[postgis-users] General question about SRID and projection

Craig Miller craig.miller at spatialminds.com
Thu May 11 14:06:29 PDT 2006

It sounds like you've pretty much got your head around it.  :-)

You are correct, the SRID refers to the Spatial Reference System of your
data which may or may not be a "projection".  It could also be stored as
geocentric data, on a local coordinate system, or as geographic.

If the data is stored as lat/long then all calculations such as area have to
be done on the ellipsoid instead of on a planar/cartesian coordinate system.
This of course requires much more complex calculations.  Everyone should
always be aware of the SRS their data is stored in and the affect this will
have on the results of queries such as area.  If the SRS distorts area, then
the area calculation will be wrong.

If the data were stored as lat/lon you would have to reproject the data on
the fly to an appropriate projection before calculating.  This works, but is
kind of costly if you know you will always be doing similar types of
queries.  Most people store their data in an SRS that their organization
typically uses, and then reproject that on the fly for specific analyses.
Depending on your typical use-cases this may or may not be the best way to
store your data.

If you are using a variety of different SRSs regularly, then it might make
sense to store it in lat/lon and reproject on the fly.

I don't think storing data with an SRID of -1 is reccomended, just common
since there is so much data out their with unknown origins.


-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Nelson
Sent: Thursday, May 11, 2006 12:18 PM
To: PostGIS Users Discussion
Subject: [postgis-users] General question about SRID and projection

Hi all,

This may be an ignorant question, but I am failing to understand something
basic.  I hope someone can help me clear this up.

first, the SRID refers to the projection of your geometry, correct?   
If this is so, then is the geometry data in postgis stored as lat / long
points or as projected coordinates?  If it is stored as lat /  
long, why does the SRID matter for any of the geometry functions?   
Shouldn't they all be performed on unprojected data?

If the data is stored as projected coordinates, then why is it suggested to
convert SRIDs to -1?  Don't you then lose important information about the

Also, wouldn't it be easier to have all of the geometry stored as lat / long
data and deal with projection for output only?

Thanks all.  I've really appreciated this group.  It has helped me a lot in
getting together my site on US roadless lands (http:// www.roadlessland.org

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