[Qgis-user] Projection

[Sergio Zárate Pedroche]

By setting the project preferences to the projection and datum of  
your files (which may be seen in the file's  preferences after  
opening them) all vector and raster maps having the same projection  
and datum will overly well. When some of the files wanted to be used  
differ in projection, using "on the fly projection" (set in  
preferences for the project) may give the proper display, but: you  
may have to change the file's projection settings in each file's  
preferences (CRS).
An exception to this I have experienced is when files are in latlon  
projection and the coordinates are given in degrees (not decimal  
degrees). This is the case of some maps supplied by INEGI  
(governmental agency in Mexico). In that case, I have found that you  
have to either manually change the coordinates by transforming to  
decimal degrees in the data base (a cumbersome procedure for large  
files), or (in polygons when coordinates are "invisible" in database)  
re-projecting them is needed, something that is more complicated.  
GRASS has a module for that but it is tricky to set the regions, etc.
Thus, first thing I do is setting the proyect's preferences and then  
to find out which files will be a problem. For example, when using a  
Landsat image in UTM with datum WGS84 zone 13N, but that spreads also  
to zone 14, the only thing to do is to set the project's maps that  
are in zone 14 (in this example, the zone of interest) to zone 13,  
changing the preferences of files in zone 14N to 13N (with on the fly  
projection set on). That causes the maps to be overlaid properly, but  
with a little slant in your coordinate axis when displayed; and, in  
problem files as such those described above, ask a friend that is an  
expert geographer to reproject them for you!
A fool-proof reprojecting tool would be of great help for users with  
little know-how about changing projections.

Dr. Sergio Zárate Pedroche
José María Arroyo 228
Morelia, 58341, Michoacán
Tel. (443) 352-5307
Cel. (443) 438-6212
szpedroche at yahoo.com.mx



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-user/attachments/20081026/480def79/attachment.html>