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Here are graphs of data seen from four landmarks
As we can see from the first graph that some sites are replicated (root servers) and we ignore them in our results. It is also seen that some values for Mexico are aberrant for example min rtt >200 for a Mexican site from Florida. So it is a good idea not to include Mexico in this region. This region will comprise of Canada and US only.
The above graph shows how alpha varies with distance. Using a single alpha may fail for small RTTs (distances). This is probably since the min_RTT is dependent on router delays and where networks peer (so the routes may be very different from the Great Circle Distance (GCD) rather than global distance. !aplha_rtt.png|thumbnail!Looking at the graph above of Alpha vs min_RTT from SLAC it appears there are two regimes separated at ~8ms. Above 8ms alpha is ~ 50km/ms, below it is more variable closer to ~10km/ms