[postgis-users] Confusion about projections and Spatial Data

[Milo van der Linden]

Rick Zoolker schreef:
> OK, I feel like I've got a fundamental block about how spatial data is
> actually used in the real world and how spatially aware databases
> actually make use of that data.  This is probably quite obvious for
> everyone so please bear with me.
>
> I've been playing with a bunch of spatial databases (Oracle, Postgis,
> DB2) and from what I can tell, all tend to roughly follow the OGC SFA
> spec (Oracle using different terminology).  Now it also seems that the
> SFA spec deals EXCLUSIVELY with Cartesian coordinates.  
Is that so? I haven't noticed using lat/lon being an issue. All my
projects in postGIS are in lat/lon and that tends to work fine. I have
also been using MI Spatialware and Oracle Spatial and as far as I
recall, lat/lon is no issue although in these environments we where
still using the dutch cartesian system.
> Now here's my
> first confusion.  If they only deal with Cartesian coordinates, why do
> they allow you to specify SRIDs that are lat/long based.  Will any of
> the SFA functions work with lat/long (other than ST_Transform(), which
> is actually a SQL/MM and not even included in SFA)??
>   
They will. I am using the BBOX functions, but no advanced functions yet,
but I am confident.
> My next area of confusion is that I've looked at a bunch of data from
> people such as Navteq, teleatlas, and a bunch of individual companies
> databases, and it seems that every single one ships and stores their
> data in lat/long.  So what the heck?  If the tools can't deal with
> lat/long, why does everyone ship and store their data in lat/long?
> Does everyone project this data before actually loading it into the
> db?  Help!?
>   
No, I don't. I just load it in lat/lon. As a matter of fact, I think we
would better support lat/lon projections by trying it before converting,
fishing out problem area's and bugs and then enhancing the software. So
please, try in lat/lon and if it doesn't work, see if the community can
help you. I do understand that spherical vs cartesian distances will be
challenging, but perhaps the gdal/ogr and proj projects will be able to
support there.
> Now, it appears that Oracle in particular can do some functions with
> lat/long directly (the distance calculation comes to mind), and I know
> Postgis has some specialized distance functions for geodetic
> coordinates, but I'm not sure if that extends to the whole function
> set (i.e. can I do an Intersection of two polygons specified in
> lat/long coordinates??).  Why doesn't the OGC SFA spec and libraries
> support all the functions with geodetic coordinates directly rather
> than require projected coordinates?
>   
Hit me, I don't know.
> Finally, how do the analysis and visualization tools that people put
> on top of these databases deal with these questions? (e.g. MapInfo,
> ArcView, etc.)  They seem to want lat/long data as well, but if you
> stored project coordinates in the database isn't there a problem??
>   
MapInfo works fine, quantumGIS works fine, cartoweb works fine.
Mapserver works fine.
> HELP!
>
>   -Rick
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>
>   


-- 


	

Milo van der Linden
skype: milovanderlinden <skype:milovanderlinden?add>
mlinden at zeelandnet.nl <mailto:mlinden at zeelandnet.nl>
milovanderlinden at gmail.com <mailto:milovanderlinden at gmail.com>
milo at 3dsite.nl <mailto:milo at 3dsite.nl>
http://www.3dsite.nl

	  	

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