GISVM's a great idea, but it has one issue that users of NAD27 and NAD83 datums (among others) should know about.
The official Debian and Ubuntu package repositories have a
broken package for the cartographic projections library that has been built without including the datum shift grids for US, Canadian, French and New Zealand datums. This impacts conversion of GIS data between those datums severely. In fact, if one attempts to use "cs2cs" to do coordinate transformations between NAD27 and NAD83 datums, one finds not only that there is an incorrect datum shift, but that there is
no datum shift applied:
cs2cs +proj=latlon +datum=NAD27 +to +proj=latlon +datum=NAD83
will demonstrate the issue --- type in any lat/lon (such as 106dW 35dN) and you'll find it does no datum shift at all. Since proj.4 is used by just about every GIS program included in GISVM, none of them will do correct datum conversion between NAD27, NAD83, NZGD49, or other datums that require the datum shift grids.
This issue has been reported in the Ubuntu bug tracker as
https://bugs.launchpad.net/ubuntu/+source/proj/+bug/368065 --- and has not been acted on in any version of Ubuntu yet. This issue has been present for many releases of Ubuntu, and the bug report was opened in April of 2009.
There is an unofficial "PPA" (Personal Package Archive) that one can use to get a fixed Proj.4 package. The path to this "UbuntuGIS" PPA is in comment #3 of the bug report. It is possible to update the proj.4 install in GISVM from this unofficial PPA. Follow the link in comment 3 and view the installation instructions. It is a bit tricky, and I found that following the instructions through synaptic package manager didn't work cleanly -- the Software Sources page didn't seem ever to accept the additional sources, so I had to edit /etc/apt/sources.list directly to get them in there, then follow the instructions from there to get the GPG key. Once the key was in place and the new source lines added to the sources list, synaptic could find the upgradable proj package.
To summarize the instructions on the PPA web site: you must add
deb
http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu hardy main
deb-src
http://ppa.launchpad.net/ubuntugis/ubuntugis-unstable/ubuntu hardy main
to /etc/apt/sources.list, then
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 123456677
where 123456677 is to be replaced by the actual key value that appears on the PPA page after you click "Technical details about this PPA"
Then use Synaptic Package Manager to reload package data, and you can click "Mark for Upgrade" next to "proj". Then click "Apply" and the new package will be installed. Since all the other packages use this as a shared library, all of them will magically be fixed --- QGIS, GRASS, etc. will do coordinate transformations between NAD27 and NAD83 correctly after this procedure is performed.
If you are in an area where datum shift grids are required for datum conversions (such as North America where NAD27 and NAD83 data are still common), using the GISVM with this broken proj.4 package will give incorrect results when attempting to convert data from one datum to the other.
Hope this helps someone.
I really wish the Ubuntu package maintainer would update the package and backport it to Hardy, but it's been months since this has been reported and there's still no movement on it. Users of GISVM who need correct datum conversion in North America (and other locations such as New Zealand) are left to rely on the unofficial UbuntuGIS software source at this point.
It would probably be a Very Good Thing if the next version of GISVM were distributed with the updated proj.4 library rather than the broken one.
