How high is the ground at the edge between huge water bodies and dry land?

We want to understand how the edge between huge water bodies and dry land moves. Sometimes this edge moves when the huge water body becomes higher. This happens more and more as the large rock we're living on is getting hotter. But sometimes the edge moves because the tiny rocks that form the ground get moved around by the wind or the water.

To find out how much the land-water-edge is moved by the large water or by the tiny rocks, we need to know how high the ground near the edge is. Our idea is to use flying space boxes. Some space boxes see how high the huge water body is. Other space boxes see where the land-water-edge is. They have been seeing all that for the past 30 years. Now we put this together: One edge and one how-high-is-the-water seen at the same point in time go together! When we know how high the land-water-edge was, then we also know how high the ground was. We repeat this for all edges so that we get a bigger picture.

But now all the how-high-is-the-ground points cover a long time! It would be much better if we would know how high the ground is for all edges, but only at one point in time. How good that someone thought very long and came up with an idea on how to put together all the edges in a cool way. This idea uses a lot of the playing-around-with-numbers that we did in school. It helps us to find out how high the ground was near the land-water-edge at one point in time.