NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: marine sextant on land
From: Frank Reed
Date: 2008 Oct 30, 00:50 -0400
From: Frank Reed
Date: 2008 Oct 30, 00:50 -0400
Paul H, you wrote: "In some directions I have a reasonable horizon -- mountains in the background but flat land nearby. But mostly the view is obstructed by buildings, trees, and a fence. That's all right. I already know my position, so the altitude intercept directly indicates the error in my horizon estimate. From that I'm learning where the horizon lies on these obstructions." You can also use converging lines on any building, road, or other structure that's been constructed horizontal. For example, suppose you have a multi-story that's been built recently and presumably with proper verticals. As you look down the length of the building, the lines of the floors and windows extended appear to converge at some point in the distance. That point rests on the true horizon. If you take a photo of the scene and carefully draw lines along those building lines, you can find the exact spot on the background mountains or whatever is in the distance. From then on, you have an accurate horizon for that spot. Even train tracks or rows of street lights will work in areas where the land is flat, but don't use lines farther away than a couple of thousand feet since at greater distances the curvature of the Earth comes into play. Incidentally, instead of finding the horizon, if you live among tall buildings, you can also find the zenith with fairly good accuracy by extrapolating the converging point of vertical lines (but be aware that some tall structures are built with slight convergence anyway for esthetic architectural reasons and then the trick doesn't work). -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---