# Choosing CRS for metric distance calculations? [closed]

I have the following ArcPy script to calculate total length of Polylines in a Feature Class/Shapefile:

arcpy.Project_management('lines.shp', 'lines_projected.shp', arcpy.SpatialReference(TODO EPSG CODE))
arcpy.MakeFeatureLayer_management('lines_projected.shp', 'lines_layer')
geometries = arcpy.CopyFeatures_management('lines_layer', arcpy.Geometry())
print 'Number of lines: ' + str(len(geometries))
total_length = sum([g.length for g in geometries])
print 'Total length: ' + str(total_length/1000) + 'km'

What is the methodology to find appropriate Coordinate Reference System assuming the data associated with well known region (For example "Berlin" or "France")?

For example why a simple approach of choosing the appropriate UTM zone out of the 60 won't work

• Berlin and France are very different things and the sum of the length of lines could vary greatly depending on the scale at which they were captured, in addition to varying by coordinate reference. This question is quite broad, certainly broader than could be answered in a paragraph or three, which makes it a poor fit for our "Focused question / Best answer" model. Commented Mar 2, 2018 at 16:45
• According to this: qgistutorials.com/en/docs/calculating_line_lengths.html "Since we are interested in calculating length, let’s select an equidistance projection" & "North_America_Equidistant_Conic EPSG:102010 as the CRS" it seems there is a subset of EPSG that are appropriate for this task. Of course accuracy will be compromised, but the question is how much? Commented Mar 2, 2018 at 17:56
• You misunderstand the meaning of "equidistant projection." Except between special points or along special lines, so-called equidistant projections are among the worst possible choices for computing general distances within any region. Perhaps the best general technique for selecting a good projection and assessing its accuracy is to study the Tissot Indicatrices of candidate projections: see gis.stackexchange.com/questions/5068, gis.stackexchange.com/questions/185100, gis.stackexchange.com/questions/50692, and gis.stackexchange.com/questions/32115. Commented Mar 2, 2018 at 18:45
• @whuber Thanks, by the why a simple approach of choosing the appropriate UTM zone out of the 60 won't work? Commented Mar 2, 2018 at 20:30
• UTM certainly will work (and I don't see anything in this thread or its comments suggesting otherwise), provided (a) your accuracy requirements are consistent with UTM and (b) your region doesn't straddle two or more UTM zones. Commented Mar 2, 2018 at 20:37