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<TITLE>Re: Why doesn't postgis do more with projections and SRID's? ( was: What is the tolerance parameter for simplify? )</TITLE>
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<DIV dir=ltr><FONT face=Arial color=#000000 size=2>I'm leaving town for a couple
days but I will look into the area_spheroid. As for simplify if you're using
lat/lon coordinates simplify(the_geom,.1) will give your dramatically different
results depending on whether you're near the equator or near the
pole.</FONT></DIV>
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<DIV dir=ltr><FONT face=Arial size=2>Anyhow I was trying to use simplify
(because it's all I could find) to clean up geometries I've built using
geomunion that have points too close together and cause GEOS to throw topology
errors. If I project each of my polygons onto an appropriate projected
coordinate system then run simplify with meters units it works great and allows
me to "clean" my geometries.</FONT></DIV>
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<DIV dir=ltr><FONT face=Arial size=2>-e</FONT></DIV></DIV>
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<FONT face=Tahoma size=2><B>From:</B> strk@refractions.net
[mailto:strk@refractions.net]<BR><B>Sent:</B> Tue 1/4/2005 12:13
PM<BR><B>To:</B> Ethan Alpert<BR><B>Cc:</B>
postgis-users@postgis.refractions.net<BR><B>Subject:</B> Re: Why doesn't postgis
do more with projections and SRID's? ( was: What is the tolerance parameter for
simplify? )<BR></FONT><BR></DIV>
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<P><FONT size=2>On Tue, Jan 04, 2005 at 07:06:56AM -0700, Ethan Alpert
wrote:<BR>> 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.<BR>> <BR>> So what are the reasons for
not providing this type of functionality in the database?<BR>> <BR>>
-e<BR><BR>Simplify is an output size reduction function. It uses
Douglas-Peuker<BR>line simplification algorithm, which is known to give the most
'visually'<BR>correct output. Simplifying based on a different projection
then<BR>the final (visual output) one does not make much sense.<BR><BR>On the
other hand, I see an area_spheroid() would be useful.<BR>If you want to give it
a try you're the welcome and I'm happy<BR>to give you any support you'll
need.<BR><BR>--strk;<BR><BR>><BR>>
________________________________<BR>><BR>> From: strk@refractions.net [<A
href="mailto:strk@refractions.net">mailto:strk@refractions.net</A>]<BR>>
Sent: Tue 1/4/2005 1:23 AM<BR>> To: Ethan Alpert<BR>> Cc:
postgis-users@postgis.refractions.net<BR>> Subject: Re: [postgis-users] What
is the tolerance parameter for simplify?<BR>><BR>><BR>><BR>> 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>><BR>><BR><BR>--<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>
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