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In other words, you are having trouble understanding the concept of interpolation, correct? The mathematical interpolation of contours goes like this. Let's say we have two spot elevations A & B. A = 32.7 and B = 54.0. The distance between A & B = 50 feet. We want to know where our 40-foot contour would fall between spot elevations A & B. First obtain the total elevation difference. This is done by subtracting A from B. 54.0 minus 32.7 = 21.3. Next, we want the difference in elevation between our 40-contour interval and the nearest spot elevation which in this case is A or 32.7. That works out to be 7.3. Now we need to calculate the distance (let's call this "d") we need to go from spot elevation A to our 40-foot contour. That takes the form of: d/7.3=50/21.3 or d=7.3*50/21.3 = 7.3*2.347 = 17.13 or the distance, in decimal feet, to our 40-foot contour. Got all that? Good. Now go start interpolating.