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ArtMan

The Measure of Manhattan

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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.

O
ne
M
ap's
D
atum
Is
A
nother
M
a
p
's
D
oom

 

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

sur
veyor'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 coordi
nates—
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 Refer
ence 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 varia
tions 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 coordi
nates?'" Doyle says his mentor worked for decades

to devise algo
rithms 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~

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*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?

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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 by 68-eldo

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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? :)

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I believe that several of the 1811 benchmarks were found. They were small iron rods set in the indigenous rock.

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