[mapserver-users] Basic Projection Help

[Paul Spencer]

There is normally a constraint set on postgis tables that specifies  
the SRID, if you want to change the SRID then you need to drop the  
constraint, set the SRID and optionally put back the constraint with  
the correct SRID.

I do not believe this is necessary to achieve what you want, though.   
An SRID of -1 tells postgis to return the geometry as is to  
mapserver.  Mapserver then doesn't know what the projection of the  
data is, but that is okay because you can provide an PROJECTION object  
on your layer in mapserver.  Then mapserver knows what the projection  
of the data is without having to consult postgis and will be able to  
reproject the data for you.

So you should be able to leave your data in the database with SRID -1  
and put

PROJECTION
   "init=epsg:26915"
END

in your layer.  Then if you use a different projection at the MAP  
level, mapserver knows that your data is in UTM zone 15 and will  
reproject it on the fly.

If you are *always* going to reprojecting your data then you should  
actually transform it in the database so that you don't suffer from  
reprojection overhead on every request.

Cheers

Paul

On 28-Jun-08, at 2:12 AM, pcreso at pcreso.com wrote:

> Hi Matthew,
>
> If you loaded it from shapefiles using shp2pgsql, then use the -s  
> flag to set the srid to the correct value.
>
> If you \d your table, I think you'll find a constraint on it setting  
> the srid, as well as the geometrycolumns entry.
>
> The simplest way to get things working with mapserver may well be to  
> leave it as -1 in the database (& geometrycolumns entry) & in your  
> mapserver mapfile data statement, specify the sql returns  
> setsrid(geom,26915) instead of geom, so postgis returns the values  
> with the appropriate SRID set, irrespective of what the database  
> actually stores.
>
> That will get things working until you load the data with the  
> correct srid.
>
> HTH,
>
>  Brent Wood
>
>
>
> --- On Sat, 6/28/08, Matthew Pettis <matthew.pettis at gmail.com> wrote:
>
>> From: Matthew Pettis <matthew.pettis at gmail.com>
>> Subject: [mapserver-users] Basic Projection Help
>> To: "Mapserver-users" <mapserver-users at lists.osgeo.org>
>> Date: Saturday, June 28, 2008, 5:47 PM
>> Hello,
>>
>> I'm trying to get a handle on projections and
>> reprojections.  I have a
>> dataset in a postgis database table that is in epsg:26915
>> (it's the state of
>> MN).  I know this from the metadata provided by the
>> provider (state of
>> Minnesota itself).
>>
>> When I loaded it into postgis, the geometry_columns row for
>> this
>> table/column combo set srid=-1.  As I know that this is in
>> epsg:26915, I
>> manually changed the geometry_columns row entry to it (see:
>> ===
>> geometry_columns === section).
>>
>> Problem is, I get an error of using two different
>> geometries.  Anybody have
>> any idea how i set the srid to the correct value of
>> epsg:26915?  I'd like to
>> set this correctly so I can reproject it as necessary.  The
>> map will render
>> with srid=-1, but then I don't think I can reproject
>> the image correctly,
>> right?
>>
>> below are what I think are the important sections to this
>> problem.  Thanks
>> in advance for your help.
>>
>> Matt
>>
>> === url ===
>> = 
>> =====================================================================
>> http://localhost/cgi-bin/mapserv.exe?map=C:\ms4w\Apache\htdocs\ol 
>> \mapreproj.map&mode=map
>> = 
>> =====================================================================
>>
>> === error message ===
>> = 
>> =====================================================================
>> msDrawMap(): Image handling error. Failed to draw layer
>> named 'state'.
>> msPOSTGISLayerWhichShapes(): Query error. Error executing
>> POSTGIS SQL
>> statement (in FETCH ALL): DECLARE mycursor BINARY CURSOR
>> FOR SELECT
>> asbinary(force_collection(force_2d(the_geom)),'NDR'),gid::text
>> from
>> shp_state WHERE the_geom &&
>> setSRID('BOX3D(141095.125 4809728.25,810342.625
>> 5478975.75)'::BOX3D,
>> find_srid('','shp_state','the_geom')
>> ) -ERROR:
>> Operation on two geometries with different SRIDs
>> = 
>> =====================================================================
>>
>> === geometry_columns ===
>> = 
>> =====================================================================
>> oid 
>> ;f_table_catalog 
>> ;f_table_schema 
>> ;f_table_name;f_geometry_column;coor_dimension;srid;type
>> 336791;"''";"public";"shp_state";"the_geom";2;26915;"MULTIPOLYGON"
>> = 
>> =====================================================================
>>
>> === mapreproj.map ===
>> = 
>> =====================================================================
>> MAP
>>
>>    ################
>>    # Map attributes
>>    ################
>>  IMAGETYPE   PNG
>>  EXTENT      184056.505 4809728.25 767381.245 5478975.75
>>  SIZE        600 600
>>  IMAGECOLOR  255 255 255
>>  FONTSET
>> "C:/ms4w/Apache/htdocs/fonts/fonts.list"
>>
>>    ####################
>>    # Web settings
>>    ####################
>>  WEB
>>    IMAGEPATH   "C:/ms4w/Apache/htdocs/tmp/"
>>    IMAGEURL    "/tmp/"
>>  END
>>
>>    ####################
>>    # State
>>    # Background shows through missing precincts to show
>> where no votes will
>> be coming from
>>    ####################
>>  LAYER
>>    NAME              "state"
>>
>>    CONNECTIONTYPE    postgis
>>    CONNECTION        "user=postgres dbname=gisvote
>> password=postgres"
>>    DATA              "the_geom from shp_state"
>>    STATUS            default
>>    TYPE              polygon
>>
>>    CLASS
>>      NAME            "state"
>>      COLOR           200 200 200
>>    END
>>
>>  END # Layer state
>>
>> END
>> = 
>> =====================================================================
>>
>>
>> -- 
>> It is from the wellspring of our despair and the places
>> that we are broken
>> that we come to repair the world.
>> -- Murray
>> Waas_______________________________________________
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>> mapserver-users at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/mapserver-users
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__________________________________________

    Paul Spencer
    Chief Technology Officer
    DM Solutions Group Inc
    http://www.dmsolutions.ca/