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Using the Viewshed Tool in MapInfo Pro Advanced ( Viewshed Operation Part 1 - Visibility)

Mapinfo Pro 17.0, MapInfo Pro Advanced 17.0
The “Viewshed” operation is one of the raster analysis tools available in MapInfo Pro Advanced 17.0. In the example below this analysis tool is examined in greater depth with an example.
 
A viewshed analysis seeks to answers the question - “from where can I be seen?”, or “what can I see from here?”. More specifically, the analysis tells you about the visibility of a small number of target locations at a large number of observation positions. The tool requires the user to supply the location of the targets and a raster representing the terrain. The terrain raster may represent bare earth, or it may be a DSM that includes building, vegetation and other cultural interference. The observation positions are defined by this terrain raster and lie at the center of every cell in the raster.
 
One application might be the placement of a wind turbine or the design of a wind farm. Viewshed analysis can determine from where those turbines are visible, the aim being to minimise the visibility of the targets. Another application is the placement of a mobile phone tower. Viewshed analysis can determine what areas can be serviced by that phone tower and help to design the placement and height of the tower so that line of sight visibility of the target is maximised.
 
In the simplest case we have a single target and multiple observers. The target is placed at some position within the bounds of the terrain raster and the tool will place an observer at the center of every grid cell surrounding the target, out to some specified radial distance. The tool initializes both the target and the observers at the local height of the terrain. Both the target and the observers can then be offset vertically by some height above the actual terrain. With this arrangement, viewshed analysis can answer the following questions:
  1. What observers can see the target?
    1. The output raster records a result for each observer as zero or one – visible or invisible. Alternatively you can output a classified raster with two classes – visible and invisible.
    2. It also answers the question – what observers can be seen from the target? A statistical analysis of the output raster can tell you magnitude of the area that can sight the target.
  2. How much higher must each observer be to see the target?
    1. The output raster records a result for each observer. Ideally this result would be zero or a negative value representing the vertical distance the observer must be raised to be able to sight the target. 
  3. For each observer, how much lower could the target be and still be seen?
    1. The output raster records a result for each observer. Ideally this result would be zero or a positive value representing the vertical distance the target can be lowered and still be sighted by the observer. 
In addition to having multiple observers, we can also have multiple targets. In this scenario we can answer the following questions:

       4.  How many targets can be seen by each observer? 
                  a.  The output raster records a result for each observer as an integer count of the number of visible targets.
                  b.  Calculator operation on this raster can answer the questions:
                          Which observers can sight a target?
                          Which observers can sight at least “N” targets?
                          Which observers can sight all targets?

The viewshed analysis answers question (1) by default operation. To answer questions (2) and (3) you need to enable the “Complex Calculation” setting, which is available as an option when you have a single target. Although the analysis answers two questions in this mode, the result is output as a single raster where the sign of the numeric values in that raster determines what the result is. This can be summarised thus – “Observers that cannot sight the target have negative values; the values returned are the vertical distance by which the observer must be raised for the target to be sighted. Observers that can sight the target have positive values; the values returned are the vertical distance by which the target can be lowered and still remain visible.”
 
To answer question (4), you need to enable the “Compute Intervisibility” setting, which is available as an option when you have multiple targets. In this mode, the output raster records the integer count of the number of targets that can be seen from each observer location.
 
An interesting example - to propose a new windfarm on the coast at Mull of Kintyre, on the south west tip of the Kintyre peninsular in Scotland. The two raster datasets from the UK Ordnance Survey – the 50 metre resolution DTM and the 1:50,000 scale topography imagery are taken to explain. Both of these are freely downloadable as a collection of small tiles, which have been merged together into MRR rasters, for ease of use. In MapInfo Pro, both these rasters are displayed in a map with partial transparency applied to the DTM.

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MapInfo Pro is used to create point locations for 15 wind turbines, following the 250 metre contour level on the topography along the coastline.
 
Primarily, it will show the locations in the UK from where these turbines can be seen. The 15 turbines are my targets, and the 50 metre terrain raster simulates an observer in every grid cell (2500 square metres). To perform this analysis, select “Multiple Points” viewshed mode, to supply multiple targets. Specify the table containing the turbine location points and leave all units in metres. There is a need to specify the height of each turbine, which here is set to 125 metres for all. This could have been specified for each turbine individually in the table, if required. A 2 metre offset is specified for each observer so that each observer is slightly raised off the terrain surface. The analysis is performed to a radius of 150 kilometres from each turbine. Again, this could have been specified for each turbine via a field in the table. Finally, using a “Normal Earth Correction” to include the curvature of the Earth in the analysis, which is recommended for all line-of-sight style analysis.

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The result clearly shows the radius of analysis, displaying as blue for observers that cannot sight a turbine and red for observers that can sight a turbine.

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There is a way of displaying only the observers that can sight the turbines.
 
There are multiple ways to do this - rendering options, the calculator or polygonisation. I used the polygonise tool to convert the areas where observers can sight a tower to outlined polygons. Now you can see that these turbines are theoretically visible from almost 150 kilometres away – from Ben More in the Scottish highlands for example, on a good day.

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Perhaps of more interest is the visibility of the wind farm from the nearby town of Campbeltown on the Kintyre peninsula. Whilst the turbines are visible from the surrounding hills, the residents of Campbeltown are unaffected.

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On the mainland, the residents of Prestwick and Ayr get a good view, but being 75 kilometres away they are unlikely to be too upset.

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The article gives you some background information on the kinds of calculations you can perform with the Viewshed analysis tool.
UPDATED:  May 11, 2018