Software

Getting Started:

• The basics of Display
• The basics of Multi-spectral Display
  
• The basics of Profiles

Guides to Specific Tasks:

• Georegistrations
• SeaWiFs
• Classification

• Contrast Stretching
• Creating a Shaded Relief
• Dark Object Subtraction
• Georegistration
• Intercalibration
• Online Help
• Open and Display Image Files
• Topographic Correction

On the Image Window, select Functions > Display Enhancements > Interactive Stretching. Adjust the sliders to adjust the stretching being used. Be sure to note the source of the histogram for your stretching; to stretch the entire image consistently, you may want to select Histogram_Source > Scroll, which means the sliders work on the histogram for the entire image.

If, for some reason, you don't see the Functions button on your Image Window, click the right mouse button, which will bring that button back.

Creating a shaded relief image, at least for ENVI, is a DEM (Digital Elevation Model) related task. To accomplish it, select Utilities > Data-Specific Utilities > Digital Elevation > Topographic Modeling from the main ENVI menu. Selet the DEM file from which a shaded relief image will be generated. In the Topo Model Parameters window, enter the solar geometry for your image, which can most likely be garnered from header files or documentation that accompanied your image. For the purpose of making a shaded relief image, choose to only compute [3] Shaded Relief. Last, choose an output file name for your generated shaded-relief image.

Dark object subtraction -- the subtraction of the smallest value in a band from every other value in that band -- can be performed very easily in ENVI; all you have to do is select Utilities > General Purpose Utilities > Dark Subtract from the main ENVI menu. A caveat though: if your image has areas where this is no data, the pixels in these blank areas are set to zero by default. This causes trouble when trying to do dark object subtraction in a band which has all positive values where there is data -- ENVI sees the zero value in the blank area as a valid value in that band, and subsequently subtracts zero (not very useful). To work around this, either select Regions of Interest that encompass only the areas of the image where there is data (which becomes problematic, due to memory limitations, when working with very large images), or calculate the full statistics for your entire image, per band, and subtract the smallest meaning value with has a 1% frequency

Before a topographic correction can be done, an image must be registered to a Digital Elevation Model (DEM). In order to select points on a DEM which correspond to the same geographic points in an image, it is useful to illuminate the DEM with the same solar geometry as the image, which creates a shaded relief image.

Once you have your shaded relief image, display it and your image in separate displays. From the main ENVI menu, select Register > Select Ground Control Points > Image to Image. Choose the display of your shaded relief image as the Base Image, and the display of your image as the Warp Image. Center the two Zoom Windows on a common point in both images -- stream confluences, ridgetops, peaks, and small water bodies all make good Ground Control Points. Click on Add Point to select your point.

The more Ground Control Points you select, the better. Delete bad points to improve RMS error (bad points are those with high RMS values), and use Predict to aid in finding more points.

It is important to select points from all regions of the image, having as even a distribution of points as possible, encompassing both high and low elevations, especially for areas with high relief.

Intercalibration is necessary when you want to get two images to match values, usually via histogram matching. This is done frequently for images that were taken only a few days apart, and which have a significant area of overlap on which to train your intercalibration factors.

By far the easiest way of choosing areas from which to derive intercalibration factors is, when dealing with georeferenced images, to select the Regions Of Interest in one image, then use Functions > Region of Interest > Reconcile ROIs via Map from the Image Window. This uses the georeferencing information for your two (or more) images to find the same geographic Regions of Interest in both images.

Next, calculate the numerical statistics for each of your Regions of Interest by clicking on the Stats button on the Region of Interest Controls window for each Region. Note that to switch between the same Region of Interest in either image, you just have to change the Display number on the Region of Interest Controls. Also remember to calculate separate statistics for each band in your image, as each band needs to be intercalibrated separately.

For a simple, least-squares-fit calculation of intercalibration factors, I used Microsoft's Excel, as ENVI doesn't seem to have an easy least-squares-fit calculator. Plot the mean values of the Regions of Interest from your image to be calibrated along the X axis, and the mean values of the corresponding Regions of Interest from your base image along the Y axis. Once you've created a plot (I recommend a scatter plot), right click on one of the data points, and choose Add Trendline. Choose Linear under the Type tab, and Display equation on chart and Display R-squared value on chart under the Options tab. Voila! The y = mx + b equation displayed contains the gain (m) and offset (b) to be applied to each band in the image which is to be calibrated to your chosen reference image.

Choose Utilities > General Purpose Utilities > Apply Gain and Offset to apply your intercalibration factors to the data.

Online help can be accessed at any time by selecting System > ENVI Online Help from the main ENVI menu.

Select File > Available Bands List from the main ENVI menu. The Available Bands List Window shows all files currently open. From the Available Bands List Window, select File > Open Image File to choose which images files are to be opened for displaying. Next, on the Available Bands List Window, choose whether to display your image as Gray Scale or as RGB (Red - Green - Blue) Color. Click on which bands, from the list, to display for each color (or just one if you chose Gray Scale)

The Scroll Window shows the entire extent of the image. A small red box on the Scroll Window corresponds to the portion of the image visible in the Image Window. A red box in the Image Window shows the portion of the image visible in the Zoom Window.

Assuming that the surface materials in your image have Lambertian properties, you can normalize the effect of changes in the illumination angle by applying the following transformation:

normalized reflectance = (calibrated reflectance) *
(cosine of sun zenith angle) / (cosine of solar incidence angle)
The result, the normalized reflectance, is as if the surface were completely horizontal.

To do this in ENVI, use ENVI's Band Math function, which performs calculations on multiple bands on a pixel-by-pixel basis.

First, calculate your sun zenith angle, which is a constant. Assuming you are given a sun elevation in degrees, convert it to radians by multiplying the value in degrees by ( pi / 180 ). Next, in radians, compute the sun zenith angle by:

sun zenith angle = ( pi / 2 ) - (sun elevation angle, in radians)
Select Basic Tools > Band Math from the main ENVI menu. Under the Enter An Expression text area, enter:

b1 * cos( your sun zenith angle, in radians) / b2
and select OK. On the Variable/Bands Pairings Window, select b1 - [undefined] and click Map Variable to Input File. Choose your image file. Next, select b2 - [undefined] and Map Variable to Input File. Choose your shaded relief file. Last, choose an output file name for your topographically corrected file.