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 8, 16:14 -0700
From: Greg Rudzinski
Date: 2008 Jul 8, 16:14 -0700
Digital Camera Cel Nav Follow-up Photos of the sun taken at high altitude through a filter can be augmented to full screen on a laptop for determining minutes of arc (M.O.A.) per pixel. Example for 4 power zoom on a Canon PowerShot A560 : Solar diameter 07/07/2008 31.46' = .291 M.O.A. per pixel Pixel solar diameter 108 Using M.O.A. per pixel enables subsequent sun to horizon pixel counts to be converted to M.O.A. for a designated zoom power. On Jul 3, 4:31�pm, Greg Rudzinskiwrote: > � � �A timed digital photo of the sun at low altitude provides enough > information to generate a line of position. For this task I use a > Canon PowerShot A560 set at 3 power optical zoom, 7 mega pixel, and > ISO 800. The high ISO setting helps reduce camera shake on a moving > platform. > � � �The digital image of the sun (using a shade filter held out in > front of the camera covering the sun) is downloaded to a laptop, > cropped, rotated, and enlarged so that the sun has at least a 20 > millimeter screen diameter on the laptop. This limits altitudes to > about 6 degrees. Measurements are made using a ruler directly on the > laptop screen. 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. Convert minutes of arc to > degrees and minutes. Correct for refraction, dip, semidiameter > (temperature and pressure if necessary). Perform normal sight > reduction for the GMT of the sun's photo to get an azimuth and > intercept. > � � �Preliminary trials from the beach for altitudes between 4 degrees > and 6 degrees show the average LOP within 4 nautical miles of GPS. > Precision is limited by rounding to the nearest millimeter on laptop > screen measurements. > > EXAMPLE: > > Semidiameter 15.73' (by navigation computer) > Full diameter 31.46' > GPS Latitude 34 09.8'N �Longitude 119 14.0'W > Height of eye 8 ft. > GMT 02H 44M 00S Date 07/03/08 > > Laptop measurements in millimeters: > > Horizon to lower limb 226mm > Sun diameter 27mm (measured horizontal to horizon to avoid refraction > differences between upper and lower limb) > > (226)(31.46) = 263.3' = 4 degrees 23.3' (hs) uncorrected > � � 27 > > Corrected and reduced to an azimuth 294.8 and intercept 0.4' away (by > navigation computer) > (not corrected for temperature and pressure) > > The Nautical Almanac A4 table shows an additional +0.5' correction for > a pressure of 29.9 in. and a temperature of 70 F > > iPHOTO was used to enlarge and crop sun images. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---