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<DIV><SPAN class=397513217-04012005><FONT face=Arial color=#0000ff size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=397513217-04012005><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=397513217-04012005><FONT face=Arial color=#0000ff size=2>Try
running simplify on the same size/shape polygon in decimal degrees at the
equator and near the pole.</FONT></SPAN></DIV>
<DIV><SPAN class=397513217-04012005><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=397513217-04012005><FONT face=Arial color=#0000ff
size=2>-e</FONT></SPAN></DIV>
<BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px">
<DIV></DIV>
<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left><FONT
face=Tahoma size=2>-----Original Message-----<BR><B>From:</B>
postgis-users-bounces@postgis.refractions.net
[mailto:postgis-users-bounces@postgis.refractions.net] <B>On Behalf Of
</B>Yusuf Siddiqui<BR><B>Sent:</B> Tuesday, January 04, 2005 10:03
AM<BR><B>To:</B> 'PostGIS Users Discussion'<BR><B>Subject:</B> RE:
[postgis-users] Why doesn't postgis do more with projectionsandSRID's? ( was:
What is the tolerance parameter for simplify? )<BR><BR></FONT></DIV>
<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></BLOCKQUOTE></BODY></HTML>