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Your points will fall on the grid lines not inside the boxes created by the grid lines.

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.