[gdal-dev] PostGIS SRID conundrum

[pcreso at pcreso.com]

Hi Martin,

I can't see why that won't work, but it does have one issue, and that is that if there is already a match, but the parameters are in a different order, you won't find it, &  then you are effectively creating a nonstandard SRID which duplicates a standard & supported projection. 

Which doesn't stop it working for your case.

And I don't believe there are any orthographic projecttions in Postgis at present, so it is a potential problem only for now.

Cheers,

  Brent

--- On Sat, 2/11/12, Martin Chapman <chapmanm at pixia.com> wrote:

From: Martin Chapman <chapmanm at pixia.com>
Subject: RE: [gdal-dev] PostGIS SRID conundrum
To: pcreso at pcreso.com
Cc: gdal-dev at lists.osgeo.org
Date: Saturday, February 11, 2012, 6:48 PM

Brent,  Thank you very much for the advice.  What I resorted to (at least for the present) is:  1.        I try to get the geogcs or projcs (if projected) authority code from OGR.  If successful, I try to find a match in the spatial_ref_sys table.  If I find a match then I compare the wkt, if it’s exactly the same I use it.  If not I go to number 2.2.       I export the proj4 text from OGR and do a select on the spatial_ref_sys table for an exact match.  If found I use that srid.  If not I go to number 3.3.       I export the wkt text from OGR and do a select on the spatial_ref_sys table for an exact match.  If found I use that srid.  If not I go to number 4.4.       I select the max(srid) where srid > 10000000.  If zero is returned then I  use that base number (10000000) and add a new entry to the spatial_ref_sys table using the proj4 text and wkt so all fields have a value…and my app name as the authority (makes
 me feel important J).  If a max above 10000000 is found then I take the max and add one and then add the new entry.  If you or anyone else can see any flaws in that logic I would appreciate some criticism.  Thanks again for your feedback!  Best regards,Martin  From: pcreso at pcreso.com [mailto:pcreso at pcreso.com] 
Sent: Friday, February 10, 2012 10:33 PM
To: Martin Chapman
Cc: gdal-dev at lists.osgeo.org
Subject: Re: [gdal-dev] PostGIS SRID conundrum  ESRI does not use a SRID, it uses a text string that needs complex parsing to work. If ESRI supported EPSG codes it would be easier...

The spatial_ref_sys table contains entries for all EPSG recognised projections. If you have a reasonable case to have a projection inluded by EPSG, submit a request to EPSG & it will finish up in the Postgis spatial_reference table.

In the short term, add a proj4txt record for your projection with an arbitrary new srid in the spatial_ref_sys table & it will work fine in Postgis. If you want it to work with GDAL/OGR then you;ll need to add it to the proj4 list of projection parameters. Where this is depends on your OS & implementation.

See this spec:

http://remotesensing.org/geotiff/proj_list/orthographic.html

The proj4 parameter list for your case may need to specify WGS84 as your ellipsoid & datum.

HTH,

  Brent Wood

--- On Sat, 2/11/12, Martin Chapman <chapmanm at pixia.com> wrote:
From: Martin Chapman <chapmanm at pixia.com>
Subject: [gdal-dev] PostGIS SRID conundrum
To: gdal-dev at lists.osgeo.org
Date: Saturday, February 11, 2012, 12:25 PMI know this question isn’t exactly related to GDAL but I thought someone on the list might be able to answer this question. So, I have a shape file that has the following spatial reference: PROJCS["Orthographic",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Orthographic"],PARAMETER["Latitude_Of_Center",40],PARAMETER["Longitude_Of_Center",-75],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["User Defined",1]] It was generated using OGR.  I want to insert it into postgres / postgis but there is no direct match for the wkt or proj4 definition in the spatial_ref_sys table.  So, I go to insert it into the table but the question arises, what SRID do I use?  Do I just fudge a random one with a high enough value that I hope it won’t conflict with some other SRID in the future, or do I
 create a table based off of the heterogeneous list of SRIDs at spatialreference.org and insert from that?  I would like to do the later but that will be a pain unless someone has a put together a batch insert sql file for all the projections on spatialreference.org.  Still, there is always the possibility that I may run into one that isn’t defined and I will have to fudge my own number anyway. Question:  What is the best way to handle this?  I wish the spatial_ref_sys table came with all the ESRI and other SRID definitions.  Suggestions are much appreciated! Best regards,Martin  
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