GPXIN-6 - Distance Calculations
===================================

[Issue](http://projects.bentasker.co.uk/jira_projects/browse/GPXIN-6.html)


Aim
-----

For each trackpoint, the class should calculate the distance travelled when
compared to the previous trackpoint. Ultimately, each track segment, track and
journey should have a new statistic added detailing the distance travelled.


Unit Of Measure
----------------

The distance travelled is easiest to calculate in Miles, but given that most GPS
systems generate a trackpoint at an interval of 5 seconds, it may be better to
record in a smaller unit (whether yards, feet or metres).


Method of Calculation
------------------------

Although it can introduce inaccuracies, the Greater Circle (haversine) method
seems most appropriate. Given the short interval between trackpoints, the effect
of inaccuracies in Greater Circle calculations (being a as-the-crow-flies
measure) should theoretically be minimal.


Assuming all degrees have been convered to radius, the formula can be expressed
as

  acos(sin($lat1) * sin($lat2) + cos($lat1) + cos($lat2) + cos($theta))

To convert to miles, we simply need to convert back to degrees and then

  $dist * 60 * 1.1515;


Implementation
----------------

The Distance calculation functionality is initially being implemented as
'experimental' functionality. The experimental key (see GPXIN-17) for this
functionality is _calcDistance_