gis - how reprojections are done with srid = -1 -


i huge doubt, have shape file introduced postgres , didn't specify srid, the_geom column has -1 in srid, when start other map viwers qgis or mapsertver don have problem see figure should be. if don't have specify srid location why evrybody srid it's heart of gis, , how points, painted shoud when reprojected, example wgs84.

thanks

if data has same spatial reference system (srs), can plot them in same space (comparing apples , apples). no transform done srid=-1. if want ignore srs , user (i.e., aren't sharing data others), go ahead , make srid '-1'.

however, if want mix data different srses (for example, data in wgs84 long/lat , data in utm eastings/northings), gis needs know transform reproject data can overlain beside each other. there thousands of different srses out there.

furthermore, if are sharing data else, data has diminished value. (i've spent hours trying brute-force guess else's projection when didn't specify it). example, there couple hundred metres of error between identical long/lat degree coordinates between nad27 , nad83 datums.

mind you, don't performance gains specifying '-1' srid. if don't know srid, better keep '-1' incorrectly guessing wrong one.


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