Compute Image Statistics
Lesson content
Lesson 1 of 1
Compute Image Statistics
In this exercise, you will use the Statistics View dialog to evaluate basic statistics and to view image histograms in a QuickBird image.
Evaluating statistics is an important step in learning more about your data, particularly before using analytical tools to process imagery. By looking at the minimum and maximum pixel values, you can quickly tell if they fall within the expected range, and if the image contains any outliers. Image histograms reveal how the data values are distributed across the image. This combined information can help you decide whether or not the image will be suitable for a given application, and if outliers need to be removed. Outliers are pixels with extremely high or low values that are outside of the normal range.
Open and Display an Image
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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. 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.
Compute Quick Statistics
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In the Layer Manager, right-click on qb_boulder_msi and select Quick Stats. The Statistics View dialog appears.
The plot window provides a quick overview of the minimum, maximum, mean, and standard deviation values for the four image bands. The color bar in the X-axis indicates the wavelength range along the electromagnetic spectrum.
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Look at the Basic Stats section below the plot window. It lists the minimum, maximum, mean, and standard deviation values for each band.
The QuickBird image has 11-bit radiometric resolution, which means that the image supports up to 211, or 2048, brightness values. The statistics table verifies that the pixels fall within the expected range across all bands. Viewing the statistics table also provides a quick way to verify that the image does not contain any outliers such as negative values or extremely high values.
Below the Basic Stats table are numeric values for each band's histogram. For example, in Band 1, three pixels have values that fall within the 138 - 143 range:
Rather than read the numeric values in a histogram, you can plot the histogram.
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If you are using ENVI 6.1 or later, click the Histograms check box located above the Basic Stats table. 2. 4
Click the Select Plot drop-down button at the top of the Statistics View dialog and select Histogram Band 1. A plot window appears. 3. 5
Right-click inside of the plot window and select Legend. A legend is added to the plot.
From this plot, you can see that most of the pixel values in Band 1 range from about 150 to 300, with the highest occurrence (100,000 pixels) around 250.
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Click somewhere on the red plot line, and hold down the left mouse button. A set of crosshairs centers on the nearest data point. Outside of the lower-left corner of the plot, red numbers show the data for the selected point. In the example below, pixel values that fall within a range of 276 to 281 occur 26,682 times in the image:
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Click the Select Plot drop-down list and view the histogram for Band 2 (green), then Band 4 (near-infrared). What differences do you see in terms of how the data is distributed in both bands? Does one band show a higher concentration of pixel values in a certain part of the electromagnetic spectrum than the other band? 2. 8
When you are finished, close the Statistics View dialog.
In summary, the Statistics View dialog provides both simple and detailed options to assess image statistics. It also offers more specialized capabilities such as changing the level of reported precision and exporting the statistics to ASCII files, PDF files, or PowerPoint presentations.
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