How to draw a line on a fixed Z height

[MJSpitzer]

Seems simple enough but I can't figure it out. I need to draw a line at a fixed Z-height, rather than at 0. Basically, I want the elevation command from AutoCad. The reason is that I'm tracing over a terrain with a texture of a satellite image, but the line I'm tracing is below the terrain, and so, I can't see it. I can move it up afterwards, but that doesn't help me to trace.

 

Thanks.

[MJL Imaging]

I think I would create a grid at the desired Z height and make it the active grid. That should work.