What is the difference between geocorrected and orthorectified imagery?

Geocorrected imagery has been fitted to mapped features and does not include a digital terrain model (DTM) to compensate for ground height variations within the image. It has the advantage of being inexpensive to produce, and gives a good approximate fit to vector map data. But since it is not controlled to the National Grid or adjusted to compensate for the ground height variations, it does not inherently provide positionally or geometrically accurate imagery. Update is also a problem since the controls, being based upon mapped features rather than true location, are not necessarily stable.

Orthorectified imagery is created using National Grid control points and a DTM. This means that the distortions inherent in a flat photograph of a three-dimensional object (the earth's surface) are more completely and accurately corrected. An orthorectified image is positionally more accurate, and geometric fidelity (shape) is retained in all terrain, including hilly areas. Also, a consistent scale is maintained throughout the image (as per a map), and future imagery updates will be in the same position.

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