"Orthorectified" (i.e. on a map) means both. It's corrected to a perpendicular to the observer view _and_ also corrected for scale distortion due to elevation (

"Orthorectified" (i.e. on a map) means both. It's corrected to a perpendicular to the observer view _and_ also corrected for scale distortion due to elevation (the top of a mountain is closer to the sensor and therefore appears larger than the valley beside it). For off-nadir imagery (i.e. not looking straight down) the view angle plays a larger role than the scale distortion part. With that said, raw elevation matters too, especially when the imagery is off-nadir. RPCs describe a view direction + lens distortion / etc. A completely flat surface at elevation will be in a different location than a completely flat surface at sea level. Things will also be a different _size_ as well depending on elevation. Think of the RPC as a vector looking a specific direction (at a single pixel level, that's kind of what it is, though each pixel gets a different one). You're placing a point on the map where that ray intersects the Earth's surface. It intersects at a different point depending on the elevation of the surface. Projected (aka orthorectified) data needs to correct all of that to be able to place imagery on a map accurately. If the airport is completely flat, then using a constant elevation is fine, but you still need to use that elevation and not assume it's at sea level. Otherwise it will be in the wrong place on a map and also a slightly incorrect size.