elevation matte

For example, the water line, where the side of a cliff/moutain/hill is washed off and exposes the mud/stone underneath. Or how snow appears at a certain altitude, or timber lines. You can use a measure distance tool to find the min max, and you can choose the coordinate system to find p. Usually world, or object.

point p = transform("object", P);
point maxp = point(0,max,0);
point minp = point(0,min,0);
point offsetp = point(0,offset,0);
result = (ycomp((p+offsetp)/(maxp-minp)));

Then connect to a spline.