[Xastir] Re: Bounding boxes

[Tom Russo]

On Mon, Oct 13, 2003 at 09:04:57PM -0700, a Mr. Richard Feyler of Fort Lee, New Jersey <archer at eskimo.com> writes 'Dear Rosanne Rosannadanna':
> On Mon, 13 Oct 2003, Tom Russo wrote:
> 
> > Um... since the geotiff files are originally scanned from paper maps
> > that are lat/lon rectangles, and small ones at that, isn't it usually
> > the case that the transformation is mostly a rotation, and not much of
> > a stretching operation?
> 
> No.  You're thinking of the paper USGS maps that you buy.  Take a
> look at one of the DRG files in a normal TIFF viewer.  They have been
> distorted into a UTM projection.  

Actually,  I think the paper maps *are* distorted, too --- they are UTM 
projections of a 7.5' lat/lon band, and that's what the collar info
even says.  I'm looking at a USGS paper quad right now, and seeing in the 
bottom left corner that it states "Projection and blue 1000-meter ticks: 
Universal Transverse Mercator."  It's not just the DRGs that have been
projected into UTM, it's the original source map.

You're right, of course, the DRGs are distorted, and I'm just too used to 
looking at maps from the area I'm in, where the quads are still *roughly* 
rectangular.  Go farther north than where I live and these effects are more 
noticable.

I believe that the only real difference between the paper maps and the 
DRGs is that they've been rotated  --- the paper maps are made so the 
N/S neatlines are as parallel to the edge of the paper as they can be,
whereas the DRGs are rotated so the UTM grid is parallel to the pixel grid.

-- 
Tom Russo   KM5VY    QRPL #1592   K2#398    http://www.swcp.com/~russo/
Tijeras, NM DM64ux   SOC #236     AHTB#1    http://www.qsl.net/~km5vy/
 "The older I grow the more I distrust the familiar doctrine that age
  brings wisdom." -- H.L. Mencken