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List:       postgis-users
Subject:    Re: [postgis-users] question on SRID setting
From:       chodgson () refractions ! net
Date:       2002-12-10 16:59:25
Message-ID: 1039539565.3df61d6d3e67f () www
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I believe that you have found the correct projection. Ideally, you should find 
the correct projection for each table of data that you load, and specify that 
SRID for your geometry column. This will allow you to do projection transforms 
very easily, within postgis. However, if you are not going to be using the 
built-in projection transformation, it is unnecessary to determine and set the 
SRIDs - you can just leave them at -1. 

It is important to note that mapserver does not use postgis's built-in 
projection transforms, it has its own (both are built on the same proj.4 
library). So it is not necessary to properly define your SRIDs in postgis, in 
order to use mapserver. 

More information about the transform() function and SRIDs can be  found in the 
postgis documentation at http://postgis.refractions.net/docs/

Chris

Quoting Michael O'Sullivan <mos@i3sp.com>:

> Hi,
> 
> I am very new to the world of postgis (day 1) - and would like some
> clarification on how the spatial_ref_sys table and SRID values in other
> tables of data I load work.
> 
> As background I have compiled postgresql and postgis, and loaded the
> spatial_ref_sys.sql
> data that comes with the postgis distribution.
> 
> As an exercise I have then downloaded some shapefile data for US ports from
> 
> http://www.bts.gov/gis/download_sites/ntad02/newusdownloadform.html
> 
> 
> It had a web page with metadata as below
> 
> > /Spatial_Reference_Information:/
> >
> > /Horizontal_Coordinate_System_Definition:/
> >
> > /Geographic:/
> >
> > /Latitude_Resolution:/ 0.000001
> > /Longitude_Resolution:/ 0.000001
> > /Geographic_Coordinate_Units:/ Decimal degrees
> >
> > /Geodetic_Model:/
> >
> >     /Horizontal_Datum_Name:/ North American Datum of 1983
> >     /Ellipsoid_Name:/ Geodetic Reference System 80
> >     /Semi-major_Axis:/ 6378137.000000
> >     /Denominator_of_Flattening_Ratio:/ 298.257222
> >
> > /Vertical_Coordinate_System_Definition:/
> >
> > /Altitude_System_Definition:/
> >
> > /Altitude_Resolution:/ 1.000000
> > /Altitude_Encoding_Method:/
> > Explicit elevation coordinate included with horizontal coordinates
> >
> 
> After looking through the spatial_ref_sys.sql file I decided on using
> the following SRID
> ---
> --- NAD83
> ---
> INSERT INTO "spatial_ref_sys"
> ("srid","auth_name","auth_srid","srtext","proj4text") VALUES
> (4269,'EPSG',4269,'GEOGCS["GCS_North_American_1983",DATUM
["D_North_American_1983",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Gre
> enwich",0],UNIT["Degree",0.017453292519943295]]','+proj=longlat
> +ellps=GRS80 +datum=NAD83 ');
> ---
> 
> Is this the correct SRID entry?
> 
> There are a number of other "NAD83"  entries, (e.g. NAD83 / UTM zone 5N)
> these seem to be subsets of the NAD83 entry. I notice these have
> PROJECTION entries which seem to be defining these subsets from what I
> can make out.
> 
> As a rule - if I am downloading data (usually in shapefile format I
> guess?) I will have to obtain the relevant metadata so I can work out
> the SRID to use?
> 
> As I said this is all very new to me so please excuse me if this
> question is rather obvious. I have been reading different bits and
> pieces of the install doco today and am still trying to put it all
> together. Feel free to point me to any  links which may be relevant.
> 
> thanks,
> 
> Michael
> 
> 
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> 





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