[Xastir] Ridge Radar images

[Tom Russo]

On Fri, Dec 25, 2009 at 12:45:36PM -0500, we recorded a bogon-computron collision of the <rtg at aapsc.com> flavor, containing:
> > I would not be surprised to see an ellipse instead of a circle --- Xastir's
> > coordinate system is known as the "Platte Carre" or "Equidistant Cylindrical"
> > projection --- this distorts the real world, and the distortion is of just
> > the form you're seeing.  According to "An Album of Map Projections (USGS
> > Professional Paper 1453), page 22, regarding the distortion:
> >
> >   Infinitessimally small circles of equal size on the globe (indicatrices)
> >   are ellipses except along the equator, where they remain circles.  Areas of
> >   the ellipses also vary.  Ths, there is distortion of both shape and area.
> >
> 
> Is that also why the Tigermaps appear quite distorted when you zoom out to 
> a continent-wide view?

That is because Tigermap online images are in Sinusoidal projection, and
Xastir makes no attempt whatsoever to reproject the images. Xastir merely asks
the TIGER server for a map that has the right corner points, and displays it
using the same sort of "stretch and translate" operation that we're using
for any other GEO map.  This is totally wrong (because the transformation
from Sinusoidal to Xastir's coordinate system is not a linear affine 
transformation), and the wrongness becomes more apparent the larger the area 
you choose.

-- 
Tom Russo    KM5VY   SAR502   DM64ux          http://www.swcp.com/~russo/
Tijeras, NM  QRPL#1592 K2#398  SOC#236        http://kevan.org/brain.cgi?DDTNM
  In some cultures what I do would be considered normal. 
                                  -- Ineffective daily affirmation