Figure 1.This is what it looks like:
I have also generated my ecoregion map of Utah using data from Omernik's ecoregion map for the entire U.S. All data were preprocessed to fit a UTM coordinate system.
The ecoregion map was segmented into coverages containing each of the ecoregions. This is not a necessary step, but will help in the discussion here.
Since I want to analyze the Normalized Difference Vegetation Index (NDVI) values for each ecoregion, I need to first remove water bodies from the dataset. Since water is not vegetation, and my objective is vegetation, I need to remove the water so it does not bias the results.
I generated a water mask by classifying the AVHRR data and identifying those clusters that identified water. I converted this classification into a binary mask (0 = water and 1 = land). Here is the mask:
Using the mask option under the Image Interpreter / Utilities menu I masked out all the water from my Utah AVHRR image:
To select the area of interest (AOI), I selected the File option from the viewer window, and chose Under the New option the AOI. I then went to File/Open and selected my vector file to overlay onto the AVHRR image. In this case I chose the Wasatch-Uinta ARC/INFO coverage (please note that Imagine cannot read ARC7.0 files, so if you are using ARC7.0 you have to export your coverage in ARC and then use the Import/Export utility in Imagine to load the data as an ARC6.0.2 coverage).
Here is the image with the Wasatch-Uinta coverage draped onto it.
Imagine as a default loads an ARC coverage as a vector layer and not as a polygon layer. To turn on polygon topology and make imagine understand that you are dealing with polygons, go to the Vector choice on the Viewer and choose Properties. The following dialog appears:
Click on the Polygon button to turn on the polygon topology. You may also want to modify the color and shade pattern by clicking on the Set... button. Hit the Apply button. The polygon covering the image will look like this:
Under the AOI selection in the Viewer window choose Tools to bring up the set of selection tools for AOI's.
Use the mouse to click inside the polygon to select it. If you have many polygons, as I do here, then on the subsequent polygons hold down the Shift button on the keyboard as you click in each polygon. This will allow you to select many polygons. Your screen should look like this:
Go to the AOI button on the Viewer and choose Add To AOI. The outside edge of your polygon should turn to a dashed line.
Use the mouse to select each AOI (again using the shift key on the keyboard to allow you to select multiple AOI's. This is what it should look like:
Once they are all selected, go to the AOI button on the Viewer and choose Group. This may not be a necessary step but I did it anyway. Your window will look like this:
Under Image Interpreter, Utilities there is a Subset button. This will allow you to subset an image based on a number of criteria.
Specify your input .img and your output .img files. At the bottom, choose the AOI button and in the subsequent viewer that pops up choose Viewer. That is tell the subset program that the AOI you want to use is in the existing viewer window. After you have done this choose the OK button and the file will be subset based on the AOI that you have chosen.
The file will look like the ones below. Be aware that if you subset using an AOI, Imagine will generate a file exactly the same size as the parent file with your data in it and the surrounding area blank. This tends to be a waste of space and is considered a logical bug in Imagine. ERDAS has stated that the next revision of Imagine will not behave this way.
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