Determine Visibility from an Area

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Lesson content

Lesson 1 of 1

Determine Visibility from an Area

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. * •

Locate areas that are simultaneously visible from multiple rooftops. To do this, you will draw filled polygons on two different rooftops, then look for areas where their visibility overlaps. * •

Create a “Full Seen/Hidden Map” layer. * •

Create a heatmap layer that shows how much each pixel in the DSM was seen from combined observation points on rooftops. * •

Save the "Full Seen/Hidden Map" layer and heatmap to disk.

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

1

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.

4

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

1

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 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°.

2

Set the following values in the Viewshed Tools properties:

Vertex Interval (m): 1 meter
Range: 100 meters 2. 3

Locate a relatively tall building in the image. To do this, uncheck the visible image layer in the Layer Manager to hide it. Then look for a building in the DSM image that is mostly white; for example:

Building shown is located at 38°54'8.03"N,77°2'54.57"W.

4

Display the visible image again, then right-click in the Image window and select New > Filled Polygon. 2. 5

Draw a polygon around the perimeter of a rooftop, then double-click to close the polygon. The rooftop is marked with yellow vertices spaced one meter apart. A high density of observation points like this simulates having one or more observers on a rooftop, walking in all directions to oversee the area.

6

Now you will add a second filled polygon. Right-click and select Add > Filled Polygon, then repeat the steps to draw a polygon around a nearby building.

Note*: Be sure to select the Add > option. The New >* option will erase the polygon that you already drew.

You should have two filled polygons; for example:

Second building shown is located at 38°54'11.82"N,77°2'59.34"W.

Create Full Viewshed Results and a Heatmap

1

Right-click in the Image window and select Full Results. After several seconds of processing, a new “Full Seen/Hidden Map” layer overlays the image layer. It shows two classes: “Seen” (green) and “Hidden” (red). Green pixels indicate areas that are visible from at least one observer point, while red pixels indicate areas that no observer points could see.

2

In the Layer Manager, uncheck the Full Seen/Hidden Map layer to hide it. A “Seen Heatmap” layer is displayed over the visible image.

*Tip*: If the view is too cluttered, uncheck the DSM layer to hide it. Then increase the transparency of the visible image layer. This makes the heatmap colors more prominent. The yellow dots in the example below are the observer points on two rooftops.

A heatmap shows how much each pixel in the DSM was seen from the combined observation points on the rooftops. Red-to-yellow colors represent pixels that were seen the most. Blue-to-purple represent pixels that were seen the least. Notice that red-to-yellow pixels correspond to the tops of the tallest objects in the area such as trees and higher buildings.

Save Full Results and Heatmap to Disk

1

Right-click in the Image window and select Save > "Full Seen/Hidden Map" to File. 2. 2

Go to a preferred output directory and name the file SeenHiddenMap.dat. Click Save. In the Layer Manager, the new file name will replace “Full Seen/Hidden Map.” 3. 3

Repeat the same steps for the heatmap by selecting Save > “Seen Heatmap” to File. Name the output file SeenHeatmap.dat. 4. 4

This concludes the exercise.

This exercise showed how to determine what is visible and/or hidden from within user-defined polygons. 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 a Single Observer Point

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Determine Visibility from an Area

Determine Visibility from an Area

Sample Data

Open and Display Images

Start Viewshed Analysis

*Create Full Viewshed Results and a Heatmap*

Save Full Results and Heatmap to Disk

Create Full Viewshed Results and a Heatmap