Measure Features
Lesson content
Lesson 1 of 1
Measure Features
Mensuration is the process of calculating the length and perimeter of linear features directly from an image, by drawing line segments. You can also calculate the distance between features or the area of a polygon. If an image is georeferenced to a standard map projection, the measured values are reported in meters, feet, or whatever distance unit you choose.
Keep in mind the accuracy limits of the data. You will be working with a QuickBird image in this exercise. According to the QuickBird product specification manual, the horizontal geolocation accuracy for any given pixel of a standard imagery product is 23 meters with a 90% circular error (CE) probability. This means that geographic coordinates for a pixel is accurate to within 23 meters in any direction, with a 90% confidence.
Reference: DigitalGlobe. QuickBird Imagery Products - Product Guide, rev. 4.7.2 (2006).
Open an Image
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Start ENVI. 2. 2
Click the Open button in the Toolbar.
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When the Open dialog appears, go to the "data" directory in your ENVI installation path.
Windows: C:\Program Files\NV5\ENVIxx\data (xx is the version number)
Linux: /user/local/NV5/envixx/data
Mac: /Applications/NV5/envixx/data
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Select the file qb_boulder_msiand click Open. This is a Quickbird multispectral image of Boulder, Colorado. The picture below shows the image at its full extent.
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Zoom to the running track near the large, bright school.
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Click the Mensuration button in the Toolbar.
The Cursor Value dialog appears, and red crosshairs are displayed over the center of the view. The cursor icon changes to a + symbol, indicating that it is ready to draw line segments. First, you will measure the distance from the track to the nearby school.
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Click on the north end of the track. A line extends from that point to wherever you move the cursor. The Cursor Value dialog reports the distance and azimuth (direction) of the line segment. 2. 8
Click on the edge of the school directly north of the track. A line is drawn between the two points. 3. 9
Right-click in the view and select Accept. The line is now complete and the cursor value changes to a + symbol, ready to draw again. The Total Distance reported in the Cursor Value dialog should be close to 76 meters. Watch the following animation:
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Press the Esc key on your keyboard to deselect the polyline annotation. 2. 11
Click anywhere on the track. Continue drawing line segments all the way around the track until you reach the starting point. You cannot end the line exactly at the first point; you must offset it by one pixel. Right-click and selectAccept. A second "Polyline" annotation item is added to the Layer Manager. The Cursor Value dialog reports the length of each segment as well as the perimeter (shown as Total Distance). It should be approximately 420 meters.
Tip:To remove the last line segment drawn, click the Backspace key on your keyboard.
The Cursor Value dialog also reports the azimuth of each line segment, with respect to North.
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Press the Esc key to deselect the polyline annotation. 2. 13
Click theUnits drop-down list in the Cursor Value dialog and select Feet. The Total Distance and individual polyline segments are now reported in feet. 3. 14
Next, you will measure the area of a building using a polygon annotation. Click the Annotations drop-down list in the Toolbar and select Polygon. 4. 15
Click on a corner of the large, bright building. A line extends from that point. 5. 16
Draw line segments around its perimeter. When you are finished, right-click and select Accept. The Cursor Value dialog reports the Area and Perimeter values, along with the length of each line segment.
Line segments are offset for clarity
Tip: If you click another button in the Toolbar (i.e., Select, Pan, etc.), the annotation layer loses focus. To resume drawing a polyline, click the Annotations drop-down button in the Toolbar and select Polyline. The cursor changes to a + symbol.
This concludes the exercise.
Refer to the Create Annotations quick guide for more information about annotation layers.
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