One of the areas to consider when completing a drone survey is how large elevation changes can affect the quality of your map.
Choosing an image overlap ratio and altitude for the drone capture can be tricky if you have differing elevations.
If you choose the correct parameters for an area although the elevation lowers, the resolution of the map in that area will decrease.
If you choose the correct parameters for the lower area and then see a rise in elevation, the image overlap on the upper area could decrease so much that it's likely the software won't have the data to stitch it correctly. Less than ideal.
It's possible to run a couple of different flights at differing altitudes or choose an average altitude and overlap ratio for the area although it can take longer and may still produce sub-par results if not executed correctly.
This is where using a capture method with terrain awareness greatly assists in improving map quality. Using software integrated with the SRTM (look it up, very cool!) the drone is able to raise and lower with the topography of the area thus creating a consistent resolution and image overlap.
There are some drones in combination with flight software that work well-using terrain awareness, and others not so much.