Ortho-rectified imagery is sometimes the most overlooked spatial component when it comes to obtaining and processing satellite imagery.
A client recently approached us to process some GeoEye-1 imagery. The client wanted to use the data for a number of things:
a) assist with geological mapping,
b) overlaying of assay results and,
c) potential access road planning.
Obviously a dataset with exceptional positional accuracy was required; however we noticed that the client was only after ‘geo-corrected’ imagery. We called the client and explained the situation and they asked us: ‘so what’s the difference?’ This is a good question which many people ask.
Geo-corrected imagery has been processed to spatially fit previously mapped features which did not include a DEM to offset terrain variations. These images are traditionally one-step above Level 1 imagery acquired by satellites and are relatively cheap to produce. They give the user a good fit, however geo-corrected images are not geometrically correct. Thus, they should not be used for controlled ground truthing exercises such as those associated with precision mapping such as for assay analysis, let alone access road planning.
On the other hand, ortho-rectified imagery is produced by using a DEM, and as a result, the image is corrected to meet higher conformity standards. This produces a true and undistorted image of the location given a better indication of terrain conditions. Moreover, a consistent scale is applied throughout the image area making future updates much easier and more precise. Ortho-rectified imagery is more expensive to produce and requires a good quality DEM, however if the imagery is to be used for applications needing more accurate results, then they are essential.
Below is a before and after ortho-rectified image of the same area captured by GeoEye-1. The results are clearly evident.