I know you should never calculate area using a geographic coordinate system. What is the reason for that though? Why is it bad to calculate area using NAD 1983 for instance? I looked online, but could not find the reason for why you don't want to do that.
Thank you, Luke Kaim
Thank you Luke Kaim (SIE) Lucas.Kaim@maine.edu (914)263-7866 �??Everything should be made as simple as possible, but no simpler�?� (Albert Einstein).
Because ArcGIS doesn't yet have a robust algorithm to calculate areas on an ellipsoid (spheroid) surface. It's actually pretty difficult, with most published solutions using various approximate methods. So, it's easier (and still gives reasonable results) to project data to a equal area projection and do the calculations on a plane. The results are very sensitive to how big the area is, and consequently, how dense the vertices are.
Thank you so much for your fast reply. Is there any literature that would explain this in more detail and why it is hard to calculate? I am going to get your book now, but is there any other helpful reading material that you would suggest?
The simple answer is that a system of 'geographic' coordinates uses angles (latitiude and longitude) as if they were measured distances, and assumes (among other things) that one unit in one place is the same as one unit everywhere else.
You want to measure things in units that are meaningful: square degrees are simply not meaningful units of area.
Every means of flattening the earth introduces distortions; one issue in chosing a projection is what particular aspect of the earth you want to preserve, or at least distort the least. An equal area projection preserves area, at the expense of sometimes severe distortion of direction and distance.
I'm going to focus on online resources. If you're talking about Understanding Map Projections by Melita Kennedy and Steve Kopp, almost everything in there is part of the online ArcGIS desktop documentation (in v10, professional section, guide books, map projections).