Pitney Bowes MapInfo Vertical Mapper - Manual (Page 140)

The Viewshed Function
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Vertical Mapper 3.7
• The Simple Calculation option enables you to create a classified grid file and assigns either the
category “Visible” or “Invisible” to each cell depending upon whether it is visible or invisible from
the viewpoint.
If multiple viewpoints are selected, each grid cell of the new classified grid is assigned a category
“NumVisible_n”, where “n” is the number of viewpoints visible from that cell; the values will range
from zero to the total number of viewpoints used in the calculation.
• The Complex Calculation option returns a value measured in grid z-units.
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).
The Looking From box enables you to set the point on the map from which all intervisibility
calculations are made (the viewpoint). The position is determined by entering known coordinates or
by using the Viewpoint Pick tool.
The View Point Height box enables you to set the value for viewpoint height if, for example, the
viewpoint represents an observation or a transmission tower of known height. The height is
automatically displayed in metres.
The Viewshed Offset box enables you to add a height to account for the size of an object that is
being viewed.
For example, if you want to measure a line of sight from a transmitting antenna to a receiving car
antenna, you may want to add a value of 1.5 metres at every grid location to better approximate the
true height of the car antenna above the surface of the ground. The height is automatically displayed
in metres.
The Viewing Radius box enables you to control the maximum radius around the viewpoint for which
Viewshed is calculated.
The Earth Curvature Model list enables you to choose an earth curvature model. Earth curvature
must be taken into account for most line of sight calculations. You can select one of the following
models:
Normal Earth Curvature calculated using an oblate spheroid model.
4/3 Earth Curvature 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.
No Earth Curvature ignores earth curvature.
Note If the grid is in latitude/longitude, you must choose a linear unit from the list. For all grids
defined in latitude/longitude, you must have a defined earth curvature.
Performing Multipoint Viewshed Analysis
You can perform viewshed analysis using several points.
1. In the map window, using any of the MapInfo Professional select tools, select point objects
representing the locations you want to perform the analysis on.
2. In the Grid Manager, click the Analysis button, and choose the Viewshed Analysis command.
3. In the Viewshed dialog box, enter the settings you want to use for the analysis and click the OK
button.