NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Digital Camera Celestial Navigation
From: Greg Rudzinski
Date: 2008 Jul 4, 14:23 -0700
From: Greg Rudzinski
Date: 2008 Jul 4, 14:23 -0700
Frank, I was able to locate the pixel count under photo info while in the edit mode of iPhoto. Precision is improved by a factor of four over the ruler method. Neat! 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 --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---