title: PostGIS spatial reference ID (SRID) for regular cartesian coordinate system?
body: <p>I would like to store a polygon as a set of 2D points represented as x, y coordinates of a Cartesian coordinate system.</p>
  
  <p>Which spatial reference can I use? Looking at my spatial_ref_sys table, all the spatial references seem to be geography related.</p>
  
  <p>The points of the polygon would represent satellite measurements which I would invoke ST_ConvexHull on to get the shape of the satellite footprint.</p>