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?