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 · Accepted solution]

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.

[RandyMcGregor_BMcD]

Facepalm. The input line was in decimal degrees. Projected it to State Plane, and 'Length' is back. Thank you!