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Frank, you wrote:
"For a given level of zoom in your camera, the angular size of pixels (at
> least near the center of the field of view) is a fixed quantity which you
> could measure quite accurately. Place a meter stick 34.38 meters from your
> camera and perpendicular to the line of sight. Take a photo. Then the
> centimeter marks will be minutes of arc. The "pixels to minutes of arc"
> ratio will not change (unless you select a different zoom setting). You
> could get really fancy and measure the slight variation in angular scale
> across the field of view --there's a little distortion like this."

Why not measuring the sun's horizontal diameter? This measure is
(almost?) not affected by refraction. The value can be calculated
quite accurately and only changes a few percent during the year. One
only has to be careful that the sun is not overexposed.

Further on Frank wrote:
> "There's something else you can try with photo navigation. Have
> you ever heard Ken Gebhart talk about getting Sun altitudes by the
> refractional flattening of the Sun? Suppose you take a digital photo of the
> Sun a few degrees above the horizon when the horizon is obscured somehow. If
> you very carefully measure the vertical diameter of the Sun and compare it
> with the horizontal, you can work backwards from the refraction tables (or
> formulae) to determine the Sun's altitude. It works, but it requires very
> accurate measurements. Since many modern digital cameras have fairly high
> optical zoom magnifications, you might be able to get good results."

This doesn't look to me as a reliable procedure. There are often
situations where differential refraction produces odd shapes like
"ballooning" (lower limb stretched down) or "squeezing" (lower limb
pressed towards the middle).

BTW: using a digital camera has yet an other advantage. Sometimes the
sun can't be seen with the eyes. However, if the clouds in front of
the sun are sufficiently thin the sensor of the digital camera still
can see the IR image of the sun. In those cases it's possible to take
"shots" without actually seeing the sun (except on the screen of the
camera).

Marcel

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