[Mapserver-users] [SUM] Postgis Transform Problem
Benjamin Wragg
bwragg at tpg.com.au
Thu Jul 3 04:28:06 EDT 2003
Thankyou to Paul Ramsey, Vincent Schut from the Mapserver list and Ed
McNierney and Gerald Evenden from the proj4 list for their help on this
one. My lack of knowledge of projections was really the problem as
converting 90 or -90 lat is impossible with the Mercator projection.
My past use of ArcView 8 convinced me it was possible but it seems ESRI
have some "idiot protection" in their projection engine which must alter
the lat value if it's to great.
Thanks again for your help
Cheers,
Benjamin
-----Original Message-----
From: mapserver-users-admin at lists.gis.umn.edu
[mailto:mapserver-users-admin at lists.gis.umn.edu] On Behalf Of Vincent
Schut
Sent: Wednesday, 2 July 2003 5:12 PM
To: bwragg at tpg.com.au; mapserver-users at lists.gis.umn.edu
Subject: Re: [Mapserver-users] FW: [postgis-users] Postgis Transform
Problem
Benjamin,
my 2c on latlon and mercator:
tmerc is 'transverse mercator', as used in the abreviation 'UTM':
Universal
Transverse Mercator.
'Exercising tmerc at the boundaries': in latlon systems (which are
essentially
unprojected, though they are sometimes called latlon or geographic
'projections'), the north and south pole are single points. In
projections
like (transverse) mercator however, the north and south pole are either
not
there (they fall outside the valide projection area) or can be regarded
as
being stretched till these 'points' occupy the entire northern and
southern
boundary line of the projection of the entire earth. You might better
understand this if you look at these projections as wrapping the earth
in a
vertical cilinder and projecting each point of the sphere of the earth
onto
its nearest point on the surface of the cilinder. There are only 2
points
that are undefined in this, because they are similarly equal in distance
to
more than one point, to be specific an entire line: the most upper and
lower
points of the spere, or any latlon point that has a latitude of 90 or
-90.
Try cs2cs with any other value as lon, and 90 or -90 as lat, and it will
give
you the same non-output as 180, 90, because all these coordinates are
the
same point: north or south pole.
I don't know how arcview does it, but it must be some workaround of the
normal
projection formula, cause that will not give you one single point for
these
coordinates. It will either give you a line, or a 'undefined' result.
On Wednesday 02 July 2003 00:46, Benjamin Wragg wrote:
> 179.99 and 89.99 work OK. The problem is that I receive on a regular
> basis minx,miny,maxx,maxy coordinates which define the area of a
> hardcopy map boundary. Since many hardcopy maps are made of the entire
> earth, the coordinates sent are -180,-90,180,90. I then create PostGIS
> features from these coordinates.
>
> If I project the data in ArcView 8 I get something like the attached
> image. The purple/pink is a box I defined as -180,-90,180,90 using
> WGS84 and then projected it to Mercator. I was hoping to achieve
> similar using PostGIS.
>
> What did you mean by 'exercising tmerc at the boundaries'? 'tmerc'?
>
> Thanks,
>
> Benjamin
>
> (I removed the image for the list. If you need it let me know)
>
> -----Original Message-----
> From: Paul Ramsey [mailto:pramsey at refractions.net]
> Sent: Tuesday, 1 July 2003 11:57 PM
> To: bwragg at tpg.com.au
> Subject: Re: FW: [postgis-users] Postgis Transform Problem
>
>
> cs2cs exercises proj4 which is what postgis uses, so the test is
> equivalent. You realize that you are exercising tmerc at the
> boundaries, right? I do not know what the correct behavior should be
> at that coordinate. What happens when you try 179.99 89.99?
>
> On Monday, June 30, 2003, at 10:53 PM, Benjamin Wragg wrote:
> > I tried the cs2cs utility. I think I'm passing the correct
> > paramaters. If I enter the following:
> >
> > cs2cs +proj=latlong +ellps=WGS84 +to +proj=merc +lon_0=150.0
> > 180 90
> >
> > I get
> >
> > * * 0.00
> >
> > If I type any other coordinate I get a figure back. So is the cs2cs
> > utility what Postgis uses to project it's coords? If it does, it
> > seems
> >
> > this is whats causing the problem.
> >
> > Thanks,
> >
> > Benjamin
> >
> > -----Original Message-----
> > From: postgis-users-bounces at postgis.refractions.net
> > [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
> > Paul Ramsey
> > Sent: Wednesday, 25 June 2003 10:55 PM
> > To: bwragg at tpg.com.au; PostGIS Users Discussion
> > Subject: Re: [postgis-users] Postgis Transform Problem
> >
> >
> > Those coordinates are right on the edge of the global plane, could
> > be hitting the edge of validity in the reprojection library. Try
> > testing the same stuff directly using the proj4 cs2cs utility.
> >
> > On Tuesday, June 24, 2003, at 10:35 PM, Benjamin Wragg wrote:
> >> I'm having a problem with the transform function in Postgis. If I
> >> run the following query:
> >>
> >> select transform(GeometryFromText('MULTIPOLYGON(((112 -45 ,112 -10
> >> ,155 -10 ,155 -45,112 -45)))',4326),54004)
> >>
> >> I get:
> >>
> >> SRID=54004;MULTIPOLYGON(((12467782.9688466
> >> -5591295.91855339,12467782.9688466
> >> -1111475.10285223,17254521.0729574
> >> -1111475.10285223,17254521.0729574
-5591295.91855339,12467782.9688466
> >> -5591295.91855339)))
> >>
> >> But if I do:
> >>
> >> select transform(GeometryFromText('MULTIPOLYGON(((-180 -90 ,-180 90
> >> ,180 90 ,180 -90,-180 -90)))',4326),54004)
> >>
> >> I get:
> >>
> >> Error: transform: couldn't project polygon
> >> Warning: Error occurred while executing PL/pgSQL function transform
> >> Warning: line 2 at return
> >>
> >> Any ideas why? I turned the postgresql log level to debug5 to see
> >> if I
> >>
> >> could get more detailed errors, but it's exactly the same error.
> >> I'm running postgresql 7.3.2, postgis 0.7.5 and proj 4.4.7
> >>
> >> Thanks,
> >>
> >> Benjamin Wragg
> >
> > Paul Ramsey
> > Refractions Research
> > Email: pramsey at refractions.net
> > Phone: (250) 885-0632
> >
> >
> > _______________________________________________
> > postgis-users mailing list postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
>
> Paul Ramsey
> Refractions Research
> Email: pramsey at refractions.net
> Phone: (250) 885-0632
>
> _______________________________________________
> Mapserver-users mailing list Mapserver-users at lists.gis.umn.edu
> http://lists.gis.umn.edu/mailman/listinfo/mapserver-users
--
______________________________________
Vincent Schut
Sarvision B.V.
Wageningen, The Netherlands
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