[postgis-users] Buffer/Nautical Miles

[Chris Hermansen]

>From Wikipedia http://en.wikipedia.org/wiki/Nautical_mile

    A nautical mile or sea mile is a unit of length. It corresponds
    approximately to one minute of latitude along any meridian. It is a
    non-SI unit used especially by navigators in the shipping and
    aviation industries.[1] It is commonly used in international law and
    treaties, especially regarding the limits of territorial waters. It
    developed from the geographical mile.
    ...
    The international standard definition is: 1 nautical mile = 1,852
    metres exactly.

So note: *approximately* one minute of latitude.  Not longitude.

Your buffer won't therefore find things within a nautical mile east-west
since one minute of longitude at 45° south or 72° north (not sure which
you're looking for) is considerably fewer metres than one minute of
latitude.

I think you should stick with metres and projected coordinates.

Blake Crosby wrote:
> I'm trying to create "circles" by using the buffer function around a
> point.
>
> The radius values i'm working with are in nautical miles. The
> definition of a nautical mile is 1 minute of 1 degree.
>
> Since this unit of measurement is based on a degree, and WSG84 is
> based on degrees of lat/long (and not meters like UTM) will the
> following be valid for a circle with a radius of 2 nautical miles
> around point (72.3,-45.3)?
>
> st_buffer(st_geomfromtext('POINT(-45.3 72.3)', 4326), 0.0166667*2)
>
> I'm trying to troubleshoot a problem I'm having and want to rule out
> any issues with my usage of buffer.
>
> Blake
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-- 
Regards,

Chris Hermansen         mailto:chris.hermansen at timberline.ca
tel+1.604.714.2878 · fax+1.604.733.0631 · mob+1.778.232.0644
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