[OpenMap Users] Converting distance to pixels?

I'm trying to render an OMPoly (series of lines), with each line representing an area of interest. Each line has a given half-width (in KM or NM), and I'd like to represent that second-dimension with the line width. However, it's not obvious to me how to compute the number of pixels required for the line stroke given the (half-)width. I'm assuming it would be based on the scale and projection. (I'd be content with a fixed line width even though, accurately, it would fluctuate along certain routes of cylindrical projections.)

Thanks,
Shahak Nagiel

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