# NavList:

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

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From: Greg Rudzinski
Date: 2008 Jul 4, 11:53 -0700

```Frank,

Thanks for the pixel measuring tips. I will give it a try. G

On Jul 4, 2:11�am, frankr...@HistoricalAtlas.net wrote:
> 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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