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How Visibility works

Release 9.3
Last modified September 7, 2011
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Visibility analyzes visual exposure and performs viewshed analysis. The table below lists the types of questions that can be answered with the Visibility tool.

Question Visibility options
Which areas can be seen from a fire lookout tower that is 15 meters high? POINT, FREQUENCY, OFFSETA
How frequently can a proposed disposal site be seen from an existing highway? LINE, FREQUENCY
Where should the next communications repeater tower in a series be located? POINT, FREQUENCY, OFFSETA, OFFSETB, VERT1, VERT2, AZIMUTH1, AZIMUTH2
Given a set of locations for fire lookout towers, what is the minimum number of towers required to see the entire study area? POINT, OFFSETA, OBSERVERS
Which locations of the raster can see only the disposal site and transmission tower 3? POINT, OBSERVERS
I want to assign different weights to each landscape feature within a viewshed based on its visual quality. How can I determine which surface locations have the best view? POINT, OBSERVERS

Controlling the visibility analysis

Default settings for the options controlling Visibility
Option Default setting
SPOT Estimated using BILINEAR interpolation
VERT1 90
VERT2 -90
RADIUS2 Infinity

The FREQUENCY option

With the FREQUENCY option, Visibility creates a raster recording the number of times each area can be seen from the feature observation points. This value is recorded in the VALUE item in the table of the output raster. All cell locations assigned NoData on the input <grid> are assigned NoData on the output raster.

Rasters with the OBSERVERS option

The OBSERVERS option stores the binary-encoded information about which observation points can see each raster cell. This information is stored in the VALUE item.

To display all the regions of the raster that can be seen only by observer 3, open the output raster attribute table and select the row where observer 3 (OBS3) equals one and all other observers equal zero. The regions of the raster that can be seen only by observer 3 will be highlighted on the map.

Raster OBSn items

In addition to the standard items Value and Count in the value attribute table, new items will be created corresponding to each observer in the input point coverage. The items are OBS1...OBSn, where n is the number of observers. They are defined as:


OBSn 2 2 B -

These items record the visibility of each cell by every <cover> observer. For example, every raster cell that can be seen by observer 8 (<cover># = 8) will contain a value of one in item OBS8. Cells that cannot be seen by the observation point are assigned a value of zero. Cell locations assigned NoData on the input <grid> are assigned NoData on the output raster.

You can use the OBS items to identify those raster cells that can be seen from a specific observation point. This is slightly different from the previous case in which a selection was made based on the value. In this case, cells that can be seen by observers 1 and 8 may also be seen by other observers (in which case, they each would have a different value).

For example, to display all areas that can be seen by observation points 1 and 8, open the raster attribute table and select the row where both observer 1 (OBS1) and observer 8 (OBS8) equal one and all other observers equal zero.

Quantifying visual quality

The OBSERVERS information can also be used to perform an analysis of visual quality. For example, you can determine the visual quality of all locations on a surface by positioning an observation point at each significant visual feature within the input raster's extent. Such points might include the city dump, auto salvage yard, local parks, and each of the power transmission towers in the region.

After running Visibility with the OBSERVERS option, use the OBSn item in the output raster's table to select those cell locations that can see each visual feature. Use any of a variety of the ArcGIS Spatial Analyst functions to accumulate positive or negative scores, depending on each observation point's visual quality and weight. After all observation points have been considered, those cell locations with the best scores will have the best visual quality.

Curvature and refraction corrections

The ISurfaceOp::Visibility method with the esriGeoAnalysisVisibilityFrequencyUseCurvature or esriGeoAnalysisVisibilityObserversUseCurvature enumerations corrects for the curvature of the earth and refraction. Corrections are made when projection information for the surface is present in the PRJ file. In addition, the ground units and surface z units must be in feet, meters, or units/meter. The formula used for the correction is:

                       Dist2               Dist2

Zactual = Zsurface - --------- + Rrefr * ---------

DiamEarth DiamEarth


- Dist is the planimetric distance between the observation feature and the observed location.

- Diam is the diameter of the earth.

- R refr is refractivity coeeficient of light.

The default value for the diameter of the earth (Diam earth ) is defined as 12,740,000 metres and the default value for the refractivity coefficient (R refr ) is 0.13.

For some applications involving visibility with radio waves, the refraction correction to be applied depends on the wavelength of the signal. This is often achieved by adjusting the diameter of the earth. While the Visibility tool has no provision for specifying the radius of the earth, a viable workaround is to adjust the units information in the PRJ file of the surface and the input point or line coverage of observation points.

Consider, for example, that the ground units and surface z units are in meters, and it is required to use 1.5 times the radius of the earth for visibility analysis. This is achieved by temporarily modifying the ground units and surface z units to 1.5 units/meter in the PRJ file. Correction for refraction of visible light is already incorporated in the calculations, which has the effect of increasing the radius of the earth by a factor of 1/0.87 = 1.15. This should be taken into account when further correction for refraction needs to be made. In the example discussed above, the actual correction for the radius would be 1.5/1.15 = 1.304. The units and z units of the surface should be set to 1.304 units/meter to achieve the net effect of 1.5 times the radius. Similarly, the units for the input coverage should also be set to 1.304 units/meter.

The location of telecommunication sites is not merely a simple matter of determining the intervisibility, but rather involves a number of parameters involved with the modeling of radio wave propagation, including reflection, refraction specific to the frequency, attenuation (signal weakening), interference, atmospheric effects, and so on. Nonetheless, the Visibility tool is appropriate during the preliminary investigation stages of assessing possible telecommunication sites and coverage.


The following example shows how to incorporate the effects of vegetation cover in a visibility analysis. This is easily accomplished if you have a vegetation raster with an attribute item containing the vegetation height.

In this case, the HEIGHT attribute is expressed in feet, but the values in the input elevation raster are in meters. You will convert the vegetation height to meters when you add the vegetation raster's height to the elevation raster to produce a raster describing the vegetation canopy (the elevation z-value plus the z-value of the height of the trees).

elevation + (vegetation.height * .3048)

Then perform the visibility analysis in the usual way. In this example, you are identifying the areas on the raster that can see the towers of a proposed transmission line. Each vertex of the input line cover represents the location of a proposed tower.

visibility(canopy, powerline, line)

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