[postgis-users] Why doesn't postgis do more with projections andSRID's? ( was: What is the tolerance parameter for simplify? )

Yusuf Siddiqui ysiddiqui at i3.com
Tue Jan 4 09:02:42 PST 2005

The simplify command uses the Douglas-Peuker algorithm.  The tolerance is
the leeway that it uses to determine whether or not to remove a vertex from
a straight line segment.  Do a web search on Douglas-Peuker to see details
about the method.  At this point in PostGIS, it is too much to expect that
it would treat a geographic projection differently (e.g. using great circle
distances instead of "degree" distances) than other projections.  At least
there is something there that works reasonably well.

Yusuf Siddiqui
Research & Development
i-cubed : information integration & imaging LLC
201 Linden Street : Third Floor
Fort Collins, CO  80524
970-482-4400 voice
970-482-4499 fax


From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Ethan
Sent: Tuesday, January 04, 2005 7:07 AM
To: strk at refractions.net
Cc: postgis-users at postgis.refractions.net
Subject: [postgis-users] Why doesn't postgis do more with projections
andSRID's? ( was: What is the tolerance parameter for simplify? )

Having simplify use the units of the projection probably was easier to
implement but what does it really mean to use a 1/10 of a degree of
tolerance? Why doesn't postgis allow the use of more reasonable values say
meters? Why don't more functions work more like distance_spheroid (where you
provide a spheroid to use for computation)?  I ran into the same problem
with area. What possible use is there knowing the degrees^2 of a polygon? It
would seem that with the SRID and the proper spheroid more intuitive values
could be used for functions like area and simplify and area. To compute
areas for my data I know have to read my data out of the database, reproject
it and compute my values. It would be much more convenient and less error
prone if the database could do this for me.
So what are the reasons for not providing this type of functionality in the


From: strk at refractions.net [mailto:strk at refractions.net]
Sent: Tue 1/4/2005 1:23 AM
To: Ethan Alpert
Cc: postgis-users at postgis.refractions.net
Subject: Re: [postgis-users] What is the tolerance parameter for simplify?

On Mon, Jan 03, 2005 at 05:41:02PM -0700, Ethan Alpert wrote:
> Just curious what the units are. I have geometries in SRID 4326 (lat/lon
> wgs84) so what units would the tolerance parameter be in when calling
> simplify?
> Thanks,
> -e

It would be the units of the projection.
Decimal degrees in your case.


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