NY Times article on USGS benchmarking of altitude

[+The AIIM Team]

I am sure this has to be a repost but I did search.

 

NYT requires a subscription so here it is in it's full text with credit given.

 

April 22, 2003

Urban Heights

By C. CLAIBORNE RAY

 

. How is the altitude of an American city determined?

 

A. The United States Geological Survey lists the altitude of a National Geodetic Survey benchmark at or near the center of civic power, like city hall or the main post office. Such a point is chosen to offer some consistency, because a city like San Francisco can have a range of altitudes.

 

A Geodetic Survey benchmark is a metal marker about 3 1/2 inches in diameter, placed on a stable foundation like a building or bridge abutment. Its actual altitude is most reliably determined by a surveying technique called leveling, explained David Doyle, chief geodetic surveyor for the National Geodetic Survey.

 

"The technique is an optical observation made using a specialized piece of surveying equipment that does nothing more than determine differences in height," he said. "It uses a telescope with a very sensitive level bubble and two people holding rods marked with different values. By observing one, then the other, the difference in altitudes is read."

 

"There is actually one ground zero," and all of the 600,000 points in the survey's database refer to it, Mr. Doyle said. "It is a single benchmark at a place called Father's Point, at the mouth of the St. Lawrence Seaway." The point, which is tectonically stable, is called the North American Vertical Datum of 1988.

 

The public can find the value for any benchmark through the Web site www.ngs.noaa.gov.

The A.I.M. Team: Ali, Ivan and Lil Mikayla, Jacksonville, Florida

Equipment: Garmin eMap 32MB memory, Palm VIIx, Old College Eastport backpack, VW Jetta Wagon and Isuzu Trooper (hers)

[survey tech]

Mr. Doyle is widely considered to be the foremost expert on geodetic surveying. He has devoted his professional career to improving the quality and usefulness of our national control network, using state-of-the-art equipment and methods. He has lead the effort to refine the precision of the network through superior mathematical adjustments, vastly improving its value in the technological era we are now entering. The method of leveling he is describing here is known as differential leveling, which has been the conventional method prior to the dawn of GPS. It is still the most reliable and commonly used method, but the ongoing advancement of GPS technology will eventually replace it, probably at some time in the new century.

[+The AIIM Team]

Thanks, ST. I really thought this would be a repost, too.

 

Good to know what ground zero is for trivia's sake, too.

 

The A.I.M. Team: Ali, Ivan and Lil Mikayla, Jacksonville, Florida

Equipment: Garmin eMap 32MB memory, Palm VIIx, Old College Eastport backpack, VW Jetta Wagon and Isuzu Trooper (hers)

[WhistlingWind]

I imagine there'd be a benchmark at 'ground zero'. If there is it'd be a cool one to log!

 

WW

[+Tacguy]

Your reading my mind! Who can "point" us to the mark? Does it have some cool designation like "Ground Zero" or something?

[NothingBetterToDo]

I did some web searching, and here's what I found...

 

Unfortunately, this point is in Canada, so it won't be in the NGS database, and consequently, can't be "logged"

 

Father's Point is much more popularly known as "Pointe-au-Pere" since it's in the French-speaking province of Quebec.

 

The Canadian Geodetic Survey does have a page dedicated to this point.. known as the "Guardian of Elevations". There was some sort of special ceremony commenorating that point, and there are pictures of the monument, but I don't know if it's the monument itself that contains (or is) the "Benchmark", or if there's an actual survey disk nearby.

 

I'll do some more checking and maybe put up a page on my website with anything else I find; it appears that the Canadian Geodetic Survey requires you to create a login account. In the meantime, here's the site that contains some pictures of the monument and info on the ceremony:

 

http://www.geod.emr.ca/index_e/info_e/guardian_e.html

 

More to come hopefully...

 

[This message was edited by NothingBetterToDo on April 28, 2003 at 12:09 PM.]

[NothingBetterToDo]

Here's the datasheet - It's interesting to note that this is the "Guardian of Elevations", yet ironically there is no vertical data on the datasheet..

 

quote:

---

SITE IDENTIFICATION

 

Unique Number : 99L9000

Name : POINTE-AU-PERE

Established By : Canadian Hydrographic Service - F & O

Province : PQ

Prov. Identifier : None

NTS Map No : 022C09

 

STATION COORDINATES

 

Method : Scaled

Latitude : N48° 31' 01"

Longitude : W68° 28' 10"

Agency : Geodetic Survey Division - NRCan

UTM : Zone = 19 N = 5373883 m E = 539180 m

 

VERTICAL DATA

 

None

 

STATION MARKER INFORMATION AND LOCATION

 

Marker Type : Survey Plaque

Inspected in : 2000

Status : Good

Inspection Comments : None

 

Accessible by passenger car or light truck and a walk of less than 50 m

 

POINT-AU-PERE

 

"THE GUARDIAN OF ELEVATIONS" A COMMEMORATIVE MONUMENT.

 

French Location Description :

 

POINTE-AU-PERE

 

"LE GARDIEN DES ALTITUDES" UN MONUMENT COMMEMORATIF.

 

HISTORICAL COORDINATES NOTE: Coordinates listed below are no longer maintained by GSD and should be verified with your provincial agency before

use.

 

None

---

 

There are a couple additional pictures located about 3/4 of the way down this page. Note the contrete column directly behind the plaque which appears to have some kind of marker on top; although the datasheet lists the plaque itself as the monument.

 

I'm driving out east thataway this fall for a scenic vacation, maybe I'll drive up here and take some pictures

 

Update - NGS DOES appear to have a nearby reference to this mark.. it has PID AH9256, but neither this site nor NGS's site is able to bring up the datasheet.

 

A search on NGS based on the coordinates of the plaque put this point 3.3 miles from the plaque, with a designation of "Pointe Au Pere".. but no other info seems to be available.

 

[This message was edited by NothingBetterToDo on April 28, 2003 at 12:21 PM.]

[+MaxEntropy]

"Tectonically stable" must be a relative term since we are all riding around on huge, moving tectonic plates. I imagine that the site is more stable that, say California where the huge rock that makes up the Sierra Nevada mountains is rising about a foot a century.

It keeps the amateurs out of the surveying business after all because if the earth was perfectly round and completely stable it would be too easy wouldn't it?

 

Mickey

Max Entropy

More than just a name, a lifestyle.

[Kerry.]

Unfortuneately the good old level and staff isn't going to be entirely replaced anytime soon.

 

Geoid separation and gravity in an absolute sense for GPS heighting principles simply isn't adequate for many purposes.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go