[Xastir] Ridge Radar images

[Tom Russo]

On Fri, Dec 25, 2009 at 10:01:29AM -0800, we recorded a bogon-computron collision of the <mcdermj at xenotropic.com> flavor, containing:
> Rick --
> 
> I wrote a Perl script to do this for me a while ago and posted it to the list.  This is what it comes up with for the DTX radar:
> 
> URL		http://radar.weather.gov/ridge/RadarImg/N0R/DTX_N0R_0.gif
> TIEPOINT	0	0	-86.4032277471369	45.3865516246449
> TIEPOINT	600	550	-80.5309854264693	40.0036628306996
> IMAGESIZE	600	550
> REFRESH		60
> TRANSPARENT	0x0
> PROJECTION	LatLon
> 
> They look very similar to your numbers.  I haven't loaded it up in Xastir, but for the Oregon and Massachusetts area radars that I've loaded up before, I haven't noticed any eccentricity in particular.

I just looked over the Xastir code for .geo processing and now see that your
(Jeremy's) geo file and my own should be equivalent.

Xastir uses the top left tiepoint to place the image, and uses the bottom-right
to compute the scaling.  My bottom-right tiepoint at 599,549 and yours at
600,550 would yield the same scaling.  Rick's would be off a tiny bit, because
he used the lat/lon of 600,550 but assigned it to pixel 599,549.

As Rick noted, however, the difference shouldn't be huge.

I thought perhaps the issue might be one of the number of digits of precision
Rick's using, but when I compared his 5-digits to our full precision numbers,
the differences would amount to a few dozen feet, not the large distance 
Rick is reporting.

So I don't know for sure why Rick is seeing things offset to the east so much.

When I use either Jeremy's or my own geo files, the positioning of the DTX
radar image appears to be the same as the positioning of the radar data on
the Ridge web site images to within eyeball norm.  The images are slightly
elliptical as expected, but not dramatically so.

-- 
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