How do surveyors measure lots on slope?

[Chris E.]

I own a couple of lots that Im curious on how deep they are. I know its 140ft deep but there is a very steep incline that runs along the entire property. Do surveyors remove the slope mathematically to find the 140 ft depth or do the basically run a measuring wheel down the hill for 140ft? (i know its done with more precise equipment than just a wheel) and I know I should get a survey obviously but Im just trying to get a general Idea of where I can expect the lot line to be. 

[Wayne Brooks]

It is Not measured along the slope, it is measured on a level horizontal plane.