View Profiles

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Lesson content

Lesson 1 of 1

View Profiles

In this quick guide, you will:

•

Display horizontal and vertical profiles of data across a QuickBird image. * •

Create a custom transect and view its profile. * •

View spectral profiles of individual pixels.

Open and Display an Image

1

Select File > Open from the Menu bar. An Open dialog appears. 2. 2

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

3

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.

Display Horizontal (X) and Vertical (Y) Profiles****

1

Select Display > Profiles > Horizontal from the Menu bar. Red crosshairs appear in the Image window, and the Horizontal Profile plot window appears. Because this is a true-color composite, the plot window shows profiles of the red, green, and blue bands. The profiles show digital number (DN) values on the Y-axis, along the horizontal line of the crosshairs, from left to right.

The left half of the plot shows rapid fluctuations in data values where the profile crosses the urban area. The region between columns 200 and 700 corresponds to dry fields. The low values beyond 700 correspond to the dark water. Notice how the red band has much lower data values than the blue and green bands over the water.

2

In the Layer Manager, click on qb_boulder_msi to bring focus to that layer. 2. 3

Select Display > Profiles > Vertical from the Menu bar. The Vertical Profile plot window appears. It shows data values along the vertical red line in the crosshairs.

4

Click in a different part of the image. The crosshairs center over that point, and the plot windows update to show the corresponding horizontal and vertical profiles. 2. 5

Close the Horizontal Profile and Vertical Profile plot windows. The windows are dismissed and the crosshairs are removed from the Image window.

Next, you will display a horizontal profile of one band and learn how to smooth its curve.

6

Select File > Data Manager from the Menu bar. 2. 7

In the Data Manager, right-click on Band 4 and click Load Grayscale. A grayscale image of the near-infrared band is added to the Layer Manager and displayed in the Image window. 3. 8

Select Display > Profiles > Horizontal from the Menu bar. The Horizontal Profile plot window appears, and red crosshairs are displayed in the Image window. Again, the plot shows data values from the near-infrared band along the horizontal line in the crosshairs.

The dramatic fluctuations in data values make interpretation difficult. You can smooth the curve to create a more visually appealing plot.

9

Click the Options drop-down button in the Horizontal Profile window and select Curve Smoothing. A slider bar appears at the top of the window, and the plot curve is smoothed.

Notice that the range of data values changes along the Y-axis. The original plot showed the actual range of data values (0 to 665), whereas this one shows a narrower range. The purpose of smoothing is to approximate the original shape of the curve when the image contains many data values. The data values along the Y-axis are not literal when the curve as been smoothed.

10

Drag the Curve Smoothing slider at the top of the Horizontal Profile window back and forth to interactively change the smoothing level. For more precise control, click and hold the slider needle while using the arrow keys on your keyboard to increment the slider one unit at a time.

11

Close the Horizontal Profile window. The window is dismissed and the red crosshairs are removed from the Image window.

Display Arbitrary Profiles (Transects)

1

Select Display > Profiles > Arbitrary from the Menu bar. The cursor changes to a + symbol.

Tip: You can also click the Arbitrary Profile (Transect) button in the Toolbar:

2

Click on a linear feature such as a road or shoreline. Then click on several spots along that feature. To end the line, right-click and select Complete and Accept Polyline. An Arbitrary Profile appears, showing the data values for the pixels along your line.

3

Close the Arbitrary Profile window. 2. 4

In the Layer Manager, right-click on [1] qb_boulder_msi.dat and select Remove. The grayscale image is removed from the Image window.

Spectral Profiles

The following steps demonstrate how to plot the spectra of individual pixels across all bands in the QuickBird image. A spectral profile (sometimes called a z profile) plots spectral information from the whole file and not just the bands displayed on the screen.

1

Select Display > Profiles > Spectralfrom the Menu bar. Red crosshairs appear in the Image window, and a Spectral Profile window appears.

Tip: You can also click the Spectral Profile button in the Toolbar:

The Spectral Profile shows data values across four bands for the pixel underneath the crosshairs probe:

The wavelength values are displayed along the X-axis since the image contains spectral metadata for each band. If you view a spectral profile of an image that does not have wavelength metadata, the X-axis would only show the band numbers.

2

Click in different locations in the Image window. Notice how the Spectral Profile plot window automatically updates to show the spectrum of each pixel you click on. 2. 3

In the Go To field in the Menu bar, enter 307, 433 and press the Enter key. The crosshair probe moves to a tree in the image. You can see a small peak in the green wavelength region, followed by a larger peak in the near-infrared wavelength region.

4

Click anywhere along the red curve in the Spectral Profile. Dashed crosshairs center on the nearest data point of the curve. Red text appears in the lower-left corner of the Spectral Profile. The number in brackets is the band number associated with the selected data point. The X value is the center wavelength, in nanometers. The Y value is Data Value.

At the bottom of the Spectral Profile is a color bar indicating the color that humans perceive when light is emitted at that wavelength. The far left part of the color bar shows the visible wavelengths. The remainder of the color bar is black because those near-infrared and shortwave-infrared wavelengths are invisible to the human eye.

This concludes the quick guide.

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