THL Toolbox > Places & Geography > Satellite Imagery
Contributor(s): David Germano, Kelly Johnson
Satellite imagery is very useful for examining regions to locate the coordinates of specific features, if the imagery is high enough resolution to see enough detail. In addition, the satellite imagery is useful to allow for visualization of the terrain.
The highest resolution free satellite imagery is Landsat 7 from NASA. The Landsat 7 sensors collect 15 meter resolution black and white imagery, but only 30 meter color scenes. Pansharpening is a software technique merging elements of the black and white magery with the lower resolution color imagery to increase visible detail. Pansharpened images are created after the fact by post-processing the Landsat imagery. The resulting resolution is a compromise between 15 and 30 meters per pixel.
Google Maps and Google Earths only uses Landsat imagery for its most distant views. As you zoom in, you are viewing commercial satellite imagery that Google has purchased. In both Google Maps and Google Earth you’ll see a copyright notice along the bottom of the screen identifying Google’s imagery source. This notice changes as you zoom in according to the imagery resolution provided by each vendor. You’ll only see NASA (i.e. Landsat imagery) in the copyright notice when zoomed out to a very distant vantage point. As you zoom in, NASA falls off the list and is replaced by higher-resolution imagery from Terra Metrics, Digital Globe, or another source.
For example, a Google Maps search for "Tibet" by default zooms to a TerraMetrics image. If you manually zoom out, NASA appears on the list. Zoom in far enough and TerraMetrics is eventually replaced by DigitalGlobe for the highest resolution. At a cost, other imagery vendors provide sub meter imagery.
Many people are familiar with Google Maps and Google Earth and may wonder why it is not simply sufficient to use them. Google Maps and Google Earth do not use the OpenGIS-standard WMS service API, but rather a proprietary API for interfacing with other data streams and projects. This means that specialized work is necessary to get Google resources to work with other GIS resources, rather than just implementing them directly as WMS as one might do for OpenGIS-based resources and services. Google also provides satellite imagery in a single Mercator projection called the “Google Projection”. (http://docs.codehaus.org/display/GEOTDOC/08+Google+Maps+Projection) This introduces distortions to a map that are very difficult to correct. This may not matter for a small place like Charlottesville, but can entail major problems for a large region like Asia. Google has to do this because otherwise they could never manage all the math of multiple projections.