The colors that go from light to dark represent the depth of the water in a meaningful way that helps us perceive depth. The USGS and NOAA have a series of color charts that have been adopted as part of the metadata used. From the cartography perspective, we like to adjust contours to whatever scale the map is presented. If it makes sense to do 3′ 6′ 9′ 12′ or whatever we choose, we control the representation. Using depth data and our GIS we can make them any way we’d like. No map tracings of any sort – ever.