[Qgis-user] use raster + GCP points + ?? as data

[Alister Hood]

> Date: Tue, 12 Jul 2011 08:53:14 +0200
> From: jr.morreale at enoreth.net
> Subject: Re: [Qgis-user] use raster + GCP points + ?? as data
> To: <qgis-user at lists.osgeo.org>
> Message-ID: <e9a67c9a9cbd67859e5f1d072eea6a68 at enoreth.net>
> Content-Type: text/plain; charset=UTF-8; format=flowed
> 
>  On Mon, 11 Jul 2011 23:27:14 -0700, Alex Mandel wrote:
> > On 07/11/2011 10:30 PM, Richard Duivenvoorde wrote:
> >> Hi all,
> >>
> >> I have these pile of scanned old maps and can georeference them
with
> >> the
> >> good working georeferencer plugin in qgis.
> >>
> >> But do NOT want to really warp the image. Why should we want to
> >> store
> >> TWO huge rasters, maybe loosing 'data' contained in the 'original'?
> >>
> >> So my question: is it possible (for Qgis, but preferebly also with
> >> other
> >> tools (wms's) to load data from the original raster, probably
> >> accompanied with a txt file with GCP points (or maybe some other
> >> format
> >> containing both gcp points and projection/transformation
> >> information)?
> >>
> >> Is there some 'common' way for the the world of gis?
> >>
> >> Thanks for any thoughts,
> >>
> >> Richard Duivenvoorde
> >
> >
> > A world file is the common way. It describes the coordinates of the
> > corners, how many units per pixel and what skew if any. See the
> > wikipedia page for details.
> >
> > I believe the georef plugin can save the a world file for you.
> >
> > Thanks,
> > Alex
> 
>  It doesn't enable for all the transformation methods.

In many situations this wouldn't be a problem.
Another issue though is that IIRC the georeferencing plugin can't create
world files with rotation.

>  Richard > this choice has a huge trade-off with most formats, the
time
>  to resample the raster each time the users move the canvas will not
be
>  accepted by them. Storage is cheaper and does not interfere with your
>  users. If the size is a huge issue and not the quality of
>  representation, you can reduce the original resolution.

Personally I haven't been able to detect _any_ slow-down with on-the-fly
raster reprojection, which did surprise me.  But obviously this can't be
a problem if the project coordinate system is the same as the scanned
map.


Alister