[Xastir] Re: Starting with a scanned USGS 7.5" paper map....

[Brad Douglas]

On Sat, 2008-01-19 at 11:27 -0700, Tom Russo wrote:
> On Sat, Jan 19, 2008 at 12:12:22PM -0600, we recorded a bogon-computron collision of the <RiverRidge at CenturyTel.net> flavor, containing:
> > I am collecting the USGS topographic maps for  my area and I am fortunate 
> > in that all except one were on Libre Map.  The one that was missing, The 
> > good folks at UW Madison Geography Library had the paper map and scanned it 
> > for me.  So now I have a 454MB scanned image in tif format.
> 
> Nice.
> 
> > Setting aside the fact that for one map, I would probably be better off 
> > just buying the proper Geotiff map files............  How do I go from this 
> > scanned image to a georeferenced digital file?  Can I get there from here?
> 
> Yes, but you need a tool that you probably don't have yet.  There are two
> tools of choice, GRASS (http://grass.itc.it/) and QGIS (http://www.qgis.org/).  
> GRASS can do more with your data, but QGIS has a simpler georeferencing tool.  
> GRASS has a learning curve as steep that looks a lot like Everest, QGIS is
> a bit of a PITA to install but is comparatively easy to use.

+1.  My opinion is a bit partisan, being a GRASS developer. ;-)

GDAL should be all you need to convert file formats and do basic
warping.  It is very comprehensive, but lacks any GUI for GCP selection.

> The trick is to carefully select points of known coordinates (in the 
> coordinate system of the map, which in this case are probably UTM) and give 
> the georeferencer the locations of those points in the image (the USGS uses 
> the 16 lat/lon graticule points).  It then computes the affine transformation 
> from image coordinates to geographic coordinates and puts in the 
> necessary TIFF tags.

GRASS can help here (i.rectify) to select coordinate pairs for warping.

> It is a fairly involved process.  I might be willing to do it for you --- I've
> done it for a few other people on the list.  It just so happens that this
> weekend I've chosen to set aside a lot of time to do GIS work, so if you 
> put your data somewhere where I could grab it today, I'd take a look.  

Learning GRASS is a fairly involved process.  It is generally geared
toward research as opposed to ease of use.  I new wxPython GUI is
actively been developed and should be mature by GRASS v7.

One of these days, I really need to put together some tutorials geared
towards Xastir.  I'm usually too busy on other projects. :(

[snip]

> Yes, it does.  Georeferencing scanned images is tricky and time consuming, so
> it is expected that few will want to do it.  I've done it many times, and I
> try to avoid doing it if possible.  But with the right tools and a little
> care it can be done.

This largely depends on your image size (cell count), resolution and
GCPs.  It can take anywhere from a few seconds to hours.


-- 
73, de Brad KB8UYR/6 <rez touchofmadness com>