Spaced Out ::

I’ve been doing a little bit of training this week on using ArcGIS for simple image processing and, whilst ERDAS Imagine remains my staple (simply because I’m familiar with it), it’s quite beneficial to find new ways of accomplishing old tasks. Indeed I should acknowledge Paul Goldsmith, as many of the ideas here we drummed up between us (”Image processign the ArcGIS way”).

ArcGIS is a bit of a mixed bag when it comes to image processing. If you don’t have ArcToolbox (and therefore access to ARC/INFO) then there are quite a few limitations. None the less there is sufficient here to make it well worthwhile. We started out by downloading Landsat ETM+ bands 2, 3 and 4 from GLCF. These are predominantly GeoTIFFs with a single band per file. After downloading and uncompressing, they were converted to GRID files (and this is the best place to start as ArcGIS prefers to work this way).

If you want to display a false colour composite this is very quick and easily achieved through the image properties and symbology. Likewisem sub-setting an image is remarkably easy. Simply zoom your data frame to the extent of interest and export the layer of interest selecting the data frame extents as the bounding coordinates (and this has to be one of the simplest sub-setting routines I’ve come across).

The real power of working with imagery comes from using (the paid for) extension Spatial Analyst. This has a “Raster Calculator” option which allows you to perform a variety of operators on raster data. Importantly, this accesses the underlying ARC/INFO and uses it’s commands. It’s therefore well worthwhile in being familiar with commands in the GRID module of ARC/INFO and these can be viewed in the ArcDoc help system (NOT the help in ArcMap). Thus you can stack multiple layers in to a single file (MAKESTACK) or perform simple operations to calculate indices such as the NDVI. A word of caution; make sure you set the working directory (under Options in Spatial Analyst) to the same as the locaiton of your data.

ArcGIS is really very capable but much of the power is hidden and not obviously visible. With ArcGIS having such a large market share of the GIS arena, it’s well worthwhile knowing how to make use of software you already have.