[Qgis-user] OnTheFly Reprojections

[Andre Joost]

Am 21.02.2014 15:41, schrieb Jonathan Moules:
> Hi List,
> Two issues,
>
> First, I'm noticing that quite a few of the layers we have that are in
> British National Grid aren't actually treated that way when I load them
> into QGIS. This seems to be because they have a very slightly different
> definition than the one QGIS has.
> For example:
>
> +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
> +ellps=airy +towgs84=375,-111,431,0,0,0,0 +units=m +no_defs
>
> +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
> +ellps=airy +towgs84=446.448,-125.157,542.06,0.1502,0.247,0.8421,-20.4894
> +units=m +no_defs
>
> QGIS definition
> +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000
> +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489
> +units=m +no_defs
>
> Three different definitions for the same thing.

Not really the same. The first uses a three-parameter datum shift which 
is less accurate than the 7-paramrters used by the other ones.

Number 2 and three are almost identical.

>
> So my question:
> Is it possible to make QGIS recognise all the different definitions as one
> single definition?
>

No, better not.

Greetings,
André Joost