[Xastir] Ridge Radar images

[Rick Green]

On Fri, 25 Dec 2009, Tom Russo wrote:

> I think this is wrong:
>> Longitude = line5 - (line1 * 600) Latitude = line6 + (line4 * 550)
>
> it should be, I think:
>
> Longitude = line5 + (line1 * 599) Latitude = line6 + (line4 * 549)
>
> That's because the affine transformation would be, normally (preserving the
> sign of all the lines):
>
> worldcoordinate1(pixelx,pixely) = line1*pixelx+line2*pixely+line5
> worldcoordinate2(pixelx,pixely) = line3*pixelx+line4*pixely+line6
>
> The world coordinates of pixel (0,0) should be (line5,line6),
> and you're trying to compute the coordinates of pixel (599,549).
>
> I think you did mostly the right calculation even though you reported the sign
> wrong in the Longitude expression, but you used 600,550 as the lower-right
> pixel value wheras you're tying it to pixel 599,549 (correctly) in the
> geo file.  That would, if I'm not mistaken, be stretching your image slightly
> along the diagonal, shifting everything to the southeast.   Try changing the
> lower right tiepoint to -80.540772, 40.013450 and see if it's any better.
>
   Thanks, It moved the image infinitesimally in the right direction, but 
the apparent center is still closer to Port Huron than White Lake.

> Ideally, Xastir should have a means of using the worldfiles directly, instead
> of using the custom geo file technique --- but that's more coding for somebody.
>
> 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?

Rick, N8BJX