I was dealing with multi-source map authoring with coordinate system conversions here in China years ago (~2004)... that&#x27;s before Google Maps came out.<p>Later I found that in China a given city&#x27;s satellite data for a will be off by some standard drift, usually pretty much effectively constant for a whole city area, and by co-relating a few known points throughout your area of interest across multiple sources&#x27; coordinate systems you can scale things well, even if you are remaining 100% projection agnostic (black box treatment).<p>I have never studied computational cartography, GIS and only have a basic self-taught understanding of matrices but didn&#x27;t find this all that difficult to hack up at the time.<p>We were using it to generate little point-of-interest area maps to help people find locations throughout the city on a sort of city-focused whats on style magazine operation. Maps were generated in multiple languages.<p>We were shut down by the publishing bureau but subsequently kept running anyway, apparently with no problems. The business continues today, though I have nothing to do with it.