title: How to use linear interpolation estimate current position between two Geo Coordinates?
body: <p>I have the following available:</p>
  
  <ul>
  <li>last reported lat,lon w/timestamp</li>
  <li>target lat,lon</li>
  <li>estimated time to target</li>
  <li>heading</li>
  </ul>
  
  <p>How can I interpolate an estimated position over time?</p>
  
  <p>I know that's enough to calculate the required average velocity for the remainder of the trip. Given a straight-line distance, it's pretty trivial. I know it has to do with vectors but I'm a bit rusty and thought it better to consult some experts.</p>
  
  <p>The reason I need this update rate is limited, so to show smooth animation I need to guess at the current position between updates.</p>
  
  <p>The target platform is a Google Maps application so I have available some basic functionality like a Geo-correct function for distance between two coordinates. Language is unimportant as I know many and can port or adapt any examples if needed. General solutions would be preferred however.</p>
  
  <hr>
  
  <p>Is this simply two independent vector calculations? </p>
  
  <pre>
  lat<sub>estimate</sub> = lat<sub>start</sub> + (Δ<sub>lat</sub> * P)
  lon<sub>estimate</sub> = lon<sub>start</sub> + (Δ<sub>lon</sub> * P)
  
  Where:
     t<sub>estimated</sub> = the reported estimated time to target
     t<sub>elapsed</sub> = time since last time estimate
     P = t<sub>elapsed</sub> / t<sub>estimated</sub>
     Δ<sub>lat</sub> = lat<sub>reported</sub> - lat<sub>target</sub>
     Δ<sub>lon</sub> = lon<sub>reported</sub> - lon<sub>target</sub>
  </pre>