Projections and Coordinate Systems - Desktop GIS: Mapping the Planet with Open Source Tools - page 135

NAD27 datum, but he wants to know the approximate geographic coor-
dinates for the point. 2 PROJ.4 can quickly answer his question. First
he has to know the parameters for the UTM projection in order to do the
conversion. There are several ways to do this—perhaps the easiest is to
look up the proj string in the . epsg file that is installed with PROJ.4. 3
On a Linux system you'll find this in /usr/share/proj/epsg . To locate the
projection, you can open the . epsg file in your favorite text editor and
search, or you can use grep from the command line:

$ grep -C 1 -i "utm zone 6n" /usr/share/proj/epsg| \
grep -i "nad27"
<26705> +proj=utm +zone=5 +ellps=clrk66 +datum=NAD27 +units=m +no_defs <>

# NAD27 / UTM zone 6N
<26706> +proj=utm +zone=6 +ellps=clrk66 +datum=NAD27 +units=m +no_defs <>

Here we used grep to search for “utm zone 6n” and told it to print one
line on either side of the match ( -C 1 ) and ignore the case ( -i ). Since
we wanted only NAD 27 projections, we piped the output to grep again
to show only those lines containing “nad27.” From the result we get a
couple of things: the EPSG number, in this case 26706, and the string
that we need to use with proj . If you can't find the . epsg file on your sys-
tem, you can search for your projection using the tools on the Spatial
Reference website. 4 Entering “nad27 utm zone 6n” as the search string
will quickly find the projection. You can then copy the proj parameters
from the website.

Now that Harrison has the projection parameters, he can convert his
coordinates from UTM Zone 6N to geographic using invproj :

$ invproj +proj=utm +zone=6 +ellps=clrk66 +datum=NAD27 \
+units=m +no_defs
312244.49 6795460.41
150d30'W 61d15'N

Harrison entered the coordinates that he read from his status bar
(312244.49, 6795460.41) and got the results in degrees and minutes
(150d30'W, 61d15'N). Lucky for Harrison, his birds like to roost on nice
clean coordinates. Just to convince himself that this works, he plugs

2. If you think there is more than one way to do this, you are right—depending the
software you are using.
3. The . epsg file included with PROJ.4 does not include a number of datums outside the
United States and Canada. You can find a complete list of datums in your GRASS install
in etc/datum.table .
4.

http://spatialreference.org