Papa-Bear-NYC Posted November 10, 2008 Share Posted November 10, 2008 (edited) A year or two ago, I decided to specialize in my benchmark hunting. Until then I was assiduously finding every NGS mark in my home area (New York City) and then for a good part of the 3 seasons (= not Winter) I'd find time to head to New England for some hiking and mountain climbing. Some of thse treks took me to the Canadian boundary or across state boundary lines. So while I was at it, I would log such survey markers that I might find on a mountain top, or such boundary monuments that I should find on GC and NGS - that is if the markers of monuments happened to be in the databases (which was not always the case. Then I thought, why not specialize and combine, and spend what free time I had for my several hobbies on hikes and climbs specifically planned to find the maximum number of interesting markers on my treks. So instead of Peakbagging at times, and benchmark hunting at times (with the occasional overlap) I would do Peak-benchmark bagging. As I told the on-line hiking group I belong to, instead of chasing another peak list (I had already done the popular ones) I would pursue "Bugs, Benchmarks and Boundaries" (the bugs come gratis ) Then in the summer of last year, as I was planning a trip to Maine and New Brunswick as a vacation with my wife, I read Holograph's excellent Wiki on the Eastern Oblique Arc, EOA for short. This described the monumental project done by the Coast Survey from about 1830 to about 1900 which surveyed the entire East Coast to an unprecedented level of accuracy. And in so doing, helped establish the "Figure of the Earth" (the exact shape of the earth's spheroid in North America) which laid the foundations of modern datums. Pretty great stuff, of which I was totally ignorant until then. Here's Holograph's Wiki: EOA Wiki. And ... (drum role) ... many of the survey markers from the 1830s to the 18570s and beyond are still sitting there, waiting for someone to find them. So I obliged. This is the fifth of a series of my reports on this forum of the progress of this quest along the EOA. Some of you may recall the earlier threads, but I've linked them here in case you want to go back and check them out. They basically outline my recoveries, and non-recoveries with background material, maps and photos included. My vacation intersects the Eastern Oblique Arc October 1, 2007 The Eastern Oblique Arc crosses Massachusetts April 25, 2008 The Eastern Oblique Arc meets the Borden Survey July 28, 2008 The Eastern Oblique Arc in Western Maine and New Hampshire August 20, 2008 The Epping Base Line and Base Net (Click on the map for an inteactive Google map) Put most simply, when you do a triangulation, you set up your stations (usually on mountain tops) and measure angles to other stations. Then you determine the accuracy (basically you do each measurement many times and take averages and determine average errors) and solve the triangles using the rules of geometry (remember from high school, the sum of the 3 angles of a triangle is 180 degrees), Then after taking into account the curved surface of the earth and other errors, you end up with a series on interconnected triangles which span the area of your survey. This was a lot of computation in pre-computer days and sometimes took months or years. But there are two things missing: the orientation of the triangles (which way is north) and the scale (how big are the triangles. Careful observations of the north star and other astronomical observations gets your the first, but for the second (how big are the triangles) you need to physically measure the exact length of a side of one or more triangles, and thus set the scale for the whole survey. This was easier said than done. The longest they could hope to measure a line was about 5 or 6 miles. Since many of the triangles had sides that were sometimes 50 or 60 miles long (look at the map at the top of the thread - the line from Mt Desert to Cooper is about 92.87 kilometers = 57.70 miles, and the longest line in New England, Mount Pleasant to Ragged Mountain (Maine) is 135.16 kilometers = 83.98 miles!), any small measurement error would get multiplied by a huge factor. So they measured the "Baseline" (as it was called) carefully. And I mean very, very carefully. They used metal bars, in this case 6 meters long and aligned them along a carefully laid out route. They compensated for temperature and inclination and height above sea level. When they could, they used a straight stretch of a railroad (as was done in Massachusetts) , or along a beach (as was done in Long Island, NY). But as we know, in Maine there are no beaches and the railroads don't go straight , so they found an area that was long and relatively flat. The blueberry barrens of Cherryfield, Maine (the "Blueberry capitol of the world"; so says the sign) fit the bill and that was where they laid out the line. The area was known as the Epping Plains or the Epping Blueberry Barrens and hence the name of the baseline. The distance was measured over 8 days in July 1857, and the accuracy of the 5.4 mile line was calculated to be on the order of 1 part in 500,000. The final corrected measurement from 1857 was 8715.9422 m with a probable error of 0.0158 m. Recent GPS measurements actually confirm this impressive accuracy. The Base Line as laid out and graded across the Epping Plains (from CGS S.P. No. 7) Of all the baselines measured in the 19th century (of which there were a couple of dozen) by the Coast Survey this is the only one which still exists - that is to say which has both ends still in existence and in good condition. In all other cases over 150 years of "progress" has obliterated most traces of these early survey marks. I found one end of the Massachusetts baseline (See here), the other end is long lost. Both ends of the Long Island Baseline (on Fire Island) are long gone. Thankfully, Maine is one state in the East where progress has not run amok (not yet anyway). The "Base Net" is the system of stations around the base Line (Tunk, Burke and Pigeon) which serve to bridge the gap between the relatively short base line and the primary triangulation, which tends to have very long lines. The mathematics was simplified by using this intermediate step. Due to the limted resources for computations, it would be impossible to simply solve the many equations for the primary network plus the base line. So they first solved the equations for the base net (56 equations - using 10 place logarithms), and then considered the quadrilateral surrounding the base net (Cooper-Howard-Mt Desert-Humpback) as fixed, and used it to set the scale for the rest of the primary net in the area. Then of course they had to match that up with the next section which had it's own baseline (Massachusetts), and on and on. The calculations involved which would probably take the average PC minutes (assuming you had lots of memory) took literally years when done manually. A posed picture of the placement of the measuring apparatus. Perhaps this is the very photo Bache referred to in the quote below (from CGS S.P. No. 7) Here are the primary sources of what they did. Luckily these reports, some over 150 years old, are on-line courtesy of NOAA. It's extremely interesting reading ("On Monday the work was, in part, interrupted by the arrangements for photographing the apparatus, on Tuesday by a fog and on Wednesday by showers in the beginning of the day" - Bache in the CS Annual Report for 1857) and they were written by some of the greats of 19h century surveying. But they are hefty and technical, so you have been warned. Bibliography CS Annual Report for 1857, Appendix 26 "Notes on the measurement of a base for the primary triangulation of the eastern section of the coast of the United States, on Epping Plains, Maine, by A. D. Bache, Superintendent United States Coast Survey" CS Annual Report for 1864, Appendix 14 "Report on the method of reduction , and results of the connexion of the Epping Base Line with the primary triangulation in the Eastern States by Charles A. Schott, assistant, United States Coast Survey" CS Annual Report for 1865, Appendix 21 "Results of the primary triangulation of the coast of New England, from the northeastern boundary to the vicinity of New York" And finally the full report, putting it all together: CGS Special Publication No. 7 (1902) "The Eastern Oblique Arc of The United States" In the next note I will give summaries of my recoveries of these stations, and in the final note on my progress. The short story is that I'm substantially finished in the EOA stations from Maine down to Massachusetts. Edited November 11, 2008 by Papa-Bear-NYC Quote Link to comment
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