Accepted engineering design is for the slope to be measured perpendicular to a baseline, hence why Civil 3D and Land Desktop calculate it this way, and probably every other civil engineering software out there.
Take a road for instance your typical cross section will show a 3:1 slope, this is measured perpendicular to the centerline. If you wanted the distance between contours to be 3:1 then the 3:1 slope on your section would have to be an odd value (ie 3.27:1) depending on the slope of your road for each and every cross section.
Look at any of your corridors with a set slope and measure the contours and you will see that this is the case, not because it is wrong but because this is how it is done due to standard engineering practice.
When the project get staked out the surveyors will create a line to go off of, usually parallel to the centerline, and set slope stakes. The Contractor will set temporary stakes perpendicular to the baseline to get his 3:1 slope. I don't think any of us want to see slope stakes be adjusted to produce 3' between contours for a 3:1 slope, because it would cost a lot of money to accomplish.
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