Projections effect on GIS computations? Area and distance measurements in the GIS.

Discussion created by dmede on Apr 25, 2013
Latest reply on Apr 29, 2013 by dmede
I'm a cartographer as well as a GIS professional and I think I have a decent handle on projections and how they effect the various attributes of map display.

What I'm less clear on is the need to match a particular kind of projection with particular shape computations in the GIS.

When a researcher or student here gets help with a project they will often be told to match their reference system to their computation, so equal area for a project looking at area calculations or equidistant for a project involving distance measurements or buffers etc.

The implication is always that you will get incorrect results for area and distance if you used a mismatched projection (equal area for distances). I'm pretty sure this is not correct and that only the visual aspect of the map is at risk when mixing up the projection like this, but that calculations taken from two projections of the the same envelope but differing shape characteristics should be the same as long as the projection has real units like feet or meters.

Can any one here verify this for me? Or correct me if I'm wrong?