I think 3D length is not supported using the field calculator (at least not this syntax). The only listed defined keywords for the shape components are: type, extent, centroid, firstPoint, lastPoint, area, length, isMultipart, and partCount. I believe you are limited to using the LENGTH_3D option of the Add Z Information tool. Then you would have to apply a unit conversion from your base units to miles. No guarantee this will work, but since the 3D Analyst tool uses the term LENGTH_3D as the keyword and I don't see that variant listed above, you could try: !shape.length_3D@miles! Thanks Richard, but I think you were correct that 3D length is not supported using the field calculator tool in this context. I tried !shape.length_3D@miles! and received the same error. Thanks for your help though!
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