http://blog.opengeo.org/2010/06/14/prj2epsg/
Prj2EPSG
June 14th, 2010
Once upon a time, a group of smart people got together to define a common
standards base for geographic map services, a “Web Map Service” specification,
if you will.
They wanted their map services to be interoperable, but different maps can be
rendered using differentprojections, and in order to overlay one map onto
another, they needed to know (and advertise) the projections of both.
There was an existing standard for representing map projections, called
“well-known text” (which is also, confusingly, the name that describes a
standard for representing geometries) but it was quite verbose. Who, after all,
could remember this:
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,
AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]
More importantly, how would this fit cleanly into a URL?
Fortunately, there already existed a large database of commonly used map
projections: the EPSG database. This provided a single numeric ID for each
common map projection. So it was decided that all map services must advertise
their projection using a unique number defined by an authority, and set EPSG as
the first authority. And so ESPG:4326 came into the world (WGS84 geographic
coordinates) along with ESPG:26910 (NAD83 UTM Zone 10 North), and many others.
But, unlike me, most GIS practitioners haven’t memorized the EPSG database. So
they frequently ask questions like “what is the EPSG number for Oregon
State-Plane South?” and “how do I find the EPSG number for this shapefile?” One
could search spatialreference.org, a site for understanding spatial reference
systems. My own answer used to be a fairly unhelpful set of directions for
doing a text search of the PostGIS SPATIAL_REF_SYS table:
SELECT srid, srtext FROM spatial_ref_sys WHERE srtext ILIKE '%oregon%';
But today, I can provide a much simpler answer: Use prj2epsg.org. With
prj2epsg.org, you can paste in full well-known text descriptions, you can type
in shorter keyword searches, and you can even read a .prj file directly.
This free public service is provided by OpenGeo and our cloud services provider
SkyGone. The code is naturally all open source and the service is built on top
of the same GeoTools library that is at the heart of ourOpenGeo Suite.
And now we’re all hopefully one step closer to living happily ever after.
Tags: epsg, prj, shapefile, site, well known text, wkt
This entry was written by Paul Ramsey on Monday, June 14th, 2010 at 10:00 am
and is filed under Products. You can follow any responses to this entry
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