[postgis-users] Latitude and longitude distances

[Vacuum Joe]

Thanks for the scoop on that Ethan.

What I will probably do then is not use PostGIS at
all, but just do this all in native Postgres. 
Fortunately for me I don't care about the polar
regions too much, so I can make some approximations to
use a "box" to narrow down my sets that I have to do
actual distance calculations on.  I already have the
formula for calculating distances between lat/lon
pairs so I'll just use that.  Fortunately also there
aren't too many data points at this stage.

Thanks for your help.


--- Ethan Alpert <ealpert at digitalglobe.com> wrote:

> 
> 
> Well postgis really doesn't support this activity
> well. You can only
> really compute linear distance using the
> distance_spheriod function but
> the buffer function doesn't support using meters if
> the data is in a
> geodetic coordinate system so that complicates the
> sql you need to
> write.
> 
> You can do what you're after with distance_spheriod
> but you need to
> compute the distance against every point unless you
> come up with a
> scheme of querying. For instance you can perhaps
> select points that
> intersect a box that has a +- some degree differnce
> and then compute the
> distance of your query point against the results
> rather than the full
> dataset. You'll have to be careful though if you
> have points at high and
> low latitudes as this approach may not capture all
> points with in a
> distance radius.
> 
> Anyhow long story short is you can do it in postgis
> but postgis doesn't
> help you out too much with it's built in functions.
> You'll need to
> understand more about coordinate systems,
> projections and datums to
> really do this accurately and come up with efficient
> implementations.
> Implementations could vary depending on the
> distrobution of your points
> within the lat/lon coordinate system. If all your
> points say fall within
> a UTM zone or a state plane coordinate system then
> you could get away
> with just projecting all your data into the
> appropriate projection.
> 
> -e
> 
> 
> 
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
>
[mailto:postgis-users-bounces at postgis.refractions.net]
> On Behalf Of
> Vacuum Joe
> Sent: Tuesday, July 26, 2005 11:48 PM
> To: postgis-users at postgis.refractions.net
> Subject: [postgis-users] Latitude and longitude
> distances
> 
> 
> Hello,
> 
> I'm new to PostGIS.  I have what I hope is a basic
> question.  I have a database with many locations in
> it.  These locations are all specified by latitude
> and longitude.  I
> need to be able to do things like select all the
> rows that are within a
> certain radius of a given latitude/longitude.  The
> results don't have to
> be precise, and the radius will never be more than a
> few miles.
> 
> I assume this type of use is perfectly within what
> PostGIS can do, but I can't figure out how to store
> lon/lat data and do searches on it.
> 
> If there are any docs or examples you could point me
> to, I would greatly appreciate it.
> 
> Thanks
> 
> 
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