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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Which diameter of the sun in digital photos ?
From: Marcel Tschudin
Date: 2009 Aug 22, 17:12 +0300
From: Marcel Tschudin
Date: 2009 Aug 22, 17:12 +0300
The reason for the difference has finally been located. However, before explaining it, I first would like to answer George's questions: > Another question relates to that 6x zoom. Is Marcel comparing the Sun and > the landmarks on the same photo, so that we can be sure that the zoom level > is the same for both? Or are they taken at different times, in which case we > need to be convinced that setting the zoom to (presumably) its maximum of x6 > is reproducibly the same on each occasion. No, those are different photos. In the case of the sun the scale is derived statistically from measurements done in over hundred photos. In the case of the landscape feature several photos have been made at different days and from different locations (and even at different times of the day). Nobody is perfect: yes it very rarely happens that the zoom is not completely set up to the mechanical stop; this can later easily been noticed since the size of the object is suddenly too small. The different photos from the landscape feature compared well with each other. > Next is the angle subtended between two landmark points. Marcel tells us the > spacing between them has been determined by Google. Does that correspond to > old-fashioned paper mapping? How precisely has his own observation post been > located, as that's a factor in the calculation? Yes, the cause of the problem is here. Let me explain how it arose; it will also show the type of difficulties one has to overcome at certain places on this planet. Since a long time I'm looking for a reference feature with known height. For this I was looking for detailed maps, but couldn't find a shop to buy them. It's only recently that I learned that it is illegal in this country to sell such maps or even hand out copies of them. A professor suggested that I indicate him the places of interest which he then would look up for me. I'm still waiting for his answer.... In this situation I started to look for a 'feature' where I could estimate the height from relationships between horizontal and vertical dimensions using GoogleEarth and GoogleMap for measuring the absolute horizontal dimensions. I finally found a small island where several private photos could be found on the Internet showing the island from different directions; to this collection I added one of mine. A governmental Internet page even mentioned its height as being 90m. From the analysis of 6 photos resulted finally a height of 87m which agreed quite well with the 'official' height. From my own photo, the only one where the viewing angle was exactly known, resulted a height of 86m. As a further verification I calculated now the height in my photo also using my pixel scale derived from the sunset photos and the distance measured with GoogleEarth and obtained only 79m. Looking also at my other photos of this island showed that the scale is 182.6 pixels per degree compared to 198.2 as derived from the sunset photos. The good agreement of the estimated height with the 'official' value made me question what could be wrong with the scale obtained from the sun. Not finding a possible reason why the pixel scale derived from the sunset photos would be so much wrong, I started finally to question the measurements of the distances which were made with GoogleEarth. In order to verify this I made in the meantime photos of a large building in 1.7km distance which has distinct features and is also well visible on GoogleEarth. Out of a careful analysis a scale resulted of 208.7 pixel per degree! This makes it now clear: measurements done with GoogleEarth don't have the accuracy as the shown value may allude. Btw: the mean value of the two scales obtained with GoogleEarth (182.6 and 208.7) is 195.7 this agrees already much better with the 198.2 (+/-1.5) as obtained from the sun. Thank you, George, Greg and Bill, for your help. Your reflections and suggestions have very much been appreciated. Marcel --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---