Hi there. This isn't so much a math question as it is a conceptual question. I can't seem to wrap my head around the need for coordinate transformations. *Why* do they need to be done? I think I really need a picture for this, so this might not be the right place to ask, but if you can describe the "why" rather than the "how" that I find in all the papers/books I've found, that would be great. Basically, the problem that I'm working on is a plane with a camera on a motorized gimbal. This gimbal needs to point to a spot on the ground and remain pointed to that spot as the plane flies through space. So, the first step is to convert the Lat/Lon into ECEF coordinates to get cartesian math. Get that part, no problem. My question is, why can't I just calculate the vector between these two points and call it a day? From everything I've read, I need to rotate the plane's coordinate system to align with the earth's coordinate system using matrix transformations. But then, if the plane is now "tipped" to be on the same "axes" as the target, and I calculate the vector, it still won't be correct because the plane isn't really tipped! Please forgive me if I'm being rather obtuse here, but any insight that you can give me would be much appreciated. thanks!