howto set ogr2ogr imported SRID to 0? Currently defaulting to SRID 1

Question

I'm currently trying to import a bunch of boundaries into a legacy system with a specific SRID. its a mySQL db.

After import and running some intersect code, i receive this error:

Binary geometry function st_intersects given two geometries of different srids: 1 and 0, which should have been identical.

As it says, there is an srid mismatch of 1 vs 0. The newly imported boundaries are SRID 1, the existing boundaries from another table (cannot be changed) are SRID 0. I have established this with mysql command

select ST_SRID(boundary_polygon) from boundaries; 

My question, what EPSG value would i use with ogr2ogr's -a_srs flag. I want to force SRID=0?

Here is the current command i'm using for import:

ogr2ogr -f "MYSQL" MYSQL:"blahdb,host=localhost,user=blah,password=blah,port=3306" boundaries.tab

I'm currently running this command which forces the import to SRID=0 which seems by convention to mean No SRID.

UPDATE boundaries SET boundary_polygon = GeomFromWKB(boundary_polygon, 0);

This then allowed my st_intersects method to work fine and find intersect between two polygons.

However, I still need a way to import as SRID=0 from ogr2ogr instead of SRID=1. So that I don't need this post import SQL step

Answer 1

The behavior with SRIDs in the GDAL MySQL driver is not very clearly explained on the driver manual page http://www.gdal.org/drv_mysql.html. A hint can be found from the "Caveats" section

SRS information is stored using the OGC Simple Features for SQL layout, with geometry_columns and spatial_ref_sys metadata tables being created in the specified database if they do not already exist. The spatial_ref_sys table is not pre-populated with SRS and EPSG values like PostGIS. If no EPSG code is found for a given table, the MAX(SRID) value will be used.

What probably happens when you save data into MySQL for the first time with an unknown SRID is that GDAL does not find the spatial_ref_sys metadata table. Because the table does not exist it is created and one row with SRID=1 is inserted. Next time the metadata table exists and value MAX(SRID)=1 will be used when new data with unknown projection are saved into new tables.

A workaround, kindly tested by user @wired00, is to make one conversion with some dummy data with GDAL so that spatial_ref_sys table appears and update the SRID from 1 to 0. Now MAX(SRID)=0 and zero will be used as SRID in following conversions.