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Aerial Photos are not Maps.
More precisely, UNCORRECTED aerial photos are not maps; an uncorrected aerial photo provides only a single-point perspective of the surface of the Earth. It also contains a range of distortions, some of which include the imperfections in the camera lens/sensor, platform ephemeris conditions (altitude, position, the velocity of the camera or sensor and the timing of the data capture), the atmosphere and the topography of the terrain (relief dostortion).
To transform an aerial photo from an “image” into a “map”, these need to be corrected. This can be done in numerous ways that fall into one of these three categories:
- Rigorous Orthrectification
- Image-to-Map rectification
- Image-to-image coregistration
The terms you will hear most commonly among geospatial professionals are Orthorectification and Georectification. There are many ways to these processes, but understanding the difference between them is critically important.
Orthorectification is a very accurate method of correcting an image to align with realworld coordinates creating a true map. Orthorectification involves precisly measuring the exact location of the photo center and the exact angle of the camera when the image is acquired. Also required are the camera calibration parameters (to remove camera and lens induced distortions) and an elevation model (DEM) of the earth (to remove terrain induced distortions).
Georectification (sometimes incorrectly called Georeferencing) is a method of stretching and warping an image to align with a map or other spatial data in a Geographic Information System (GIS). Ground Control Points … are taken at visible physical features on the landscape. The resulting accuracy can never exceed the accuracy of the reference data (a 1:24000 USGS topo is only accurate to within 20 meters).
Image-to-map rectification and image-to-image coregistration methods are most commonly used by those who register their applied geospatial research with SAL. If you would like more information, or assistance with accessing geospatial data to transform and image you wish to work with, you can register your research and come and see us 🙂
To get an aerial photo from a “map” to “data”, you will need to undertake another series of analytical processes into consideration. To get you started, here are some questions to ponder:
- What type of data are you hoping to extract from your original image?
- How accurate was your rectification/registration?
- How many bands are available in the image and what software do you have available for working with them?
- How are you going to ground truth what you derive?
- Can you differentiate the GCP’s from your rectification/registration work from those that will be used for your data creation work?
Again, if your research is registered, you know where to find us if you want to have a chat!
