Variation in terrain altitude affects both the resolution of your data and the reliability of image stitching. Understanding these concepts helps you produce high-quality map outputs when flying over quarries, hills or tall structures.
Ground sampling distance and resolution
Ground Sampling Distance (GSD) represents the distance between pixel centers measured on the ground. When your subject matter varies in altitude, the GSD changes because the distance between the drone and the surface is inconsistent.
If you take off at the bottom of a deep quarry or hill, the surrounding landscape may be higher than the drone's flight altitude. If you take off from a rim, the subject at the bottom consists of fewer pixels. This results in a lower resolution and a higher GSD in the final orthorectified image.
The Terrain Awareness feature allows the drone to maintain a consistent elevation above the ground as the terrain changes.
Overlap percentage
Stitching maps with high-quality elevation generally requires at least 8–9 images of each point on the ground. A small change in elevation can cause the overlap to drop below this requirement.
When flying from the base of hilly terrain or areas with tall buildings, trees or stockpiles, increase your takeoff overlap to compensate.
This diagram shows how rising terrain reduces image overlap as the drone maintains a constant altitude relative to the takeoff point.
If terrain changes affect your relative overlap, use the Terrain Awareness feature. For objects or trees that Terrain Awareness does not adjust for, use the Overlap Optimizer app to ensure your combination of overlap and flight altitude supports successful stitching.
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