Explore Pixel Values

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Lesson 1 of 1

Explore Pixel Values

Knowing the data type and expected values of your imagery can help determine whether or not you can run certain analytics. Examples of data types include byte, integer, floating point, and complex.

This exercise demonstrates how to check the data type and pixel values of an image.

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 figure below shows the image at its full extent.

4

Look at the Property Sheet in the lower-right corner of the ENVI interface. The Dimensions field indicates that this image has 1,024 columns and 1,024 rows. It has four bands in the visible to near-infrared (VNIR) wavelength range. Its interleave is band sequential (BSQ). "Uint" means that the data type is unsigned integer.

For integer data, you should know the number of bytes per image pixel and whether or not the integer is signed (can have negative values) or unsigned (can only have positive values). If you apply a mathematical expression to unsigned integer data that yields a value greater than 65535, it will result in an overflow error.

The pixels in this image represent digital numbers (DNs). DN values indicate varying levels of brightness in an image. Similar to photos taken with a cell phone camera, each pixel has a brightness value in one or more bands. QuickBird images are collected with a radiometric resolution of 11 bits, which means the DNs can have 211 (2,048) unique values.

Refer to the View Image Statistics quick guide to learn about data ranges, minimum and maximum values, and other image statistics.

Use the Status Bar to View Pixel Values

1

Locate the Status bar at the bottom of the ENVI interface. It contains three segments. The left segment indicates the current geographic coordinates as you move the cursor around the image. (You must first click inside of the image to display the coordinates.) The middle segment indicates the projection type: UTM Zone 13 North, with a WGS-84 datum.

2

Right-click in the middle segment of the Status bar and select Raster Data Values. 2. 3

Move the cursor around the image and note the Data values in the Status bar. The first number is the DN value of the current pixel location in the red band. The second number is the DN value in the green band. The third number is the DN value in the blue band.

You might want to know more information about pixels besides their DN values. You can use the Cursor Value dialog for this.

Use the Cursor Value Dialog to Get Pixel Information

1

Click the Cursor Value button in the Toolbar.

The Cursor Value dialog reports information about the pixel underneath the center of the red crosshairs. This intersection is is called a probe. Since the QuickBird image is georeferenced to a standard map projection, the Cursor Value dialog reports position and map information.

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Geo: Latitude/longitude coordinates * •

Map: Northing and easting coordinates * •

MGRS: Military Grid Reference System coordinates * •

Proj: Projection name * •

File: X and Y image coordinates, where the upper-left pixel in the image is the origin of the coordinate system * •

Data: The data values of the pixel in the three bands that are displayed

The On demand button is activated in the Cursor Value dialog. This means that you can only retrieve pixel information by clicking in the image to move the red crosshairs to that location.

2

Click in a different part of the image. The crosshairs center over that location, and the Cursor Value dialog reports information for the corresponding pixel.

Let's say you want to record data retrieved from different pixels. While On demand updates are enabled, you can copy the data for the current pixel to the system clipboard, then paste it into an application of your choice.

3

In the Cursor Value toolbar, click the Copy probe text drop-down button and select Copy All.

4

Start a text-editing application; for example, Notepad on a Windows system. 2. 5

Use the Ctrl+V keyboard shortcut or another method to paste the pixel information into a new document.

You can also copy and paste cursor value information into text annotations in ENVI. See the Create Annotations quick guide for more information about text annotations.

In addition to viewing individual pixel values, you can calculate statistics for the entire image. This is highly recommended before using analytical tools to process your imagery. See the View Image Statistics quick guide for details.

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