I'm working on a PHP application that needs to obtain data from a topographical map (different elevations denoted by different colors). I need to do two things:

Using this map (800x600), find a way to determine the exact pixel location of a particular city. For example, San Francisco is located at precisely 121x585.

Using the location from (1) above, I need to read the exact color at that location.

Note: The map provider does not provide location-based data, only a colored map. I suspect multiple libraries would be needed to map coordinates to locations on the map (via a ratio?) and then use OCR to read the color.

Are there any PHP libraries/tools that do this? How would you pull this off?