It is unclear what area you are looking at but the stockpile measurements are designed to handle individual piles on a flat-ish surface. If you are measuring the whole area at one time it is bound to be off. If there is any curvature at all in your model that would certainly affect things. A stockpile the size of a house or slightly bigger would usually work fine. If you are looking at a pile that is the size of a football field it would likely be pretty accurate if the pile was tall. But if you are looking at a large area like a football field and hoping to resolve a few feet the method we use isn't going to be very accurate.