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Georeferencing vs. Georectification vs. Geocoding

Mark Alonzo

I posted recently about confusion in the geospatial community around the terms DSM, DEM, and DTM.  Another set of terms that are used in different ways by different people are those that refer to the geo information associated with an image.  I thought it might be useful to outline the way that I use these terms. Be aware that the ENVI documentation, and other Exelis VIS employees, as well as plenty of people in the geospatial community, sometimes use these terms differently.  But if I were king of the world, I’d define them like this:

Georectify:  To take an image that has not been adjusted to be in a known coordinate system, and put it into a known coordinate system.  Usually this means taking an image that is in its original geometry, and putting it into a map projection.  There are different ways to do this.  Perhaps the most common way is to identify a set of points in the image for which the latitude and longitude or map coordinates are known, and use them to warp the image into a map projection.

Georeference:  To take an image that is already in a known coordinate system, and provide the information necessary for software to understand which coordinate system it is in.

Geocode: Same as georeference.

Geometrically Correct: Same as georectify.

Orthorectify: To take an image in its original geometry and very accurately adjust it so that it is in a known coordinate system, with distortions due to topographic variation corrected.  An orthorectified image has uniform scale throughout the image.  A DEM (and by this, I mean an image in which the pixel values represent the ground elevation above sea level) is required for true orthorectification.

How do you use the terms georeference, georectify, geocode, geometrically correct, and orthorectify?

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