New answers tagged length
!shape.length3D! does work. I had to make sure that I selected the PYTHON_9.3 Expression type in the Calculate field Options dialog. (I'm running 10.2.)
According to the ArcGIS Help website for a related tool, this field should be in the units of the output coordinate system, which in your case is meters. When the output is a feature class in a file geodatabase or a personal geodatabase, the values in the Shape_Length field are always in the units of the output coordinate system specified by the ...
Late to the party, but with a hopefully useful contribution. Building on scw's answer using geopy, I wrote a small function that does the calculation for a shapely LineString object with arbitrarily many coordinates. It uses a pairs iterator from Stackoverflow. Main feature: the docstrings are much longer than the snippets. def line_length(line): ...
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