map algorithm question

I am working on an application which draws maps on a screen. This is done with using sets of x,y coordinates which are made into polygons and then drawn in a paintComponent. The user now wants to add elevation (a "z" component). The question at hand is how to quickly index into a data structure to find an elevation point based on the mouse location (x,y) of the screen.

Which is the best data structure to use to look up an x,y point in order to get a z point (elevation). If the exact x,y is not present then return the closest.

I was hoping that someone might either relate how they might have done something similar or a pointer to reference. Thanks - Lou
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Relevant Pages

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    ... I am working on an application which draws maps on a screen. ... This is done with using sets of x,y coordinates which are made into polygons and then drawn in a paintComponent. ... The question at hand is how to quickly index into a data structure to find an elevation point based on the mouse location of the screen. ... Although not exactly a GUI question I was hoping that someone might either relate how they might have done something similar or a pointer to reference. ...
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