title: How to calculate Cross-Track error (GPS/Core Location)
body: <p>Does anyone know how to determine determine the "Cross-Track Error"? </p>
  
  <p>For those who are unfamiliar: You are driving along a line from Point "A" to point "B". When in transit, when you veer off that line, the distance from your current position to the line is the cross-track error.</p>
  
  <p>I have a simple algorithm now which works, using basic geometry with the latitude and longitude of the three points - the problem is that it does not take "great circle" calculations into account (i.e. actual meters-per-degree longitude varies depending on your latitude, and does not equal that of the latitude).</p>
  
  <p>In other words - if you know of a "great circle" formula for determining this, please let me know - but it is not a straight Cartesian geometry problem.</p>