Ground sampling distance (GSD) is the measurement of the pixel size in real life units. What this means is that as the GSD increases, the spatial resolution of the image will decrease and objects will become less visible, making it harder, or impossible to trace them. The GSD is determined by the altitude of the flight and resolution of the camera.
To use the AirWorks Web App the GSD in the orthomosaic must be less than 0.1 feet. For best results, AirWorks recommends the GSD to be less than 0.05 feet.
(interactive display showing GSD vs Camera vs Altitude)