Why is there no 'Length' option for Calculate Geometry Attributes?

RandyMcGregor_BMcD

I made a tool that includes the calculation of length. The input is always a line, and (as you see in the image below) other length options are available (3D, geodesic...). I got an error and it turned out that the simple 'Length' option is not available. It sometimes is, so what is it that causes it to not be an option. I can't see what's different about the input that causes this option to not be available. 

Thank you,

Randy McGregor

 

DanPatterson

Length .... shows up for me if the input dataset is in a projected coordinate system and it is not Z-enabled

The other options allow for other options given the input data spatial reference and data type.