I'm have a Level 1 Thermography certification. There are some basics of infrared concepts that apply regardless of application.

1. Infrared or thermal imaging cameras only read temperature patterns on a surface in relationship to other surfaces. If the surface of the object is blocked from view the thermal camera cannot detect it normally...there are exceptions but that has more to do with air movement and reflectivity.
2. The surface of the subject object can be affected by a number of outside factors and internal factors that either increase or decrease it's temperature
3. The camera reads the temperature range of an area and objects within that area....the temperature gradient change then shows based on the selected scale of the camera...often a pattern or object is easily recognized because of a wide temperature variation. Objects at the same temperature are very difficult to detect and distinguish.
4. If you are flying above a tree canopy then the density of the canopy will directly affect what the camera can detect.
5. The altitude and quality of readings will really depend on the image sensor. I have a middle grade handheld FLIR that I use for my construction consulting work on water intrusion and construction materials....I've picked up deer and other animals for hundreds of yards looking down gas or power lines. The further away the less recognition of the subject...becomes more of a temperature blob the further away you are. Conversely, the closer you are the images can be very detailed.