Let's suppose you need to calculate the volume of a stockpile of crushed gravel. If you were to measure or survey enough points on the surface of the stockpile, you could determine its volume. But it's a pretty big pile of gravel and you can't see the whole pile from one place. In fact, it's so big that, in order for you make measurements over the entire surface, you would have to move your survey equipment to several different places. Well, the first thing you would want to do would be to establish several points with known positions and elevations, or control points, around the base of the pile, each one in sight of the other. Not only do you want to measure accurately to these points, but also you may want to find them again later. To make sure you can find them, you will need to put a marker, such as a nail, in the ground at each point.

Once this is done, you can set up your survey instrument on any point and, then using the control points as a reference, make measurements, or 'survey in' all the surface points of the gravel stockpile that can been seen from that point. Next, you would move to another established point (this is where the nails come in handy…) where you can see more of the stockpile and then repeat the process. Once the entire surface has been measured, the volume can be calculated. Provided you've accurately surveyed the control points and the markers (nails in this case) are set solidly, you could then come back in 6 months and do a second survey to see how much gravel has been removed since the last survey. This time you would save time (and money) because your 'control network' is already established.

See how it works?