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

[Otto Dassau]

Hi Carson, 

many thanks for your help. I added your improvements and tried to use the
correct terminology for the whole section. Maybe you can have a final look at
the updated PDF? 

http://download.osgeo.org/qgis/doc/manual/qgis-1.0.0_user_guide_en.pdf

And now that we already improved the terminology, it would still be great
if more people can have a look at the projection part again, and check if the
section describes the current projection/CRS support in QGIS correctly.

thanks a lot
 Otto

On Tue, 20 Jan 2009 11:00:01 +0000
Carson Farmer <carson.farmer at gmail.com> wrote:

> 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