ENVI Quick Start - Tutorial of ENVI Software - Completely GIS, GPS, and Remote Sensing Lecture Material - facegis.com
ENVI Quick Start


This quickstart tutorial is designed to give users trying ENVI for the first time a quick demo script. The following exercises briefly introduce you to ENVI's graphical user interface and basic capabilities. The exercises assume that you have downloaded and installed ENVI on your hard disk or have installed from CD-ROM. Please contact Research Systems directly for a fully functional CD-ROM or see the ENVI download instructions to obtain the software via the Internet.

Files Used in This Tutorial

You must have the ENVI TUTORIALS & DATA CD-ROM mounted on your system to access the files used by this tutorial, or copy the files to your disk.

The files used in this tutorial are contained in the CAN_TM subdirectory of the ENVIDATA directory on the ENVI TUTORIALS & DATA CD-ROM.

Required Files

The files listed below are required to run this exercise.

CAN_TMR.IMG	Boulder, CO, TM Data 
CAN_TMR.HDR	ENVI Header for Above
CAN_LST.EVF	List of EVF Files

Start ENVI

Before attempting to start the program, ensure that ENVI is properly installed as described in the installation guide.

  • To start ENVI in Unix, enter " envi " at the UNIX command line.
  • To start ENVI on Microsoft Windows systems, double-click on the ENVI icon or select ENVI from the Windows 95 or Windows NT Task bar.
  • To start ENVI on Macintosh systems, double-click on the ENVI icon, or drag and drop the ENVI icon onto the IDL applications icon.

Load a Grayscale Image

Open a Multispectral Landsat Thematic Mapper (TM) data file representing Canon City, Colorado, USA.

Open an Image File

To open an image file:

  1. Select File -> Open Image File from the ENVI Main menu.
  2. Navigate to the CAN_TM subdirectory of the ENVI tutorial data directory just as you would in any other application and select the file CAN_TMR.IMG from the list and click "Open".

Note that on some platforms you must hold the left mouse button down to display the submenus from the Main Menu.

The Available Bands List dialog will appear on your screen. This list allows you to select spectral bands for display and processing.

Note that you have the choice of loading either a grayscale or an RGB color image.

  1. Select "TM Band 4" in the dialog by clicking on the band name with the left mouse button in the Available Bands List..
  2. Click "Load Band" in the Available Bands List to load the image into a new display.

Band 4 will be loaded as a grayscale image

Familiarize Yourself with the Displays

When the image loads, an ENVI image display will appear on your screen. The display consists of a Image window, a Scroll window, and a Zoom window. These three windows are intimately linked; changes to one window will be mirrored in the others.

All windows can be resized by grabbing and dragging a window corner with the left mouse button.

  • Resize the Image window as large as possible. Note that the Scroll window disappears. Make the Image window smaller than the image data and the Scroll window reappears. Try resizing the Zoom window and note how the outlining box changes in the Image window.

The following describes the basic characteristics of the ENVI display group windows:

Image Window

The Image window shows a portion of the image at full resolution. The zoom control box (the highlighted box in the Image window) indicates the region that is displayed in the Zoom window.

  • To reposition the magnified region, position the mouse cursor in the zoom control box, hold down the left mouse button, and move the mouse. Alternately, you can reposition the cursor and click the middle mouse button to move the magnified area instantly.

Note: On Microsoft Windows systems with a two button mouse, click Ctrl-Left Mouse Button to emulate the middle mouse button.

Note: On Macintosh systems, click Option-Mouse Button to emulate the middle mouse button.

Scroll Window

The Scroll window displays the entire image at reduced resolution (subsampled). The subsampling factor is listed in parentheses in the window Title Bar at the top of the image. The highlighted box indicates the area shown at full resolution in the Image window.

  • Move the mouse cursor and click the middle mouse button in the Scroll Windows to reposition the displayed area, or drag with the left mouse button as described above.

Zoom Window

The Zoom window shows a portion of the image, magnified the number of times indicated by the number in parentheses in the Title Bar of the window. The zoom area is indicated by a highlighted box (the zoom control box) in the Image window.

  1. Click the right mouse button in the Zoom Window to zoom in (increase the magnification).
  2. Click the left mouse button in the Zoom Window to zoom out (decrease the magnification).
  3. Move the mouse cursor in the Zoom window and click the middle mouse button to reposition the magnified area by centering the zoomed area on the selected pixel.

Display Functions Menu

The Functions menu gives you access to many ENVI features that relate directly to the images in the display group.

  • Click the right mouse button in the Image window to toggle the Functions menu button on and off.

Note: On Macintosh systems, click Command-Mouse Button to simulate the right mouse button.

Apply a Contrast Stretch

By default, ENVI displays images with a 2% linear contrast stretch.

  1. To apply a different contrast stretch to the image, select Functions->Display Enhancements->Default (quick) Stretches to display a list of default stretching options for each of the windows in the display group.
  2. Select an item from the list to apply a contrast stretch to one of the windows.

Alternatively, you can define your contrast stretch interactively by selecting Functions->Display Enhancements->Interactive Stretching in the Image window.

Apply a Color Map

By default, ENVI displays images using a grayscale color table.

  1. To apply a pre-defined color table to the image, select Functions->Color Mapping->ENVI Color Tables to display the ENVI Color Tables dialog.
  2. Select a color table from the list at the bottom of the dialog to change the color mapping for the three windows in the display group.

Note: On 24-bit color systems you must click the Apply button at the bottom of the dialog to apply the selected color table to the image.

  1. Select Options->Reset Color Table to return the display group to the default grayscale color mapping.
  2. Select File->Cancel to dismiss the Color Tables dialog, keeping your changes.

Cycle Through all Bands of the Image (Animate)

You can display all the bands in an image sequentially, creating an animation.

  • Select Functions->Interactive Analysis->Animation and click "OK" in the Animation Input Parameters dialog.

Each of the bands six bands from the TM scene are loaded into a small animation window. Once all the bands are loaded, the images are displayed sequentially as a "movie".

  1. You can control the animation using the CD-Player-like controls in the upper left of the Animation Controls dialog, or the speed of the display by adjusting the "Animation Speed" slider in the upper right of the dialog.
  2. Click "Cancel" to end the animation.

Load a Color Composite (RGB) Image

ENVI allows simultaneous multiple grayscale and RGB Color displays.

  1. To load a color composite (RGB) image of the Canon City area, click on the Available Bands List. If you dismissed the Available Bands List during the previous exercises, you can recall it by selecting File->Available Bands List from the Main ENVI menu.
  2. Click on the "RGB Color" radio button in the Available Bands List. Red, Green, and Blue fields appear in the center of the dialog.
  3. Select Band 7, Band 4, and Band 1 sequentially from the list of bands at the top of the dialog by clicking on the band names.

The band names are automatically entered in the Red, Green, and Blue fields.

  • Click "Load RGB" to load the image into the Image window.

Scatter Plots and Regions of Interest

Scatter plots allow you to quickly compare the values in two spectral bands simultaneously. ENVI Scatterplots allow quick 2-band classification.

  • To display the distribution of pixel values between band one and band four of the image as a scatter plot, select Functions->Interactive Analysis->2-D Scatter Plots.

The Scatter Plot Band Choice dialog appears.

  1. Under "Choose Band X" select Band 1. Under "Choose Band Y" select Band 4. Click "OK" to create the scatter plot.
  2. Place the cursor in the Image window, then press and hold the left mouse button and move the cursor around in the window.

Notice that as you move the cursor, different pixels are highlighted in the scatter plot. This "dancing pixels" display highlights the 2-band pixel values found in a 10-pixel by 10-pixel region around the cursor.

  • Define a Region of Interest in the Scatter Plot by clicking the left mouse button several times in the Scatter Plot window to select points forming the vertices of a polygon, then clicking the right mouse button to close the polygon.

Pixels in the Image and Zoom windows whose values match the values contained in the selected region of the scatter plot are highlighted.

  1. To define a second class, select a second color from the Class menu of the Scatter Plot window and repeat the above steps.
  2. Select "Export Class" from the Options menu in the Scatter Plot window to export the most recently created classification. The Region of Interest Controls dialog appears.

By default, ENVI chooses "Scatter Plot Import", followed by the color of the region and number of points contained in the region as the name for the region of interest. This shows up in the "Regions of Interest Controls dialog".

  • Click "Cancel" to dismiss the dialog. The region definition is saved in memory for the duration of the ENVI session.

Classify an Image

ENVI provides two types of unsupervised classification and several types of supervised classification. The following is an example of one of the supervised classification methods.

  1. Select Classification->Supervised->Parallelepiped from the ENVI Main menu. When the Classification Input File dialog appears; select CAN_TMR.IMG and click "OK".
  2. When the Parallelepiped Parameters dialog appears, select the regions of interest you created from the "Select Classes from Regions" list at the left of the dialog.
  3. Select "Output Result to Memory" in the upper right corner of the dialog.
  4. Click on the small arrow button in the right-center of the Parallelepiped Parameters dialog to toggle off Rule Image generation, and then click "OK".

The classification function will calculate statistics and a progress window will appear during the classification. A new entry titled "Parallel (CAN_TMR.IMG): (M1)" is added to the Available Bands List.

  • To load the result of the classification into a new display group, select the new entry in the Available Bands List, select "Gray Scale" for the display, click "New" (to create a second display group), and click "Load Band".

A new display group is created, containing the classified image.

Dynamically Overlay One Image over Another

You can link two display groups, allowing comparison of the images directly by displaying one over the other.

  1. From either Image window, select Functions->Link->Link Displays. The Link Displays dialog appears. Click "OK" to link at the origin.
  2. In either Image window, position the cursor outside the zoom control box and hold down the left mouse button and move the cursor.

A portion of the other image is superimposed.

  1. You can change the size of the superimposed area by holding down the middle mouse button and dragging until the superimposed area is the desired size.
  2. Close the two image displays by clicking on the Cancel button in each Image window.

Overlay Vectors on Image and Get Vector Information

ENVI provides a full suite of vector viewing and analysis tools, including input of ArcView Shapefiles, vector editing, and vector querying.

  1. Redisplay the grayscale image by clicking on TM Band 4 in the Available Bands List, clicking on the Gray Scale radio button, and then on Load Band.
  2. Open a vector file by selecting File->Open Vector File->ENVI Vector File from the File menu in the ENVI main menu. Navigate to the CAN_TM directory and choose the file CAN_LST.EVF" which contain a list of vector files for this area.

The Available Vectors List dialog will appear listing the vectors corresponding to the CAN_TMR.IMG display.

  • Click on the"Select All Layers" near the bottom of the dialog to choose the vectors to plot. Click on the "Load Selected" button at the bottom of the Available Vectors List and the Load Vector Layer dialog appears. Click on "Display #1" at the top of the dialog, and then OK.

The vector layers are listed in the Display #1 Vector Parameters dialog.

  1. Click on Apply in the Display #1 Vector Parameters dialog to load the vectors onto the image, then choose Options->Vector Information to start an information dialog about the vectors.
  2. Click and drag with the left mouse button in the Main Image Display to track the currently selected vector layer and list basic information about the vectors. Click on another layer name in the Vector Parameters dialog and then in the Main Image Display to track a different layer.

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