What your megapixels aren’t telling you
The camera market is driven by delivering ever-increasing camera resolution. It is common to have 24, 36 and even 42MP cameras mounted on UAVs. Of course, this can deliver lots of detail in your photos, but more megapixels doesn’t deliver more accuracy, nor does it tell you the level of detail on the ground.
The Ground Sample Distance (GSD) is the dimension of what each pixel represents on the ground. This depends on the physical pixel size (pixel pitch) on the chip inside your camera, the focal length of the camera, and your height above ground.
For example, a 42MP camera with a large chip will produce a pixel size of 4.51µm within the camera. With a focal length of 28mm and a flying height of 75m, this gives a ground resolution (GSD) of approximately 1.2cm. However, a 12MP camera with a small chip will give you the same pixel size and GSD with the same focal length and flying height. So a 42MP camera will reduce the overall number of photos you need, but doesn’t increase your precision on the ground.
Similarly, a very standard 12MP camera with a 14mm focal length will produce varying ground resolution (GSD) depending on flying height. At 75m, your GSD is a nice 2.3cm. But increase the flying height to 275m and your GSD is up to 8.5cm. Again, the tradeoff is less photos for less detail.
Also consider that the finest pixel size on a camera’s chip is almost always coupled with the shortest focal length. These are the “micro” cameras like the sports and adventure cameras commonly deployed on UAVs. A 12MP camera can offer a very fine 1.5µm pixel size. But couple that with a mere 2.9mm focal length, and your GSD at 75m becomes a coarse 4cm, compared to the 4.5µm cameras above.
So, it is extremely important to know the details of the camera you hope to use, including the number of pixels, the physical size of the chip, and your focal length. Only then can you determine your GSD and appropriate flying height. Having an ultra-high resolution camera is only part of the equation.
The other balancing act to consider is that of the overall footprint of the image, which will determine how many photos you need to take for a given mapping area. A standard 12MP camera with focal length of 14mm captures an area of 1.1ha at 100m flying height. But the same camera with a 25mm lens will capture only 0.4ha. The 42MP camera with a similar 28mm lens captures a 1.1ha area. The larger the footprint of the photo, the fewer the photos (are needed?) to cover your area of interest. So it is important to consider the balance between your camera, focal length, and flying height to capture a manageable number of photos.