ArtMan Posted June 3, 2013 Posted June 3, 2013 A few weeks ago, The New Yorker published a very brief review of a new book by Marguerite Holloway, The Measure of Manhattan (Norton 2013), a biography of surveyor John Randel, Jr., who laid out the street grid in Manhattan. So I got the book, and I got a big surprise midway through it. The book begins in Central Park on a hot June day in 2004, when a then-graduate student, Reuben Skye Rose-Redwood — I love that name! — plus a surveyor and a social geographer were on a mission. The three stopped repeatedly, "checking GPS coordinates, scrutinizing maps, examining a rock sitting behind a fence and a PLEASE KEEP OUT sign, and rummaging through a patch of periwinkle until they became worried they would attract onlookers or, worse, someone who worked in the park." Sound familiar? Yes, Rose-Redwood and his team were hunting for a benchmark, one set by Randal in 1811! Reading of their quest, I was struck by a severe case of déjà vu. Or, if your prefer, been there, done that. Randal set monuments at each city intersection he surveyed, including those in what became Central Park decades after his survey.The author then takes us back to the 19th century, as she describes the painstaking effort John Randal made, including commissioning high-precision instruments of his own devising to aid in his survey work in Manhattan and elsewhere. Randal's commission included, of course, preparation of maps based on his survey, and many of the intersections he marked on the ground are marked on his maps with an elevation. But what exactly did the "29.99" feet marked at Fifth Avenue and 59th Street refer to? Feet above sea level? Feet above some arbitrary mark? Author Holloway explains, beginning on p. 140. I hope she will not mind if I quote at some length the section, in which an individual well-known to this forum makes an appearance. One Map's Datum Is Another Map's Doom It took Rose-Redwood two days to record every elevation from the farm maps that he had not recorded on his earlier visits; there were 600 elevations in total; fewer than half the intersections had them. He was also missing a crucial piece of information. At the intersection of Eighth Avenue and 65th Street, for instance, Randel recorded the elevation as 59.06 feet. But in relation to what? If Rose-Redwood entered the elevations into a computer program and recreated the topography, he would get peaks and valleys only in relation to one another. The figures would have no meaning in terms of Manhattan's current landscape unless he was able to correlate the old data with today's. Rose-Redwood had hit the datum. [p.141] "Datum" is the term used by engineers, surveyors, navigators, GPS experts— by anyone involved in interpreting or traveling landscapes—fora reference system that locates points in space. A datum anchors information about position by grounding a surveyor's measurements in relation to the earth's physical spheroidal reality and by providing a common coordinate system. Datums are based on bench marks, which serve as the referents to which other measurements are compared: how much higher or lower is something than a certain bench mark. A vertical datum,as one would expect, describes elevation and is typically based on sea level. The U.S. version is called the North American Vertical Datum of 1988, or NAVD 88. It replaced the National Geodetic VerticalDatum of 1929, which averaged information from twenty-six sea-level monitoring stations. Over decades, regional variations in sea level and shifting land-based bench marks undermined the 1929 datum. Experts at the National Geodetic Survey—the government agency that concerns itself with location, location, location—established the NAVD 88 using satellite information. The satellite data are more comprehensive and can correct for variations in sea level due to, for example, gravitational pull. Horizontal datums provide latitude and longitude, or coordinates—How far north or south or east or west, as opposed to how high, how low. The current horizontal datum is called the North American Datum of 1983,or NAD 83. It was preceded by two others, those of 1879 (which came to be called the 1901 datum) and 1927, which were based on then current scientific understanding of the shape of the earth and on surveying markers set in the nineteenth and early twentieth centuries. The origin point for the 1927 datum, a metal disk embedded in a concrete slab, was fixed on a ranch in Kansas, near the geographic center of the country. With the advent of GPS, the datum went global. Whereas one could visit and touch the 1927 datum, the North American Datum now resides very close to the center of the planet. "Unless you happen to be [p.142] Jules Verne, you can't get there," says David R. Doyle of the National Geodetic Survey, who is described by some surveyors as the "godfather of geodetic surveying in the United States." The NAD 83 is linked to an international model of the earth: the Geodetic Reference System 1980, or GRS 80. That model, based on mathematical calculations, is called the reference ellipsoid and serves as the best contemporary estimate of the true figure of the earth. National and global datums have evolved as models of the earth changed, as understandings of sea level and regional variations thereof changed, as technologies changed—a process called datum shift. The same is true of regional and local bench marks, and many cities have a series of datums. Chicago, for instance, used the average level of Lake Michigan instead of average sea level, the sea being impractically far away. Elevations were thus higher or lower than the lake, which was the zero value. This Chicago datum can now be related to the national sea-level-based datum if needed: in 1929 it was 579.48 feet above the national zero value. New York City is dense with datums, dizzyingly so. Seven vertical datums and at least thirteen coordinate, or horizontal, datum systems existed even before state and national datums were introduced, according to Scott S. Zelenak, a surveyor with the Port Authority of New York and New Jersey and an expert on the region's datums. So when engineers or others describe a place, they must specify which datum they are using. Making one datum correspond to another can be straightforward, as in the case of Lake Michigan. But such correspondence can be mind-bendingly complicated when a city's history resides in many systems, maps, and measurements, all using different datums and all made in different ways with different instruments or techniques. Doyle, who strives with Randelesque persistence for datum perfection and unification,says he wishes he kept a bottle of single malt in his desk for the times when people call about New York City. "I can hear that whimper in their voice when they say, 'I am working in [p.143] New York City and ... ,'"he says. After the September 11 attacks, datum profusion posed problems for the Federal Emergency Management Agenc yand other agencies, he explains. "You would hear things like, 'If I have heights in this coordinate system, do I add 1.3 feet or subtract 1.9 to get it to correspond to my GPS coordinates?'" Doyle says his mentor worked for decades to devise algorithms that would convert every New York City datum into every other— and into the national datums. It still hasn't happened. I think many followers of this forum will find much of interest in The Measure of Manhattan, including an appreciation of the challenges faced by 19th century surveyors. ~Art~ Quote
+T.D.M.22 Posted June 4, 2013 Posted June 4, 2013 *Snip* ~Art~ I'm not good with names, but the name David R. Doyle is familiar. I'm guessing that's who you're to referring to in which an individual well-known to this forum makes an appearance. So who is that? Quote
68-eldo Posted June 4, 2013 Posted June 4, 2013 (edited) Mr. Doyle is an esteemed member of this forum and the Senior Geodesist of the National Geodetic Survey. You can read more on Google here: http://goo.gl/J1XXB Sounds like a fascinating book. Thanks ArtMan Edited June 4, 2013 by 68-eldo Quote
foxtrot_xray Posted June 5, 2013 Posted June 5, 2013 Mr. Doyle is an esteemed member of this forum and the Senior Geodesist of the National Geodetic Survey. Ex-Senior Geodesist, I believe. I thought he finally retired Jan of this year? Quote
+shorbird Posted June 10, 2013 Posted June 10, 2013 I believe that several of the 1811 benchmarks were found. They were small iron rods set in the indigenous rock. Quote
+secondgunman Posted June 13, 2013 Posted June 13, 2013 That sounds like an interesting book. I'll have to pick it up. Thanks!! Quote
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