Determine Visibility from an Observer Point
Lesson content
Lesson 1 of 1
Determine Visibility from an Observer Point
In this quick guide, you will use the Viewshed Tool to:
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Open and display a Digital Surface Model (DSM) and orthophoto that are co-registered with each other. * •
Determine visibility from a single observer point in an urban setting.
Sample Data
Download sample data below. Then extract the contents of the .zip file to a local directory.
[OpenDC_DSM_Ortho.zip
393.6 MB
DownloadArrow down with horizontal line beneath it](assets/OpenDC_DSM_Ortho.zip)
Open and Display Images
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Select File > Open from the Menu bar. An Open dialog appears. 2. 2
Go to the directory where you saved the sample data. Select OpenDC_DSM.datand *click* Open. 3. 3
Click the Zoom to Full Extent button in the Toolbar.
The DSM covers a small area in Washington, D.C. The source files are available from the OpenDC Data Catalog(opens in a new tab). Pixel values represent height (in meters) above sea level.
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Select File > Open again and select the file OpenDC_Orthophoto.dat. A true-color orthophoto overlays the DSM. It covers the same geographic extent as the DSM.
The orthophoto has a much finer spatial resolution (3 inches, or 7.62 centimeters) than the DSM (1 meter).
Start Viewshed Analysis
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In the Toolbar, click the Viewshed Analysis button.
ENVI adds a “Viewshed Seen Area” ROI layer to the Layer Manager. This layer will show the areas that can be seen from the observer points. The ROI layer is not attached to any rasters in the Layer Manager. This allows you to move it up and down in the layer stack and independently adjust its level of transparency.
An Options tab opens in the bottom-right corner of the application. The Viewshed Tool properties are editable. They include the observer point height, horizontal field of view, and vertical field of view. You can change these properties at any time during viewshed analysis.
The following diagrams illustrate the first five properties, starting with Range:
The elevation and height values associated with the viewshed are extracted from the DSM, not the visible image. The default range of the viewshed is 250 meters, and the horizontal field of view is 360°.
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Click on any rooftop in the image that is adjacent to a major corridor. A yellow vertex is added as the observer point, and green pixels show what is visible from that point. The Vertex Color and Seen Color settings in the viewshed properties define these colors.
Next, you will add an observer point, change some of its properties, and move it to different locations on the image to observe how it affects the viewshed.
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Click and drag the vertex to another rooftop along the same road to move the observer point. The results show the new areas of visibility from the observer point. 2. 4
Leave the observer point at its current location. In the next step, you will change some properties to see how they affect the viewshed results. 3. 5
In the Viewshed Tool properties, change the Show Layers value to Hidden. The view updates to show only the areas that are hidden from the observer point. In the Layer Manager, the layer name changes from “Viewshed Seen Area” to “Viewshed Hidden Area.” With this change, pixels that are visible from the observer point are not highlighted with a color, and pixels that are not visible from the observer point are colored red. The following example shows a zoomed-out view, with a range of 250 meters.
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In the Layer Manager, select the Viewshed Hidden Area layer. Then set its transparency (in the Toolbar) to 50. You can see the visible image through the hidden area.
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In the Viewshed Tool properties, change the Show Layers value to Both. The view shows pixels that are visible and hidden from the observer point. A green “Viewshed Seen Area” layer is added to the Layer Manager. (You may need to increase the transparency of the “Viewshed Hidden Area” layer again.)
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In the Viewshed Tool properties, change the Show Layers value to Seen. The “Viewshed Hidden Area” layer is removed from the Layer Manager, and the “Viewshed Seen Area” layer remains. 2. 9
Experiment with different Observer Azimuth and Horizontal Field of View values. In the following example, the Observer Azimuth is 110 degrees and the Horizontal Field of View is 180 degrees. This faces the observer toward the road that runs along a southwest to northeast direction
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This concludes the exercise.
This exercise showed how to determine what is visible and/or hidden from a stationary point. See the following quick guides for other viewshed capabilities:
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Viewshed Analysis: Determine Visibility Along a Route * •
Viewshed Analysis: Determine Visibility from an Area
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