Geography Reference
In-Depth Information
EPSG
If you are wondering about the EPSG notation that has popped
up in the Well-Known Text of coordinate systems, it represents a
dataset of coordinate systems formerly distributed by the for-
mer European Petroleum Survey Group. In 2005 the European
Petroleum Survey Group was absorbed into the OGP Surveying
and Positioning Committee. The OGP continues to distribute the
dataset.
The Geodetic Parameter Set contains a unique code for each
coordinate system, as well as details about the projection.
Many OSGIS applications can use the EPSG code as input
when doing transformations. If you have PROJ.4 installed, you
should have a copy of the .
epsg
file on your system. This file is
a simplified subset of the EPSG definitions and maps the EPSG
number to the corresponding PROJ.4 parameters. The full EPSG
database and documen
tation is available from OGP.
∗
∗.
If on-the-fly transformation isn't an option, you will have to manually
transform the data to get it into a projection you can use. If you are
using PostGIS, you can create a SQL view of your data that transforms
the geometries using the OGC
transform
function. You would then load
the view into QGIS or your other application, and the transformation
would be done automatically, at the expense of a bit of performance.
For example, if we have a
towns
layer in our PostGIS database that we
would like to use with another layer that is geographic, we can create a
view to do the job.
1
First let's look at the schema of the table:
gis_data=
# \d towns Table "public.towns"
Column | Type | Modifiers
------------+-----------------------+--------------------------------------
gid
| integer
| not null default
|
| nextval('towns_gid_seq'::regclass)
name
| character varying(20) |
class
| character varying(35) |
pop
| integer
|
shape
| geometry
|
...
gis_data=
#
1.
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