NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant vs. Digital Camera
From: Marcel Tschudin
Date: 2008 Aug 9, 12:06 +0300
From: Marcel Tschudin
Date: 2008 Aug 9, 12:06 +0300
It might be of interest to mention at this point how I do the photos of the sunsets and how I do the scaling of the pixels. Note that I do not put a filter in front of the lens to dim the sun, however, depending on the height of the sun I reduce the exposure by 2 units. The reason for this is the following: my camera indicates the time of the photo only in hours and minutes. In order to have the time in seconds I hold in addition to the camera also a watch between the fingers. When taking a timed photo the viewer and the watch have to be observed alternatively in order to have the sun in the picture, the horizon horizontal and the exact time. As a result of this my photos of the sun in a clear sky give often the impression of being slightly overexposed. Not knowing how good the photos are for measuring, i.e. for finding the scale (pixels per degree) I wondered how well they would reproduce the annual change of the sun's diameter (about 4 percent). For finding this scale the sun's diameter in degrees has been calculated for each day where photos have been made. The diameter in pixels was determined from the mean of 4 measurements of the right limb minus the mean of 4 measurements of the left limb. "First iteration": For this I selected photos where the sun seemed to be sufficiently dimmed by thin clouds, haze or smog, thus photos where I thought that the picture should provide a good measure. At this stage I did not yet filter any colour in the photo before measuring the pixels. This "first iteration" was done with something over 70 photos from different days over a year. The median of those measurements provided a first estimation of the scale (pixels per degree). Note that as from here the difference between measurement of the sun's diameter and predicted size of the sun's diameter (in pixel) could be calculated with the median of the measurements and the calculated value of the sun's diameter in degrees. The result showed that about 20 percent of the photos were outliers, with mainly too large diameters. "Second iteration" The photos of the outliers were again measured and - if still too large - removed from the set. "Third iteration" The photos have now been filtered measuring only either the green or the blue image. That filter is selected where the difference to the expected size is minimal. Applying the filters increased now the useful photos to, at the moment, over hundred. (There are still much more to be analysed.) Statistics: The statistics has now been arranged in such a manner that the median is taken only from those measurements where the difference expected minus measured is less than a given limiting input value. At the moment I have this value set to 1 pixel (compared to the sun's diameter, changing between 104 to 108 pixels within a year). I think it might actually be better to increase this limit in future slightly to may be 1.5 pixels Result, Scale factor: a) Limit: +/- 1 pixel Number of meaurements: 121 Scale (Median): 198.2 Std. Dev: +/- 0.9 b) Limit: +/- 2 pixel Number of meaurements: 128 Scale (Median): 198.2 Std. Dev: +/- 1.1 Correction for under exposure or overexposure: The difference between expected sun diameter to diameter measured in the photo is used for correcting the measurement of the upper limb which I use for my purpose, i.e. for measuring the refraction. Overexposure: This analysis revealed that the severe overexposures are related to special phenomena like ducting or omega sun which occur dominantly in spring and autumn. In all such cases the sun is much brighter than normal thus leading to photos with a sun diameter which is roughly up to about 5 percent larger. Camera: Panasonic DMC-LZ5 Optics from Leica with optical zoom 6x (Zoom is set always at maximum, i.e. at mechanical stop) Selected internal scene menu: "Landscape" Size of photos set at 1MB resulting in photos slightly less than 0.5MB Exposure value is reduced during the setting event starting with 2 units when the sun is fully above the horizon and then gradually reducing it to zero correction when the upper limb is near the horizon. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---