[postgis-users] Buffer/Nautical Miles

Peter Foley pwfoley at gmail.com
Thu Sep 11 14:35:50 PDT 2008


Nautical miles are roughly based on 1 minute of longitude, but that only
equals (approximately) one minute of latitude at the equator, so you won't
really have a 2NM radius circle if you use WSG84. You should probably use a
meter-based projection and create the circle based on a nautical mile being
1,852 meters (this, not 1 minute of longitude, is actually the official
definition).
I'm just getting started with Postgis myself, so I can't critique your
command, but someone else can probably chime in with that info.
-Peter

On Thu, Sep 11, 2008 at 1:36 PM, Blake Crosby <me at blakecrosby.com> 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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