[Xastir] georeferencing maps on osx?

[Tom Russo]

On Tue, Dec 28, 2004 at 10:13:40PM -0500, we recorded a bogon-computron collision of the <shadow at dementia.org> flavor, containing:
> On Tue, 28 Dec 2004, Tom Russo wrote:
> 
> >Ouch.  Well after the 100th or so you'll be so familiar with the procedure
> >it'll be a piece of cake to do the remaining 800.  I can't think of any 
> >simple
> >way to automate it with rasters that don't start out georeferenced and that
> >you scanned yourself.  If you find one, I'd be delighted to hear of it.
> 
> If I only did 4 points i could cheat and software-recognize the neatline 
> corners. I'd still have to give values but it would be better than 
> clicking. And it would be rough, but for maps this old it's only going to 
> be an approximation anyway.

Yeah, you could probably do that.  All depends on how accurately you need them
geocoded.  You'd probably have to figure out for yourself how i.points
passes the control points to i.rectify --- the program itself is designed
to be interactive and makes you click points, and provides no mechanism for
reading them in from a file.  However it does write out files to the 
imagery group with the points in them, so you could reverse engineer.  Would
definitely be faster to do the reverse engineering than to click 900*5 points.

4 points doesn't give you a whole lot of least-squares freedom to get the
affine transformation for a best fit, but it's sufficient for *a* fit.

> >1)  Scan map with appropriate resolution to give the meters-per-pixel you 
> >want.
> >  (for a 1:24K map 300dpi leads to about 2m/pixel)
> 
> They're all 200dpi, iirc, and there's not a chance i'm rescanning; i only 
> scanned about 1/3 of them in the first place.

Not necessary to rescan.  You just have a different target RMS error to
deal with.

-- 
Tom Russo    KM5VY     SAR502  DM64ux         http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236 AHTB#1 http://www.qsl.net/~km5vy/
 "When life gives you lemons, find someone with a paper cut."