GIS Scripting - Desktop GIS: Mapping the Planet with Open Source Tools - page 249

Figure 13.5: Results of loading earthquakes into PostGIS
add them back into his GPS and integrate them into his bird database
(Harrison likes to revisit the exact spots year after year). No problem,
you say—just put the latitude and longitudes from the paper map into
the GPS. But Harrison, being an up-and-coming GIS savant, realizes
that his paper maps are in a different datum than his GPS uses. The old
maps were created using the NAD27 datum, where his GPS now uses
WGS84. If he just plugs the latitudes and longitudes into the GPS, the
locations could be off by as much as 400 feet or more. Since Harrison
has a really accurate tool for determining latitude and longitude on his
paper maps, he's not happy with a datum problem.
It turns out it's a simple matter to transform coordinates using PostGIS
and GeoRuby. You don't even have to store anything in the database.