[postgis-users] Confusion about projections and Spatial Data

[Obe, Regina]

Very good questions.  I may be wrong on some of my assumptions, but this is my understanding
 
1) Everyone ships in lat long format because it covers the widest area of space and can be easily transformed into other SRIDs or out of all SRIDs. Its not even entirely true that everyone does.  Most state municipalities for example often ship their data in a coordinate system specific to thier area or state.
 
2) As far as I know most functions will work with lat long except distance gives you funky results because it is measuring around circles for lat long.
 
3) Why do databases have this whole cartesian thingy and not work straight with lat long?  Because screens are flat, printed maps are flat and when you look at isolated areas you can make them look flat and not skew measurements and layout.   So I guess its a hack to make your problem simpler.  If you look at the upcoming SQL Server Katmai which is supposed to have Geography - it will challenge that and try to deal with geodetic data as a "first class" reference system. 
 
4) Not everyone stores their data projected, but often will transform on the fly if they do not project.  I do when I know I am only dealing with a very isolated region like a single state or city municipality.
 
5) Its not true that tools like ArcView and MapInfo want lat long.  Actually MapInfo is pretty sophisticated I think when it comes to projections so its almost transparent to the user.  I'm not quite sure how it does it, but encoded in each tab file is the projection info and you can lay on a bunch of layers in different projections and it will magically pick the most suitable one and reproject all the layers to that.  My knowledge of ArcView is less, but I think ArcView tends to be more picky about projections so requires more knowledge on the user's side to know how to deal with multiple layers in different projections. 
 
 
Hope that helps,
Regina
 

________________________________

From: postgis-users-bounces at postgis.refractions.net on behalf of Rick Zoolker
Sent: Fri 8/10/2007 1:34 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] Confusion about projections and Spatial Data



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.  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)??
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!?

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?

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??

HELP!

  -Rick
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