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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.)

  • I would suggest "Uncle" Google :-) – com Aug 17 '10 at 14:01
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In your example of calculating area, the answer is yes, you can do it in projected space---if the projection preserves area (i.e. is equal area). For an overview of projections and their properties, see:

http://egsc.usgs.gov/isb/pubs/MapProjections/projections.html

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  • Accepted not because of the link, but for the first sentence in your answer. It gave me an essential hint how to read and think about map projections and what their properties mean. – stakx Aug 18 '10 at 20:54
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One of the best intro texts I've seen is one previously put out by ESRI >

Understanding Map Projections

PDF can be downloaded here

It not only gives a good overview of projections, coordinate systems, Datums, transformations, etc: it also lists all of the projections supported in ArcGIS (8) and their best uses.

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IMO, John Snyder is the "Uncle" of Map Projections. Page 5 from this pdf excerpt from his "Map Projections - A Working Manual" might be worth reading.

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  • The information in Snyder's manual is great: over the years it has helped me answer many questions about projections and to understand specific projections much better. It is perhaps the most heavily used GIS reference on my shelves. – whuber Aug 20 '10 at 14:56
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wiki.gis.com has an extensive glossary, including a technical introduction to coordinate systems.

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I agree that Snyder is good (Flattening the Earth for example) but I find him fairly dry and theory based.

In many cases I use the web site of Natural Resources Canada. I met the guy who wrote the pages at a conference, and he was both enthusiastic and a mine of information on all things geodesy/spatial reference.

Check out http://www.geod.nrcan.gc.ca/index_e.php

It's based on the Canadian Spatial Reference System and references GPS a lot - but the background info applies to most cases. For example the section "GPS and the Geoid" had some great info on Geoids and Elipsoids, and the publications section has a ton of papers, articles and reports.

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I really like this page:

PROJECTION REFERENCE at radicalcartography

It has various projections shown visually, which gives you a quick idea of what makes them different.

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I would suggest ESRI ArcGIS Help files, topic Map Projections and Coordinate Systems. It is well written with good illustrations.

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