I have a series of zip codes with latitude and longitude coordinates in Excel
that represent company stores.

I'd like to overlay those on a map of the USA, using those coordinates as
the centerpoints of semi-transparent circle autoshapes. First, this allows me
to dynamically (VBA) change the diameter of the circles, to give a visual
representation of coverage based on different people's ideas of what the
maximum coverage distance should be, and second, it shows where there is
overlap between circles.

Right now, I'm thinking of just importing a map image onto a worksheet,
figuring out the basic coodinates on the map edges and translating them into
worksheet coordinates, and calculating each center point based on the
relative difference in map coordinates. That isn't as precise as I'd like,
and I was wondering if anyone else has done something similar, and what
approach you used.

Thank you,
Keith