Point in polygon test is one of the most useful functions when processing Geospatial data. Unfortunately, there is no official built-in function from Mathematica yet. Here is an example of showing usefulness of this function.

We have some data on Greenland icesheet thickness in format of {lon,lat, thickness}.

Our goal is to make a map to show the thickness of icesheet.

First try with ListDensityPlot:

You can see it is not working well, we need to limit the plot region inside the boundary of Greenland. Here is the place we can use the point-in-polygon test. In this example, I use inPolyQ2 from the answer by Simon Woods:

Still not right? What's wrong? The trick is to increase MaxPlotPoints to 100 at least:

ListDensityPlot[data, PlotRange -> All,

ColorFunction -> (ColorData["Rainbow"][1 - #] &),

RegionFunction -> (inPolyQ2[greenland_ploygon[[1, 1]], #1, #2] &),

MaxPlotPoints -> 150, MeshFunctions -> {#3 &}, Mesh -> 10]

It looks like a real map now.