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A Benchmark Not In The List?


jbeale

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While walking in the Stanford foothills I noticed a metal rod set in concrete just off the path, at N 37.403491 W 122.168803 according to my Garmin Summit GPS. There is an inscribed X on the top surface of the rod, not resolved in this low-res photo. There were two small flags in the ground, one on either side, but it was otherwise just sitting in the grass. I thought surely this is a "known" mark so I would get some feeling for the accuracy of my GPS unit by comparing my observation with a datasheet I was hoping to find here. But when I typed the coordinates into the "find a benchmark" page, the closest reference is about a mile away. Should I have expected to find it here? Is there a more complete collection of surveyed marks I could use?

 

SurveyMarkFeb18-2006.jpg

 

The white concrete pad of this marker is actually visible in Google Earth as a small white dot at these coordinates, 6 meters to the NW of the path.

Edited by jbeale
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jbeale -

 

Welcome to benchmark hunting.

 

There are gozillions of marks that are located all over the place but that have not been documented to the standards required for inclusion in the NGS' database (and the Geocaching database is a snapshot of the NGS' database as of about the year 2000). The NGS database has 736,000 documented marks, but I suspect that there are probably 5,000,000 geodetic and cadastral control points in the US that are not in the database.

 

Undocumented (aka no-PID) marks (disks only) can be informally documented at the Things>Benchmarks>Recovered US Benchmarks sub-category at the Waymarking site. So far, almost 400 no-PID disks have been documented as Waymarks.

 

The thing you found looks very interesting, and probably is used by local authorities or land surveyors for control purposes. But the fact that it's not in the Geocaching database is not surprising: there are lots of similar things out there.

 

If you want to compare your GPS' accuracy against a known point, I suggest you do a radial search on your home coordinates and locate a benchmark that has ADJUSTED horizontal coordinates and that has been recovered by another benchmark hunter. The location of marks with ADJUSTED coordinates is documented to a fraction of an inch. The difference between the actual and documented location for marks with SCALED horizontal coordinates can be several hundred feet.

 

I recommend that you read the "Me First" thread pinned at the top of this section, read the FAQ, and then jump back in here if you have more questions.

 

Good hunting,

Will

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Ditto what Will said. Unfortunately, most of the benchmarks near the mysterious object you found do not have exact Latitude and Longitude coordinates. (These are indicated by the word SCALED in the description.)

 

One exception might also be one of the most interesting locations. It is about 1.3 miles from the object you photographed. Here is the Data Sheet from the National Geodetic Survey computer, with the Latitude and Longitude given in Degrees, Minutes, and Seconds:

 

National Geodetic Survey, Retrieval Date = FEBRUARY 20, 2006

HT3270 ***********************************************************************

HT3270 DESIGNATION - RATEL 1961

HT3270 PID - HT3270

HT3270 STATE/COUNTY- CA/SANTA CLARA

HT3270 USGS QUAD - PALO ALTO (1997)

HT3270

HT3270 *CURRENT SURVEY CONTROL

HT3270 ___________________________________________________________________

HT3270* NAD 83(1992)- 37 23 47.66689(N) 122 11 23.56944(W) ADJUSTED

HT3270* NAVD 88 - 91. (meters) 299. (feet) SCALED

HT3270 ___________________________________________________________________

HT3270_MARKER: DD = SURVEY DISK

HT3270_SETTING: 0 = UNSPECIFIED SETTING

HT3270

HT3270 HISTORY - Date Condition Report By

HT3270 HISTORY - 1961 MONUMENTED USGS

HT3270 HISTORY - 1975 GOOD NGS

HT3270 HISTORY - 1988 GOOD LOCENG

HT3270

HT3270 STATION DESCRIPTION

HT3270

HT3270'DESCRIBED BY US GEOLOGICAL SURVEY 1961 (WRC)

HT3270'LOCATED ABOUT 4 MI. (AIRLINE) S. OF MENLO PARK AND ABOUT 0.5

HT3270'MI. SE. OF LADERA SHOPPING CENTER.

HT3270'

HT3270'TO REACH FROM THE USGS OFFICE IN MENLO PARK, DRIVE NE. 0.1 MI. TO

HT3270'MIDDLEFIELD RD., TURN LEFT ON MIDDLEFIELD RD. FOR 0.3 MI. TO

HT3270'RAVENSWOOD AVENUE, TURN LEFT ON RAVENSWOOD AVENUE FOR 0.6 MI. TO

HT3270'EL CAMINO REAL, TURN RIGHT ON EL CAMINO REAL FOR 0.1 MI. TO SANTA

HT3270'CRUZ AVENUE, TURN LEFT ON SANTA CRUZ AVENUE FOR 2.5 MI. TO ALPINE

HT3270'RD., TURN RIGHT ON ALPINE RD. FOR 2 MI. TO A RD. FORK AND GATE

HT3270'AT S. END OF LADERA SHOPPING CENTER, TURN LEFT OFF ALPINE RD.,

HT3270'PASS THROUGH GATE CROSS LOS TRANCOS CREEK AND TURN RIGHT AT RD.

HT3270'FORK FOR 0.25 MI. TO STANFORD UNIVERSITY RADIO TELESCOPE

HT3270'INSTALLATION. STATION IS 10 FT. S. OF S. END OF WALK

HT3270'ALONG N.-S. LINE OF RADIO TELESCOPE ANTENNAE.

HT3270'

HT3270'STATION MARK--STANDARD TABLET SET IN CONCRETE POST PROJECTING

HT3270'1.5 FT. ABOVE THE GROUND, STAMPED RATEL 1961.

HT3270'

HT3270'A 4 X 4 IN. BRASS DISK SET IN CONCRETE FLUSH WITH THE GROUND

HT3270'MARKS THE INTERSECTION OF THE N.-S. AND E.-W. LINE OF RADIO

HT3270'TELESCOPE ANTENNAE.

HT3270

HT3270 STATION RECOVERY (1975)

HT3270

HT3270'RECOVERY NOTE BY NATIONAL GEODETIC SURVEY 1975 (RDM)

HT3270'FOUND IN GOOD CONDITION.

HT3270

HT3270 STATION RECOVERY (1988)

HT3270

HT3270'RECOVERY NOTE BY LOCAL ENGINEER (INDIVIDUAL OR FIRM) 1988 (PS)

HT3270'RECOVERED IN GOOD CONDITION.

 

Notice the word ADJUSTED beside the coordinates. This assures you the coordinates will be "right on the money", with far more accuracy than a Consumer-grade GPS can deliver.

 

-Paul-

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