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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

Fishboat 04-12-2004 02:13 PM

figuring distance from Lat/lon
How about this?

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"


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, "Dutton" 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.

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.


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