I am making an Android app that has bathymetric lake data on a google map. I am new to GIS work. I get this info from the state of nebraska as an already formatted kml file.

In the file they simply have LineStrings, but I have manually converted them to Polygons. I want a visual that clearly shows depth of the lake at a glance.

The simple solution I came up with uses a blue color for the polygon fill and an alpha of "11". I love this approach because it still shows map features through my polygons while still showing depth well. Each successive polygon shows as a darker and darker fill conveying depth quite well. Alas, the problem is that sometimes some of those polygons are actually shallower and not deeper.

For instance, below are two screenshots. The first is the way it should look where there is a small underwater "island" (I manually edited the file). The second screenshot shows what it looks like if I simply use the default alpha and color for each polygon.

enter image description here Correct way

enter image description here Incorrect way

I cannot edit each kml file manually as there will eventually be thousands of lakes with more added over time.

Is there a known good way of solving this type of problem?

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