ERDAS IMAGINE provides many tools designed to extract the necessary information from the images in a database.
This section briefly describes some basic image file techniques that may be useful for any application.
Subsetting and Mosaicking
Within ERDAS IMAGINE, there are options available to make additional image files from those acquired from satellite sensors. These options involve combining files, mosaicking, and subsetting.
ERDAS IMAGINE programs allow image data with an unlimited number of bands, but common satellite data types—Landsat and SPOT—have seven or fewer bands. Image files can be created with more than seven bands.
It may be useful to combine data from two different dates into one file. This is called multitemporal imagery. For example, a user may want to combine Landsat TM from one date with TM data from a later date, then perform a classification based on the combined data. This is particularly useful for change detection studies.
You can also incorporate elevation data into an existing image file as another band, or create new bands through various enhancement techniques.
To combine two or more image files, each file must be georeferenced to the same coordinate system, or to each other. See Rectification for information on georeferencing images.
Subsetting refers to breaking out a portion of a large file into one or more smaller files. Often, image files contain areas much larger than a particular study area. In these cases, reduce the size of the image file to include only the area of interest (AOI). This not only eliminates the extraneous data in the file, but it speeds up processing due to the smaller amount of data to process. This can be important when dealing with multiband data.
You can use the Subset option from ERDAS IMAGINE Raster tab to define a subset area. You can also define a subset area of an image to preview or import in many ERDAS IMAGINE Import options.
On the other hand, the study area in which you are interested may span several image files. In this case, it is necessary to combine the images to create one large file. This is called mosaicking.
To create a mosaicked image, use MosaicPro or Mosaic Express options.
Image enhancement is the process of making an image more interpretable for a particular application (Faust, 1989). Enhancement can make important features of raw, remotely sensed data and aerial photographs more interpretable to the human eye. Enhancement techniques are often used instead of classification for extracting useful information from images.
There are many enhancement techniques available. They range in complexity from a simple contrast stretch, where the original data file values are stretched to fit the range of the display device, to principal components analysis, where the number of image file bands can be reduced and new bands created to account for the most variance in the data.
See Enhancement for more information on enhancement techniques.
Image data are often used to create thematic files through multispectral classification. This entails using spectral pattern recognition to identify groups of pixels that represent a common characteristic of the scene, such as soil type or vegetation.
See Classification for a detailed explanation of classification procedures.