[GRASSLIST:5595] Re: how to import pre-projected map?

[Glynn Clements]

Christian Keßler wrote:

> Just taking my first steps on grass... What I'm missing so far is the 
> way to import a scanned, pre-projected map (Mercator projection, known 
> parameters) and convert it into a grass map (as I understand this would 
> be n,e angular coordinates - is this right?).

GRASS uses arbitrary coordinate systems, categorised as either X-Y,
Lat-Lon or Geographic (e.g. Mercator, UTM etc). AFAICT, most real use
uses a geographic coordinate system (rather than X-Y or Lat-Lon).

The exact projection is normally chosen to either match the source
data, or for minimal distortion (e.g. a conical projection for a
"wide" region or transverse Mercator for a "tall" region).

> I've taken the tutorial steps on converting a scanned map:
> - importing of scanned map into a temporary x,y location
> - converting to a second location (with projection setup same as paper 
> map projection) via i.points/i.rectify. To my understanding this step 
> (at least for 1st order transformation) only corrects for basic 
> transformation error (rotation, scaling, translation). What is still 
> missing is the conversion from Mercator projection to direct angular 
> coordinate raster map data.
> 
> What I would i.rectify to do is to look up the projection in the target 
> location, and do an additional transformation to correct for this. like 
> that: a) scaling/rotation/translation for scanning error b) 
> elipsoid-projected x,y coordinate to n,e angular coordinates (in my 
> case of mercator projection in target location).
> 
> So i.rectify didn't bring me there so far. What's the way to go for 
> calibration/conversion from a pre-projected map to a calibrated grass 
> raster map?

First decide upon the projection in which you wish to work, and create
a suitable location.

If the data to be imported matches your location, no re-projection is
required. You just import the map, and (if necessary) move it with
e.g. r.region.

If you need to import data from a different projection, create an
addtional location whose projection matches that of the data, import
the data into that location, switch to your work location, then
project the data with e.g. r.proj.

If you need to import data which doesn't use a known projection (e.g. 
aerial/satellite imagery), then the data needs to rectified using e.g. 
i.points/i.rectify.

For a scanned map, I suspect that whether you need to perform the
rectification step depends upon the accuracy of the scan. If there is
no rotation, and the scale factor is known, rectification is probably
unnecessary.

-- 
Glynn Clements <glynn.clements at virgin.net>