Satsig.net web: 
Line of sight, azimuth angle bearing and great circle distance calculator 
Overwrite the default numbers in the bright blue boxes below with the latitude or longitude of your location. Input positive numbers only (e.g. 0 to 360 deg).
If you have a GPS receiver the display may be Longitude 117 degrees and 29.842 minutes. In this case use the first two blue boxes above and put figure 0 (i.e. zero) in the seconds box. If you have a GPS receiver display like 117 degrees and 29 minutes and 50.5 seconds then put the numbers in all three blue boxes above in the same manner as shown with the default numbers.
On many GPS receivers it is possible to switch the latitudelongitude display settings format. Investigate your GPS options and if you can get the display into decimal degrees like 117.4974 degrees, then you do not need to use this calculator.
Test by clicking to calculate the result with the default numbers. The answer should be 117.4974 decimal degrees. i.e. just under 117 and a half degrees. Three alternative answers are provided, the first to 4 decimal places (accuracy 11m distance on the equator), the third to 6 decimal places (accuracy 11cm distance on the equator). If the calculator does not work, you need to enable javascript in your browser.
Accuracy: If you notice significant discrepancies between your lat/long location as shown using Google maps compared with GPS readout or physical paper maps then you should check that the coordinate systems used are based on the same datum. The datum called WGS84 is common, but on many older paper maps different local datums are used. Your GPS receiver configuration may allow one of many alternative datums to be selected.
A map datum comprises a physical location with an associated pair of lat/long figures, plus a mathematical model of the shape of the earth. The physical location might be a trig point on a convenient mountain top. The assumed shape of the earth, equatorial diameter, polar diameter etc may vary slightly according to the assumptions made. Remote islands tend to have their own datums referenced to a local mountain top.
Any problems or comments, or reports of copyright infringement, please email me Eric Johnston This calculator is copyright (c) 2005. 2013 Satellite Signals Ltd Original : 16 March 2005. Amended 5 October 2006: If you input a negative number for the whole degrees it assumes you are West Longitude and considers the minutes and seconds parts (input without minus signs) as taking you further to the west. The output decimal number is displayed negative, indicating degrees west. Higher accuracy 5 decimal places output added. Accuracy assumes the earth circumference around the equator is 40075.16km. So 1 deg = 1000 x 40075.16 / 360 = 111319.88889 m So 0.0001 deg = 11m and 0.00001 deg = 1m. Longitude accuracy improves nearer the poles as the longitude lines are closer together. Latitude error is more or less the same everywhere, but if you are pernickety you might want to work out the error based in a polar earths circumference of 40008 km. Amended 1 November 2010: If you have a longitude greater than 180 deg then that refers to locations in the range 0 to 180 degrees WEST. The set of three boxes on the right above are the result of subtracting the longitude for 360 deg. Amended 2 April 2013. Reverse process added. Amended 12 Sept 2013. Details about accuracy, datums and use of GPS added. Amended 27th Sept 2013. In view of the fact that Google maps use negative numbers for degrees West and negative numbers for degrees South I have added the facility in the lower half, reverse calculation, to insert negative numbers. When you do this the calculator gives you an alert that the result probably refers to West longitude or South latitude with figures as far as 90 deg and to West longitude in respect of numbers beyond 90 deg. Amended for small screen and mobile devices 25th July 2015
