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Using the CalculateField_management tool, one can specify the unit of measurement when calculating shape length, for example Shape.length@FEET. I was worried about doing this on data in an unprojected CRS. Related to that, Esri Help gives the following note:

If the data is stored in a geographic coordinate system and a linear unit (for example, feet) is supplied, the length calculation will be converted using a geodesic algorithm. --Esri

My question is, what does this note mean and what things should be considered when using this method to calculate a length on unprojected data?

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  • What would be the use case or benefit of calculating length on a unprojected layer?
    – artwork21
    Commented Jul 13, 2017 at 16:42
  • The benefit would be that the data would not have to be projected just to run this one calculation. If this calculation should not be done on an unprojected layer, then there would be no use case for this, hence my question.
    – Rex
    Commented Jul 13, 2017 at 20:46

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By geodesic algorithm, Esri means that the distance will be calculated on the ellipsoid/spheroid surface. Geodesic calculations will be slightly slower than 2D Cartesian calculations done in a projected coordinate reference system.

Disclosure: I am an Esri product engineer on a team that maintains the geodesic functionality.

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