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I get a huge doubt, I have a shape file that I introduced to postgres and I didn't specify an srid, so the_geom column has -1 in the srid, when I start any other map viwers like QGis or MapSertver I don have any problem to see the figure exactly where it should be. So if I don't have to specify any srid to a location why evrybody say that srid it's the heart of GIS, and how points, are painted exactly where they shoud be when are reprojected, for example to wgs84.

Thanks

AIM
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If all your data has the same spatial reference system (SRS), then you can plot them all in the same space (comparing apples and apples). No transform is done with SRID=-1. If you want to ignore the SRS and you are the only user (i.e., you aren't sharing the data to others), go ahead and make the SRID '-1'.

However, if you want to mix data with different SRSes (for example, data in WGS84 Long/Lat and data in UTM Eastings/Northings), then the GIS needs to know what to transform to reproject the data so it can be overlain beside each other. There are thousands of different SRSes out there.

Furthermore, if you are sharing the data to someone else, the data has diminished value. (I've spent hours trying to brute-force guess someone else's projection when they didn't specify it). For example, there is often a couple hundred metres of error between identical Long/Lat degree coordinates between NAD27 and NAD83 datums.

Mind you, that you don't get any performance gains by specifying a '-1' SRID. If you really don't know the SRID, then better keep it '-1' than incorrectly guessing the wrong one.

Mike T
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  • Ok, I understand what you are telling me, and – AIM Jul 05 '11 at 13:35
  • Ok, I understand what you are telling me, in fact I won´t doubt I´ll specify the srid everey time. But the thing that still keep me uncomfortable is that I have three layers in postgres the three where loaded with shp2pgsql one with srid -1, other with 2085 the same projection of the file, and the last one with srid 3819, the problem is when I visualize them with QuantumGIS taking the layers from postgres, they look exactly the same, there is no error figures are one over the other. – AIM Jul 05 '11 at 13:54
  • In QGIS, go to: Settings > Project Properties > Coordinate Reference System (CRS) and turn on "Enable 'on the fly' CRS transformation" (this is off by default) – Mike T Jul 05 '11 at 19:19
  • thanks a lot now I understand, I assume this check box make QGIS to paint the point with the projection and without it, now I understand. Again I´m very thankful. – AIM Jul 08 '11 at 13:13
  • no prob .. click the check-mark on the left to ["accept the answer"](http://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work), and in the future you should consider asking GIS questions over at http://gis.stackexchange.com/ – Mike T Jul 08 '11 at 23:01