<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD><TITLE>Re: [postgis-users] What is the tolerance parameter for simplify?</TITLE>
<META http-equiv=Content-Type content="text/html; charset=us-ascii">
<META content="MSHTML 6.00.2900.2523" name=GENERATOR></HEAD>
<BODY>
<DIV dir=ltr align=left><SPAN class=566425716-04012005><FONT face=Verdana
size=2>Ethan,</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=566425716-04012005><FONT face=Verdana
size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=566425716-04012005><FONT face=Verdana
size=2>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.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=566425716-04012005><FONT face=Verdana
color=#0000ff size=2></FONT></SPAN> </DIV>
<DIV dir=ltr align=left><SPAN class=566425716-04012005><FONT
size=2>_______________________________________________<BR><BR>Yusuf
Siddiqui<BR>Research & Development<BR>i-cubed : information integration
& imaging LLC<BR>201 Linden Street : Third Floor<BR>Fort Collins, CO
80524<BR>970-482-4400 voice<BR>970-482-4499
fax<BR>www.i3.com<BR>_______________________________________________<BR> </FONT>
</DIV></SPAN><BR>
<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left>
<HR tabIndex=-1>
<FONT face=Tahoma size=2><B>From:</B>
postgis-users-bounces@postgis.refractions.net
[mailto:postgis-users-bounces@postgis.refractions.net] <B>On Behalf Of </B>Ethan
Alpert<BR><B>Sent:</B> Tuesday, January 04, 2005 7:07 AM<BR><B>To:</B>
strk@refractions.net<BR><B>Cc:</B>
postgis-users@postgis.refractions.net<BR><B>Subject:</B> [postgis-users] Why
doesn't postgis do more with projections andSRID's? ( was: What is the tolerance
parameter for simplify? )<BR></FONT><BR></DIV>
<DIV></DIV>
<DIV id=idOWAReplyText51609 dir=ltr>
<DIV dir=ltr><FONT face=Arial color=#000000 size=2>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.</FONT></DIV>
<DIV dir=ltr> </DIV>
<DIV dir=ltr><FONT face=Arial color=#000000 size=2>So what are the reasons for
not providing this type of functionality in the database?</FONT></DIV>
<DIV dir=ltr><FONT face=Arial size=2></FONT> </DIV>
<DIV dir=ltr><FONT face=Arial size=2>-e</FONT></DIV></DIV>
<DIV dir=ltr><BR>
<HR tabIndex=-1>
<FONT face=Tahoma size=2><B>From:</B> strk@refractions.net
[mailto:strk@refractions.net]<BR><B>Sent:</B> Tue 1/4/2005 1:23 AM<BR><B>To:</B>
Ethan Alpert<BR><B>Cc:</B>
postgis-users@postgis.refractions.net<BR><B>Subject:</B> Re: [postgis-users]
What is the tolerance parameter for simplify?<BR></FONT><BR></DIV>
<DIV>
<P><FONT size=2>On Mon, Jan 03, 2005 at 05:41:02PM -0700, Ethan Alpert
wrote:<BR>> Just curious what the units are. I have geometries in SRID 4326
(lat/lon<BR>> wgs84) so what units would the tolerance parameter be in when
calling<BR>> simplify?<BR>> <BR>> Thanks,<BR>> <BR>>
-e<BR><BR>It would be the units of the projection.<BR>Decimal degrees in your
case.<BR><BR>--strk;<BR><BR>For standing up against patentability of
software,<BR><BR> Thank You, Poland!<BR><BR>Read the
intervention: <A
href="http://kwiki.ffii.org/ConsPolon041221En">http://kwiki.ffii.org/ConsPolon041221En</A><BR>Send
your thanks:
thankyoupoland.info<BR>Read/do more:
<A
href="http://www.noepatents.org/">http://www.noepatents.org/</A><BR></FONT></P></DIV></BODY></HTML>