The first step to determining the correct projection of any layer, is to find the projection information, if any, that came with your layer. In the case of a
Shapefile, like what you downloaded from Census.gov, that information is contained in a
.prj file, short for Projection.
Here are the contents of the projection file from the census data:
The key parts of this, are the first word GEOGCS means
Geographic Coordinate System, the string DATUM:
GRS_1980, and the last part UNIT:
These easily match up to the Proj4 definition that you received:
+proj=longlat +ellps=GRS80 +datum=NAD83 +no_defs
You can see that the datum and ellipsoid, match up to the datum and spheriod in the ".prj". The
Longitude/Latitude, which is measured in Units of
Degrees, which matches the ".prj" file.
There are "no defs" because no additional information is necessary to identify the Geographic Coordinate System.
A site to search for spatial references is: SpatialReference.org
The Geographic Coordinate System in NAD 83 is hard to come up with directly, because it is a part of many other projections.
To make it easier, the one you need is EPSG 4269.
This link presents the spatial reference information in a number of different formats. The one specifically for PostGIS is:
INSERT into spatial_ref_sys (srid, auth_name, auth_srid, proj4text,
srtext) values ( 94269, 'epsg', 4269, '+proj=longlat +ellps=GRS80
+datum=NAD83 +no_defs ', 'GEOGCS["NAD83",DATUM["North_American_Datum_1983",SPHEROID["GRS
To address your last point regarding the SRID = -1, I think it is important to test your data before you assume they are useless. Try to bring it up in some GIS software, like QGIS, or ArcGIS. Look at the coordinates that are shown as you move the cursor around. If you see familiar ranges of numbers, like y = 0 – +90 and x = -180 – +180, you should recognize these as Latitude/Longitude. Beginning to recognize the coordinate ranges that occur in the common projections and coordinate systems in your area will help you greatly as you move forward in your career.
If you are interested in learning more about projections and coordinate systems, this is a fantastic reference: A Working Manual (PDF) - John P. Snyder