Why is there an upper limit to the SRID value in the spatial_ref_sys table in PostGIS

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I was trying to insert a reference system in PostGIS from http://spatialreference.org/, and got a constraint violation for spatial_ref_sys_srid_check. I checked that constraint and found that, when inserting in spatial_ref_sys, the srid must never be greater that 998999.

I understand that the root cause of the problem is that spatialreference prepend a 9 to the srid. Probably to avoid collision srid in the srid column, or something of the sort. I don't really care about that, its easy to solve: removing the 9 make it work most of the time.

But why that arbitrary upper limit on the srid number? It is way below the maximum value for a Postgresql integer, so it doesn't seem to be a technological limit, and the number itself look quite arbitrary. Introducing that limit broke the code in http://spatialreference.org/ so there is an obvious downside.

So what was the rational for introducing such an upper limit, at that specific value? Is that a limit imposed by a standard body?



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