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-   -   figuring distance from Lat/lon (http://www.sailnet.com/forums/seamanship-navigation/9552-figuring-distance-lat-lon.html)

 c172guy 04-12-2004 11:26 AM

figuring distance from Lat/lon

I checked a couple of books without an answer. My neighbor is a math teacher and showed me a way. But there''s got to be a better way. I just used trigonometry converting minutes into miles and solving for the hypoteneuse using Pythagorum''s theroem. But we all know the earth is a sphere so as the distance increases so does the error. Sort of transforming the lat/lon into vectors with the earths radius and calculating the distance is there a way to calculate the distance between two points using only lat/lon coordinates??? Please forgive the spelling.

 ccboston 04-12-2004 12:56 PM

figuring distance from Lat/lon

some discussion here
http://www.movable-type.co.uk/scripts/LatLong.html

 Fishboat 04-12-2004 02:13 PM

figuring distance from Lat/lon

http://jan.ucc.nau.edu/~cvm/latlongdist.html

 GordMay 04-12-2004 02:51 PM

figuring distance from Lat/lon

By definition, 1 minute of longitude at the equator is equal to 1 nautical mile.

For a chart - goto “The Earth according to WGS 84&quot;
http://home.online.no/~sigurdhu/Grid_1deg.htm

Regards,
Gord

 geohan 04-12-2004 04:55 PM

figuring distance from Lat/lon

c 172guy: My Garmin GPS will compute the Great Circle distance for me after I enter the lat and long for each point. Maybe yours will also. Check the owners manual for instructions.

If you have to do it the hard way, &quot;Dutton&quot; gives the Haversine Formula as:
hav dist = (hav DLo)(cos L1)(cos L2)+ hav(L1-L2)
DLo is the difference in longitude and L1-L2 is the difference in latitudes.
If you don''t have Haversine Tables, substitute
(1-cosX)/2 for hav X
Have Fun, George

 c172guy 04-13-2004 05:49 AM

figuring distance from Lat/lon

Thanks for the formulas. They seem to be the same as my math teacher friend provided. My GPS will calculate distances I just was curious about how to do it with a calculator. Actually my knot meter is not that acurate so that it''s error is probably my greatest source of error.
Thanks
Tom

 Sailmc 04-13-2004 06:00 AM

figuring distance from Lat/lon

Wouldn''t it be easier to just measure it on the chart?

 c172guy 04-14-2004 11:56 AM

figuring distance from Lat/lon

I wonder about distances from a chart. Only the latitude is accurate and if it''s a mercador projection a straight line is about 12% longer than a great circle route.
This came up because I didn''t have the correct chart handy and wanted to estimate time in route.
If I am actually on the boat I just enter the destination into the gps and it gives all the navigation data. But what if it dies??? I like to be able to dead reckon. I figure the more stuff I know the better.

 ekraemer 06-10-2004 04:02 PM

figuring distance from Lat/lon

This is a reliable page for Great Circle info.

http://williams.best.vwh.net/avform.htm

Thanks,
Ed

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