I am new to PostGIS and QGIS. I am trying to convert geom to geog and then cast it back to geom in order to buffer my streets by 1,000 ft with this query:

SELECT a.gid, (st_buffer(a.geom::geography,304.8)::geometry) as geom, a.street_ext
FROM public.orangeburgroads a
JOIN public.orangeburg b ON st_within(st_centroid(a.geom), b.geom)
LIMIT 1000;

And here is what I am getting:

enter image description here

Does anyone have a clue why I am getting this result?

FYI orangeburg is a municipal boundary and orangeburgroads is a street center-line file.

enter image description here

  • SELECT ST_AsText(ST_Centroid(geom)) FROM orangeburgroads LIMIT 1; – Paul Ramsey Oct 5 '16 at 12:23
  • You are joining the buffers with the output of ST_Within. You are not keeping the original record ids and therefore the query joins buffer 1 with the first element from the output from ST_Within. That brings your whole allocation in disorder. – Matte Oct 5 '16 at 12:31
  • 1
    Just curious: Why are you converting to geog? It shouldn't be needed, at least not in this way. – tilt Oct 5 '16 at 12:37
  • @tilt I am converting so that I can use meters as a buffer unit instead of degrees. – Matt Oct 5 '16 at 13:00
  • 1
    Took me a while to see that the output was in the screenshot ;-) Your points already seem to be in a meters grid, so no need to do a conversion. What happens if you do just st_buffer(a.geom,304.8) ? – tilt Oct 5 '16 at 16:16

The coordinates of your data are not geographic, they are Stateplane, SC, Feet, EPSG:3361. I know this because I used your test coordinate and back-projected it to geographics, to see if I landed in Orangeburg:

SELECT ST_AsText(ST_Transform(
  'SRID=3361;POINT(1940865 660341)'::geometry, 4326));

Fix your data, first of all (do this for all the tables where you have projected coordinates, but an SRID of 4326):

ALTER TABLE orangeburgroads 
  TYPE Geometry(MultiLineString, 3361)
  USING ST_SetSRID(geom, 3361);

Now, knowing that your coordinates are in feet, just do your buffer query with that knowledge and skipping the trip into geography:

  ST_Buffer(a.geom,304.8) AS geom, 
FROM public.orangeburgroads a
JOIN public.orangeburg b 
ON ST_Intersects(ST_Centroid(a.geom), b.geom)
LIMIT 1000;

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