Hans Vidkjer wrote:
> I need to explain to someone why they should store their lon/lat coords
> out to 6 decimal places. Is there a formula for calculating the increase
> in positional accuracy when you go from 4 to 6 decimal places?
A fairly close approximation of the size of the earth (and one
that is easily remembered) is the old French definition of 10,000
km from equator to pole (MapInfo's model says it's 10,007.54 km,
so it's close enough).
That gives you 111.111111 km to the degree (=10000/90) along a
great circle route. So one millionth of a degree (1.0E-06) is
about 0.111 m to 3 significant figures (it's the same using
MapInfo's model, too.)
Using the wonderful metric system you can do these in your head
and amaze people at parties (though they might soon ask you to go
soak your head.) Each 0.00001 degree is a little over a meter,
each 0.0001 degree is about 11m, 0.001 about 111m, and so on.
Pretty easy, eh?
Distances per degree along a line of latitude (which are not
great circles, except at the equator) are reduced by a factor of
the cosine of the latitude, so at 40N, for example, that 0.000001
deg distance in the east-west direction is 0.0851 m.
So 6 decimals is good for desktop mapping, 5 decimal precision is
about as good as you can get with a uncorrected GPS, and 4
decimal precision is for "broad brush, big picture" types of
visionaries who make maps with crayons. However for plotting maps
with sufficiently small scale, any precision can be appropriate.
- Bill Thoen
------------------------------------------------------------
GISnet, 1401 Walnut St., Suite C, Boulder, CO 80302
tel: 303-786-9961, fax: 303-443-4856
mailto:[EMAIL PROTECTED], http://www.ctmap.com/gisnet
------------------------------------------------------------
_______________________________________________________________________
List hosting provided by Directions Magazine | www.directionsmag.com |
To unsubscribe, send e-mail to [EMAIL PROTECTED] and
put "unsubscribe MapInfo-L" in the message body.
