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Trying to get a handle on how length and area are calculated in different scenarios in ArcGIS. I don't know why I can't find a answer on the feature class fields, but I can't find a precise answer, unless I'm not understanding something and I know there is some history. Can you help me fill in the question marks? Or tell me why I'm going about this all wrong ; )

GCS = Geographic coordinate system PCS = Projected coordinate system
All links are to 10.1 help docs --

  1. Feature Class shape_length and shape_area fields
    a. GCS - ?
    b. PCS - Using simple planar
    c. Is it always auto-updated, except for shapefiles? yes

  2. ArcMap Measurment Tool
    a. GCS - default geodesic, alternatives are Loxodrome and Great Elliptic, but not planar. Area calculation not available!
    b. PCS - default planar, alternatives Geodesic, Loxodrome, and Great Elliptic
    http://resources.arcgis.com/en/help/main/10.1/index.html#//00s500000022000000

  3. Attribute Table Calculator
    a. GCS - not available
    b. PCS - planar
    http://resources.arcgis.com/en/help/main/10.1/index.html#//005s00000027000000

  4. Calculate Field Tool (Data Management toolbox)
    a. GCS - geodesic linear, area available but questionable
    b. PCS - planar
    http://resources.arcgis.com/en/help/main/10.1/index.html#//00170000004m000000

  5. Buffering Tool (and other tools coming)
    a. GCS - geodesic
    b. PCS - planar or specify GCS output http://resources.arcgis.com/en/help/main/10.1/index.html#//000800000019000000

  6. Javascript API Clientside
    a. GCS - geodesic area and length functions
    b. PCS - can convert from web mercator to geographic (or use geometry service) http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi/namespace_geometry.htm

  7. Flex API Clientside
    a. GCS - geodesic area and length functions, "The length [or area] will be calculated using a custom cylindrical equal-area projection". This is not mentioned in the javascript api!!
    b. PCS - can convert from web mercator to geographic http://resources.arcgis.com/en/help/flex-api/apiref/com/esri/ags/utils/GeometryUtil.html

  8. ArcGIS Server REST API - Geometry Service
    a. GCS - geodesic
    b. PCS - planar
    http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi/geometryservice.htm

Another question, what exactly is a geodesic measurement? I thought it meant a 3D trig formula on a spheroid (haversine?). And is it too slow to use in calculating an area and that is why equal area projections are used?

Another question, when determining length and area -- is an equal area projection more accurate than a geodesic calculation using the same datum, spheroid? And briefly why?

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Concerning the last question, please see What is the most accurate coordinate system for calculating areas of polygons?. For the penultimate, because there exist equal area projections for ellipsoids, it is much easier to compute areas with such projections than to write ellipsoid-specific code. The situation is not as nice for computing distances, because no projection faithfully reproduces all distances: thus, direct spherical and ellipsoidal distance formulas are often implemented in good GISes. – whuber♦ Nov 15 '12 at 3:02
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1.b, 3.b and 4.b use the projected coordinate system, so therefore planar. 1.c is always auto-updated when you use a geodatabase (personal/file/SDE). – Jens Nov 15 '12 at 15:52
Removed some questions marks based on comments. Just found this post with some good info as well (more open source) gis.stackexchange.com/questions/711/… – awesomo Nov 15 '12 at 19:22
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I think it might be best to split your questions. That way you will get the best answers for each one. It would be easier to vote on the answers that way, too. – R.K. Nov 30 '12 at 12:36
FIRST CHANGE YOUR PROJECTION TO UTM WITH THIS... DATA MANAGEMENT TOOLS--> PROJECTION AND TRANSFORMATION-->FEATURE--> PROJECT. IT WILL CHANGE THE PROJECTION OF YOUR FEATURE AFTER THAT GO TO ATTRIBUTE TABLE OF THE TRANSFORMED PROJECTION FEATURE AND ADD FIELD "LENGTH" OR "AREA". RIGHT CLICK ON IT IT AND CLICK CALCULATE GEOMETRY. FROM HERE YOU CAN CALCULATE GEOMETRY OF LINE, POLYGON – Shirish Kumar Mar 20 at 14:56

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