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Thus we need to find a suitable correlation between the delay measurements and the distance values. There are various factors affecting the RTT values including: propagation delays; router forwarding and queuing delays; unavailability of great circle path; presence of satellite connections etc. These make it impossible or at least difficult to reach to a single common factor which could be used in the delay to distance mapping. The paper on "An Investigation of Geographic mapping Techniques for Internet Hosts" claims there is considerable correlation. Examples of how distance correlates to RTT can be seen in the CAIDA RTT vs Distance results. Also see The Large-Scale Geography of Internet Round Trip Times .
We know that digital information travels in fiber at a speed of 0.6 times the speed of light in vacuum. Thus we can say that 1ms of RTT can equal roughly 100Km distance. But in order to tackle the additive distortions in RTT values due to the various delaying factors mentioned above, use of this 100Km/ms alpha value results in a large over estimation. As a result many geolocation location techniques, such as Octant, Constraint Based Geolocation, Topology Based Geolocation use much smaller values of 40-60 Km/ms for alpha.
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