Source: Ran a network accelerator with more than 50 million unique players for 5 years. Bad network performance is much more pervasive than you think. https://networknext.com
The reason you use the maps in the matchmaker is so you can look up the latency from client to datacenter in constant time and you don't have to waste any time on the client pinging ping servers to get the results.
Plus, it costs money to run pings through the network accelerator, and they always (from experience) converge to the latency map anyway.
And you're about to send 50 packets per second for an hour during the game. Is sending one packet per datacenter really a cost worth mentioning?
Given lat,long is a guess and even if it is accurate, doesn't correspond very well to network latency, why wouldn't you use something like the source /24 or /48 rather than lat,lon. You don't get a pretty picture that way, I guess.