Viewshed from a Tower

A viewshed is the portion of a landscape visible from a particular vantage point. That point may be on the ground or, like this example, above ground level in a tower. Viewsheds are commonly used in environmental planning (scenic views/historic vista preservation), communications tower site prospecting, and wildlife radio collar telemetry.

A viewshed raster is created from a DEM and an a point shapefile, in this case, a 30m DEM and an observation tower elevated 100m above the ground. The Viewshed tool (Spatial Analyst) in ArcGIS requires you make a simple edit to the point’s attribute table in order to run. The Spatial Analyst extension is required.

The following are general instructions. Filenames, coordinate system info, and tower height will likely differ for your project.

ArcGIS Instructions

1.) ADD DATA
Add your dem to a map. DEM_clipped is the name of the example 30m DEM used here.

2.) OPTIONAL
Create a contour layer (or a Hillshade) from the DEM using ArcToolbox > Spatial Analyst > Surface tools > Contour tool (or Hillshade tool). This may help you better locate your tower.

3.) CHOOSE LOCATION FOR OBSERVATION TOWER
Read the terrain and decide where to locate a new tower. The goal is to maximize the viewshed area of each.

Place a point marker (a graphic) at a tower viewhigh point on the map. Do this by Customize menu > Toolbars > Draw tools, select the Marker tool button (its located in the pulldown arrow menu, under the Rectangle), and click on the map to create a point. You can reposition the graphic or delete it.

4.) CONVERT POINT GRAPHIC TO A SHAPEFILE
Use the black arrow to select the point graphic and convert to a shapefile.
To do this:
– Select the point graphic.
– Right-click on Layers > Convert Graphics to Features.
– Name output point shapefile Tower_1.
– Save the new shapefile to your filespace.

viewshed example

Red pixels represent areas of the landscape visible from the monitoring station that stands 3m high (yellow triangle). All non-red areas are blocked by the terrain.

5.) EDIT ATTRIBUTE TABLE
Open Attribute Table for Tower_1.
– Click Table Options > Add Field, OK the warning, Name the new field = OFFSETA, Format = Long integer, Click OK.
– Note: OFFSETA needs to be in all caps. This is a required field for the tool to run.
– Customize menu > Toolbars > Editor. The Editor toolbar should appear.
– Next, we’ll choose the vertical offset from the DEM surface (height in meters above ground). The example number 100 (OFFSETA = 100 meters). Your value will probably differ.
– For this example, set the vertical height for the tower by changing 0 to 100 to the attribute table.
To do this:
– On the Editor toolbar, choose Start Editing from drop down arrow.
– Choose the Tower_1 layer from the list, OK the warning window.
– Type in 100 in the table for the offset height (OFFSETA field). Since the example DEM is in meter units, 100 = 100 meters.
– Note: Alternatively, you could populate the cell using the Field Calculator.
– Return to Editor toolbar > Stop Editing > Save Edits. Make sure you always end editing sessions before running tools. Nothing works right if editing sessions are left open.
– Close the Attribute Table.

6.) VIEWSHED ANALYSIS
Open ArcToolbox > Spatial Analyst > Surface > Viewshed, then…
– Input raster = DEM_clipped.
– Input point feature = Tower_1.
– Output raster = VS_1.
– Click OK to run the tool.

viewshed towers

The size of each tower’s viewshed can be calculated from the pixel count provided in the attribute table for each viewshed raster.

7.) INSPECT VIEWSHED RASTER ATTRIBUTE TABLE
When the viewshed raster is generated (VS_1), open its Attribute Table to see what has been done.
– Inspect the VALUE field. A VALUE of 1 = Count of pixels visible from the tower. A VALUE of 0 = Count of pixels not visible from tower. These classes should correspond with the display colors (default is green and pink).
– Remove fill color for the Not Visible class.
– Change the layer transparency to 55% (Properties > Display) and display over a hillshade.

8.) OPTIONAL: RECLASSIFY TO REMOVE NOT VISIBLE PIXELS
If you wish, use the ArcToolbox > Spatial Analyst > Reclass > Reclassify tool to remove the Not Visible (0) pixels from the viewshed raster. Create 2 classes, set 0 to NoData, leave class 1 alone, call the new raster something like VS_1_re.

9.) AREA OF VIEWSHED
– To tabulate the total viewshed area, use Properties > Symbology > click Categories in left side frame > click the Classify button.
– SUM, in the Classification Statistics box, shows you the pixel counts for Visible (1), Not Visible (0).
– Since our DEM is a 30m x 30m pixels*, we can find the actual viewshed area in km2 by:

Number of pixels x 900m2 / 1,000,000 = _____ km2

** NOTE: Projected 30m DEMs will have pixels that are slightly different than 30m x 30m. Check Properties > Source > Cellsize for the exact dimension of pixels.

viewshed overlaps

Summed values of reclassified pixels for 3 overlapping viewsheds. Values correspond to number of towers that can seen.

10.) OPTIONAL: TABULATE OVERLAPPING VIEWSHED PIXELS
If you have created multiple viewsheds and want to identify/tabulate the overlapping areas.
To do this:
– Use Spatial Analyst > Map Algebra > Raster Calculator to add all viewshed rasters together. You’ll create an equation something like the one below by clicking the raster names (double-click) and buttons (single-click).

“Tower_1″ + “Tower_2″ + “Tower_3″

– Name the output raster something like VS_overlap. Probably not the greatest looking or useful map, except for the data it provides.
– The attribute table for the resulting output raster does not contain the tower numbers. Instead it contains values for the number of overlapping viewsheds and the respective overlap pixel counts. For example, 0 = no overlapping pixels; 1 = on tower covers some number of pixels; 2 = two towers’ viewsheds overlap some number of pixels; etc….
– Open the Attribute Table to view data for VS_overlap…

Refs:
Franklin and Ray (1994)
Search terms for ArcGIS Desktop Help:
“Viewshed”,
“Using Viewshed and Observer Points for visibility analysis”
“Analyze visibility”

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