[Proj] Re: Distance measured in Mercator projection

Christopher Barker Chris.Barker at noaa.gov
Wed Jul 9 17:53:15 EDT 2008

strebe at aol.com wrote:
> Not so. The Mercator is conformal. The scale factor is the same in all 
> directions at any particular point.

yes, but it does change with latitude, so if you are measuring between 
two points that are very different (by some definition of very) in 
latitude, then the scale varies over that distance. If you use the 
latitude of the center of the line, you'll get pretty close, though.

If you're measuring really long distances, you're better off using 
spherical (or ellipsoidal) geometry.

Also note that a straight line on a Mercator projection is not a great 
circle so, for long distances, you need to be clear about what you mean 
by distance!

And as a note, the Mercator projection is pretty handy for 21st century 
navigators, too!


Christopher Barker, Ph.D.

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chris.Barker at noaa.gov

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