Important Note: The distance calculator on this page is provided for informational purposes only. The calculations are approximate in nature
http://www.movable-type.co.uk/scripts/latlong.html
Distance
This script calculates great-circle distances between the two points – that is, the shortest distance over the earth’s surface – using the ‘Haversine’ formula.
It assumes a spherical earth, ignoring ellipsoidal effects – which is accurate enough* for most purposes… – giving an ‘as-the-crow-flies’ distance between the two points (ignoring any hills!).
The Haversine formula ‘remains particularly well-conditioned for numerical computation even at small distances’ – unlike calculations based on the spherical law of cosines. (It was published by R W Sinnott in Sky and Telescope, 1984, though has been known about for much longer; the ‘half-versed-sine’ is (1-cosθ)/2, or sin²(θ/2) – don’t ask, I’m not a mathematician).
In fact, when Sinnott devised the Haversine formula, computational precision was limited. Nowadays, JavaScript (and most modern computers) use IEEE 754 64-bit floating-point numbers, which provide 15 significant figures of precision. With this precision, the simple spherical law of cosines formula gives well-conditioned results down to distances as small as around 1 metre. In view of this it is probably worth, in most situations, using either the simpler law of cosines or the more accurate ellipsoidal Vincenty formula in preference to Haversine! (See notes below on the limitations in accuracy of the spherical model).
Since I tend to grab an area of the global or regional country maps I want, and then get the Boundary coords in MapCreator2, I then turn them into decimal and sort out the centre coords and then need to calc the Radius after that, which is where I found some serious 100% inaccuracies from the NOAA calculator especially in close. 29km NOAA > 74.18km Real.
Graeme
