Projections and Coordinate Systems - Desktop GIS: Mapping the Planet with Open Source Tools - page 134

System Conversion, on page 194 and Section 11.2 , Raster Conversion,
on page 196 .

9.3 The PROJ.4 Projections Library

PROJ.4 is a cartographic projections library that is used in many, if
not most, open source GIS applications, both on the desktop and on
the Internet. It was originally developed by the USGS and is now main-
tained by a group of volunteers.

You may be wondering why we are mentioning a library—turns out
that PROJ.4 also comes with some handy utilities for experimenting
with projections and doing interactive transformations:

proj and invproj
Performs forward and inverse transformations for a large number
of projections. With your projection parameters in hand, you can
perform a forward calculation using proj (geographic to projected)
or an inverse calculation using invproj (projected to geographic).
Neither of these utilities does datum shifts.

cs2cs

Performs transformations between coordinate systems, including
datum shifts. With cs2cs , you supply the parameters for both coor-
dinate systems, specifying which is the target or “to” system.

geod and invgeod
Performs forward and inverse Great Circle (geodesic) transforma-
tions. This allows you to calculate latitude, longitude, and back
azimuth given a starting point, azimuth, and distance. You can
also determine the azimuths (forward and back) and distance be-
tween two known points.

nad2nad
Performs datum conversions between the North American 1927
and 1983 datums. The same conversions can be accomplished
using cs2cs .

Let's look at an example. Say Harrison has loaded up some of his bird
data in his favorite desktop application and it contains a DRG. If you
remember correctly, DRGs come in a UTM projection. Harrison is curi-
ous about a couple of bird observations on his map. When he moves
his cursor over the points, he sees big numbers for the coordinates in
the status bar of his application. He knows his DRG is in UTM Zone 6,