# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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From: Frank Reed
Date: 2008 Jul 04, 05:11 -0400

```Greg, you wrote:
"Measurements are made using a ruler directly on the laptop screen."

Does your software show pixel positions (x,y) when you move your mouse
around? That should be a little more accurate than measuring with a ruler.
If not, just a little fishing around for graphic software should turn up
something that does display the exact pixel location.

And:
"The almanac diameter of the sun is a given at
approximately 32 minutes of arc. Knowing this will allow the ratio of
the laptop screen measurements to yield an altitude of the sun's lower
limb above the horizon in minutes of arc."

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.

And:
"Perform normal sight reduction for the GMT of the sun's photo to get an
azimuth and intercept."

Very nice! 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.

-FER

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