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

Ethan Alpert ealpert at digitalglobe.com
Tue Jan 4 09:34:49 PST 2005

```I see you're point but using decimal degrees for a tolerance is not
good, obviously. So this leaves the user to have to figure out how to
project their data inorder to properly simplify the polygons.

Try running simplify on the same size/shape polygon in decimal degrees
at the equator and near the pole.

-e

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Yusuf Siddiqui
Sent: Tuesday, January 04, 2005 10:03 AM
To: 'PostGIS Users Discussion'
Subject: RE: [postgis-users] Why doesn't postgis do more with
projectionsandSRID's? ( was: What is the tolerance parameter for
simplify? )

Ethan,

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
www.i3.com
_______________________________________________

________________________________

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Ethan Alpert
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 database?

-e

________________________________

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.

--strk;

For standing up against patentability of software,

Thank You, Poland!