NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Digital Camera Celestial Navigation
From: Frank Reed
Date: 2008 Jul 04, 05:11 -0400
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 --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---