Even after some time working in this GIS field, I still don't have a solid understanding of spatial references, how they are projected, and most importantly, if you still need to worry about the fact that coordinates are geographic once they have been projected. In which cases should one pay attention when handling coordinates in a projected coordinate system?
For example, assume I am working in a projected coordinate system. I have a polygon whose vertices are now specified with X / Y coordinates. Let's say I want to manually calculate the polygon's area — could I now take any normal algorithm (for Cartesian/Euclidean coordinate spaces) for computing the polygon's area, or do I still have to account for the fact that these are geographic coordinates? What would I have to do to get an accurate result for my area computation?
It's questions such as the above example that I still don't understand. Any cues to introductory texts explaining such details are welcome.
(I was already considering looking at the PROJ.4 catrographic projection library and how it is used to deduce from that how to deal with projected coordinates, but that would've probably been too early.)