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

Paul Selormey paul at toolscenter.org
Tue Jan 4 09:59:22 PST 2005

Re: [postgis-users] What is the tolerance parameter for simplify?Hello,
I think Ethan's point should be taken more seriously. The same could be said of
the buffer operation, in this case if the coordinates is lat/long, does the 2 degree
buffer mean?

JUMP avoids such situations by supporting only projected coordinates.

Best regards,

  ----- Original Message ----- 
  From: Yusuf Siddiqui 
  To: 'PostGIS Users Discussion' 
  Sent: Wednesday, January 05, 2005 2:02 AM
  Subject: RE: [postgis-users] Why doesn't postgis do more with projectionsandSRID's? ( was: What is the tolerance parameter for simplify? )


  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 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?


  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.


  For standing up against patentability of software,

    Thank You, Poland!

  Read the intervention:    http://kwiki.ffii.org/ConsPolon041221En
  Send your thanks:         thankyoupoland.info
  Read/do more:             http://www.noepatents.org/


  postgis-users mailing list
  postgis-users at postgis.refractions.net
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20050105/12255a3a/attachment.html>

More information about the postgis-users mailing list