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It is [[Best map editing practice]] to align the [[glossary|geometry]] of the road with the aerial image. Routes created by GPS can be inaccurate. By matching the road tightly to the image of the road it is more likely that a client device will correctly lock to that road.
But aerial images are taken by satellite from great heights and sometimes extreme angles. Very clever algorithms are used to correct distortions, but these adjustments cannot be perfect and in some areas the aerial images are not accurate. [http://waze.com/editor/?zoom=5&lat=59.36954&lon=24.65771&layers=BTFTFFFTFFFFFFFFFFTTTT Example]. In such cases the GPS tracks are a better guide to the actual location of the roads, and it would be wrong to align the roads to the aerial image.