That's so a user can apply datum transformations from one coordinate system to another (ie, NAD27 <-> NAD83 <-> NAD83(CSRS)). They are usually compiled into Proj4 through grid shift binary files (.gsb) often available from federal or provincial and local agencies.
For example, in British Columbia, a lot of data comes in EPSG:26910 (UTM Zone 10N / NAD83) or EPSG:3005 (BC Albers / NAD83). But converting either of those to EPSG:3153 (BC Albers / NAD83,CSRS) which uses the Canadian Spatial Reference Standard/System requires a custom "transformation" that can only be applied when using NTv2 grid shift files. Or old NRCAN Atlas of Canada data can come in a Lambert Conformal Conic projection in the NAD27 datum. This is a long awaited feature and I'm very excited for it! This way I don't have to drop back to command line to do my vector data transformations. http://webapp.geod.nrcan.gc.ca/geod/tools-outils/index.php ftp://ftp.gdbc.gov.bc.ca/pub/world/NTv2.0 http://forums.arcgis.com/threads/4267-Custom-transformation-nad83-to-nad83-CSRS-for-British-Columbia SaultDon On Fri, Jan 3, 2014 at 2:38 AM, Lene Fischer <[email protected]> wrote: > Hi, > > Using QGIS weekly. > > When I open a new layer with a different CRS I get a dialog “Select datum > transformations for layer ..” > > I´ve already set “Enable on the fly” – > > Just clicking OK it works- but I don´t understand this dialog – can anyone > help for an explonation? > > Regards > > > > > > > > Lene Fischer > > Associate Professor > > > > Department of Geosciences and Natural Resource Management > > University of Copenhagen > > > > MOB +45 40115084 > > [email protected] > > > > > > > > > > > > > > _______________________________________________ > Qgis-user mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/qgis-user _______________________________________________ Qgis-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/qgis-user
