[Qgis-community-team] sec 8.2.1 - CRS automatically set by 1st loaded layer

[Carson Farmer]

Hi Otto et al.,
> usually not, but I think this is an exception and since we haven't started the
> translation yet, I changed it in trunk, in the tagged version and the pdf. 
>
> I changed quite a lot in that chapter to make the projection part hopefully
> clearer and it would be great, if some people can have a closer look at the
> chapter "Working with Projections" pp. 71. and check if I haven't made any 
> mistakes. You find the PDF here:
>
> http://download.osgeo.org/qgis/doc/manual/qgis-1.0.0_user_guide_en.pdf
>   

I realise I'm coming in at a relatively late date, but reading through 
the section 'Working with projections', I notice some of the terminology 
is not quite right. In many cases, you refer to all CRS as projections, 
when in fact the projection is really only one component of the CRS. The 
coordinate (or 'spatial' as I prefer) reference system is a combination 
of the ellipsoid used, the datum associated with that ellipsoid, and 
some sort of geographic, or projection coordinate system. Of course the 
projection also always has a geographic coordinate system associated 
with it. There is a pretty good blog entry available here that does a 
nice job of presenting this information:

http://www.sharpgis.net/post/2007/05/05/Spatial-references2c-coordinate-systems2c-projections2c-datums2c-ellipsoids-e28093-confusing.aspx

As an example of what I'm talking about, the first couple of paragraphs 
should really read something like:

8 Working with Projections

QGIS allows *users* to define a global and project*-*wide CRS 
*(Coordinate Reference System)*
for layers without a *pre-defined* CRS. It also allows *the user to 
define custom coordinate reference systems*, and supports on-the-fly 
(OTF) projection of vector layers. All these features allow you to
display layers with different CRS*,* and have them overlay properly.


8.1 Overview of Projection Support

QGIS has support for approximately 2,700 known *CRS*. *Definitions for 
each of these CRS* are stored in a SQLite database that is installed 
with QGIS. Normally you do not need to manipulate the database directly. 
In fact, doing so may cause projection support to fail. Custom *CRS* are 
stored in a user database.
See Section 8.4 for information on managing your custom *coordinate 
reference systems*.
The *CRS* available in QGIS are based on those defined by EPSG and are 
largely abstracted
from the spatial_references[1] table in PostGIS version 1.x. The EPSG 
identifiers are present in the
database and can be used to specify a CRS in QGIS.
In order to use OTF projection, your data must contain information about 
its coordinate *reference* system or you have to define a global, layer 
or project*-*wide CRS. For PostGIS layers QGIS uses the spatial 
reference identifier that was specified when the layer was created. For 
data supported by OGR, QGIS relies on the presence of a format specific 
means of specifying the CRS. In the case of shape-files, this means a 
file containing the Well Known Text (WKT) specification of the CRS.
The projection file has the same base name as the shapefile and a prj 
extension. For example, a
shapefile named alaska.shp would have a corresponding projection file 
named alaska.prj.

8.2 Specifying a Projection

QGIS *no longer sets* the map CRS to the coordinate *reference* system 
of the first layer loaded.
When you start a QGIS session with layers that do not have a CRS, you 
need to control and define *the* CRS definition for these layers. This 
can be done global*ly*, or project-wide in the...

where *bold* items have been added
[1] note the use of *spatial* reference here ;-)

I don't have the time right now to go through the rest, but I think it's 
important to make sure that these things are correct. Many people will 
be using QGIS as an initial foray into GIS, and IMHO it's important that 
they start out using the correct terminology.

Cheers,

Carson