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I am a beginner at ArcGIS and have a technical question.

I need to assign a projected coordinate system to my boundary/polygon data in order to calculate square miles. There are many to choose from. Which should I be using? Here are my geographic coordinate system details:

Angular Unit: Degree (0.017453292519943295) Prime Meridian: Greenwich (0.000000000000000000) Datum: D_North_American_1983 Spheroid: GRS_1980 Semimajor Axis: 6378137.000000000000000000 Semiminor Axis: 6356752.314140356100000000 Inverse Flattening: 298.257222101000020000

Thanks in advance for any consideration.

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We need to know which geographic region your data is covering. – underdark May 31 '11 at 14:16
There are other factors that might impact your choice in spatial reference. For example, if you also intend to serve out your data on the web, you may want to store data using web mercator, and calculate areas using an equal area projection for the dataframe this link posted by Mapperz. Often performance is not a big deal in one-time calculations, but is important for web map display, so storing your data in equal-area projection and projecting on the fly into web mercator might not be a good idea. – Kirk Kuykendall May 31 '11 at 14:49

Calculating area, length, and other geometric properties (Short-cut CTRL+SHIFT+G)

"It is recommended that an equal-area projection be used when calculating areas."

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An alternative method that will work for the purpose.
1. insert your data into an arcmap document,
2. right click on the TOC (table of contents) layers and select properties,
3. Select the coordinate systems tab and assign CRS as suggested in other posts here.
4. open the attribute table and add miles2 field if you don't already have one defined.
5. Right click on that field in the attribute table window and select calculate geometry.
6. Next select the use crs of data frame radio button, and the units you would like.

calculate geometry dialog

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Thank you! Exactly the answer I was looking for. Thanks, again. – megan May 31 '11 at 16:36
@megan please accept this as the answer to your question, thanks – Mapperz Jun 1 '11 at 2:45

Selecting a coordinate system is all about the purpose of what the map will be used for. ESRI has a brief overview of some different map purpose considerations when choosing a coordinate system.

Coordinate systems for map display

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I need to calculate square miles of an area. The projected coordinating system I was using is inaccuately calculating the area. Any ideas? – megan Jun 7 '11 at 14:46

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