topic Re: Projection area question in Geoprocessing Questions

Projection area question

Anonymous User — Wed, 30 Nov 2011 18:55:03 GMT

Original User: luke.kaim

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).

Re: Projection area question

Anonymous User — Wed, 30 Nov 2011 19:23:17 GMT

Original User: mkennedy

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.

Melita

Re: Projection area question

Anonymous User — Wed, 30 Nov 2011 20:03:08 GMT

Original User: luke.kaim

Hi Melita,

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?

Thank you,
Luke Kaim

Thank you
Luke Kaim (SIE)
Lucas.Kaim@maine.edu
(914)263-7866

Re: Projection area question

markdenil — Mon, 05 Dec 2011 13:54:06 GMT

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.

Re: Projection area question

Anonymous User — Mon, 05 Dec 2011 20:37:39 GMT

Original User: mkennedy

Hi Luke,

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).

Geographer's Craft has sections on coordinate systems and geodetic datums.

Much more than you probably want, but the "bible" of map projections for someone who wants to see all the math: John P. Snyder's Map Projections: A Working Manual.

Any book on map projections or even includes a chapter on them will talk about what feature characteristics are maintained by different map projections and what characteristics are distorted.

Carlos Furuti has some good information and pictures.

Melita