[postgis-users] simplify

[Paul Ramsey]

Mercator won't do for large areas, since it lacks fidelity of distance. 
  If you truly have the "whole western hemisphere" then that's a hard 
call, time to get out the book of map projections and find something 
with fidelity of distance over large areas.  Or, if the area is large 
but the shapes are small, simplify them individually in locally correct 
  projections. (Foreach feature, transform to relevant UTM zone, 
simplify, transform back and repeat)

Stephen Crawford wrote:
> OK, I lied about the "last question for today" thing:
> 
> I understand about simplifying on decimal degrees data, but I really want to
> keep the data in WGS84 and then reproject in Geoserver as needed.  To
> simplify, should I first Transform() to, say, Mercator, then Simplify(),
> then transform back to WGS84? That seems like ti would be the same problem.
> And most other projections are not appropriate for such a large area....my
> data are for the entire Western Hemisphere.
> 
> Thanks,
> Steve
> 
> 
> 
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Paul
> Ramsey
> Sent: Thursday, March 01, 2007 5:09 PM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] simplify
> 
> The units for Simplify() are the units of the data, so yes, if you
> Simplify() decimal degrees data, you'll want to supply a decimal degrees
> tolerance.
> 
> That said, DO NOT use Simplify() on decimal degrees data unless you live on
> the equator.  The difference in scale horizontally and vertically in more
> northerly areas will result in "odd" simplifications when in decimal
> degrees.
> 
> Paul
> 
> Stephen Crawford wrote:
>> One last question and I'll leave you all alone for the day :)
>>
>> If I want to generalize my data, I should simplify()?  So if the data 
>> are in decimal degrees, then is the tolerance decimal degrees?  
>> Something like this?
>>
>> CREATE TABLE accipiter AS SELECT 
>> 	gid,engl_name,sci_name,origin,Simplify(the_geom,0.01) as the_geom 
>> FROM all_species WHERE family='Accipitridae';
> 


-- 

   Paul Ramsey
   Refractions Research
   http://www.refractions.net
   pramsey at refractions.net
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   Cell: 250-885-0632