.
- Terrain Offset - This indicates the offset value which will be added to each cells within the viewing radius for viewshed calculation.
- Viewing Radius - This indicates the maximum search distance around the observer's location. You can increase or decrease the search distance for analyzing viewshed coverage.
- Earth Curvature - This indicates the earth curvature correction option to consider in viewshed calculations.
- Use Sweep Azimuth - The sweep azimuth denotes horizontal angle measured clockwise from north base line. The sweep azimuth proceeds in a clockwise direction. The values for the angle are given in degrees from 0 to 360, with 0 oriented to north.
- Sweep Angle - This specifies the angular width of the sector to be scanned. The angles are expressed in degrees.
- Value Classification - Normally the output from the viewshed is a raster with values of 0 for cells which cannot be seen from the viewpoint origin (tower), and 1 for cells which can be seen The Null values from the input raster are also preserved in the output grid. The classification option is only applicable if the smoothing option is used. In this case, the values of the grid will be smoothed to a floating point value around 0 and 1, for example, 0.756123. So, the Value Classification option allows these floating point values to be converted back into the values 1 and 0 (in/out) or 1, 0.5, 0 (in/fringe/out). You can classify the line of sight grid values into the following options:
- None - Denotes no classification of cells in the output viewshed raster from the viewshed origin.
- In/Out - Denotes whether a cell is visible or not from the viewshed origin.
- In/Out/Fringe - Denotes whether a cell is visible, not visible, or on the fringe when viewed from the viewshed origin.
- Use Complex Calculation - The value represents either the height the cell should be raised to make it just visible from the viewpoint (a negative value because it lies below the sight line), or the height that the viewing points should be lowered in order to become just visible from the grid cell (a positive value because it lies above the sight line).
- Null Cells Outside Viewshed Radii - Denotes whether to set the cells lying outside the viewshed area to null values.
- Clip to Viewshed Radii Bounds - Denotes whether to clip the radius of a viewshed area to another raster.
- Create Classified Raster - Output viewshed area raster as classified Vertical Mapper or MRR.
Earth Curvature Correction
This value is taken into account to calculate the scaling of the earth's radius to the correct units. MapInfo Pro Advanced supports the following curvature correction models:
- No earth correction
- Normal earth correction. In this case, the corrected earth's radius will be (6,378,137) meters * 1.
- 4/3 earth correction. In this case, the corrected earth's radius will be (6,378,137) meters * 4/3. This curvature correction allows radio frequency engineers to model the slight curvature of a radio wave as it travels over the earth’s surface due to diffraction effects.
- 2/3 earth correction. In this case, the corrected earth's radius will be (6,378,137) meters * 2/3.
For the purposes of viewshed computation, the Z value for each grid cell is adjusted downwards based on its distance from the center of the viewshed origin using this formula.
ZCellAdjustment = -sqrt(CorrectedEarthRadius^2 + CellDistanceFromViewshedOrigin^2) – CorrectedEarthRadius
Where:
- CorrectedEarthRadius = 6,378,137 * CorrectionFactor. Correction factors can be 1, 4/3, or 2/3.
The figure below shows the effect of earth curvature on a flat grid:
For a non-flat grid, the cells are adjusted down further away from the grid center, as shown below, for a total tower height (tower height+terrain height).
The following examples show how the earth's curvature correction impacts the viewshed on a flat grid, taking the parameters below:
- The (X,Y) viewshed origin = (536900.0, 5237000.0)
- The viewshed radius = 60,000 meters
- The view point height = 10 meters
- The viewshed offset = 0 meters
- The sweep azimuth = 0
- The sweep angle = 30 degrees
- Gaussian filter width = 3
- Vertical units = meters
No Earth Correction
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Normal Earth Correction
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4/3 Earth Correction
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2/3 Earth Correction
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Vertical Units
Units (Meters, Feet, Yards, etc.) represent the vertical units of the raster. Ideally, the vertical and horizontal Z units should be the same for the operation to calculate viewshed correctly. If the horizontal and vertical units do not match, the units are scaled to match. Depending on the unit chosen, a scaling factor is applied to make both the units match.