Use GPS.Decimal Degrees as your site co-ordinates. The Visio Page will have
to have two references points (also in GPS decimal degrees). As each site is
placed on the map it's X, Y page cordinates will have to deduced via the
relationship its GPS co-ords have with the page GPS co-ords.
You will have to design a macro to automate the placement of the site upon
the page using the Pages reference in conjunction with the site co-ords.