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I'm working on ArcMap 10 and need to get the lenghts (in meters) of the segments composing my line feature class. GCS of the line is WGS_1984. Extent of this data should be: Left 11.236520 dd, Right 11.247580 dd, Top 43.777520 dd and Bottom 43.771040 dd (I have no idea what "dd" means). I thought the correct way was to switch the Data Frame to PCS: WGS_1984_World_Mercator in order to get metric lengths with Calculate Geometry in a newly added field. I guess I was wrong since as I use the ruler to check if lengths are correct I see they are not (When I used the ruler with Data Frame set on GCS the lengths were correct). Am I doing it wrong or it is impossible to get the same lengths with GCS and PCS?

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    No, you should never use Mercator for length measurement. Please edit your question to specify the extent of your dataset (min & max X & Y) – Vince Jul 23 '15 at 11:00
  • Editing done (I specified the extent of the feature line in question) Which projection would suit me better? – Michele Cordini Jul 23 '15 at 11:37
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    "dd" = "decimal degrees". – whuber Jul 23 '15 at 14:43
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You can use Field Calculator to get highly accurate geodesic lengths of features in a feature class, regardless of the coordinate system. This means that Arc will find the shortest route across the surface of the ellipsoid when connecting vertices of your lines. Here's how to do it:

  1. Open the attribute table of your line feature class.

  2. Add a new double-precision floating point field to the attribute table.

  3. In this field, use field calculator with the Python parser and the the expression !Shape!.getLength("GEODESIC","METERS"). This assumes that your shape field is called Shape.

Field calculator screenshot

Besides METERS, there are a few other units you can use: CENTIMETERS, DECIMALDEGREES, DECIMETERS, FEET, INCHES, KILOMETERS, MILES, MILLIMETERS, NAUTICALMILES, POINTS, UNKNOWN, and YARDS.

Besides GEODESIC, there are a few other measurement types you can use: GREAT_ELLIPTIC, LOXODROME, and PRESERVE_SHAPE. But generally GEODESIC is the best in terms of accuracy.

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