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Details for Benchmark: HT0496

N 37° 31.767 W 122° 21.733 (NAD 83)

Altitude: 305.6

Coordinates may not be exact. Altitude is VERTCON and location is SCALED. (more info)

Location:
In SAN MATEO county, CA View Original Datasheet
Designation:
P 386
Marker Type:
bench mark disk
Setting:
massive structures
Stability:
Probably hold position/elevation well.

Must Read!

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As of about a month and a half ago work began on reconstruction of the Crystal Springs Dam roadway. The original roadbed was demolished, taking the two benchmark disks in the process.

I attempted to contact the agency overseeing the project in the hopes of making a physical recovery of the two disks in order to return to the NGS. Was greeted warmly but was not allowed to recover the two disks. I was informed that the agency would be recovering the disks and that it may "re-install" the two disks after the new roadbed was completed.

I informed a contact at the NGS regarding this situation and we agreed that without following a specific recovery process, that the marks, while they may be physically restored to their original locations, would be worthless to a subsequent survey. I made one more attempt to make a physical recovery, spoke with the agency's field rep regarding specific steps that would need to be followed in order to properly restore the two benchmark disks after reconstruction work had been completed but was again informed that "it would be taken care of".

Sadly, this rare "twin" disk monument is destroyed.

very easy to spot

[Photos:]
photoHT0496 reset

Found P386 and P386 reset while doing the Earthcache.

Still there...

[Photos:]
photoP386 (near)
P386 (near)

Found it as well.

Found along with the P 386 Reset Marker.

[Photos:]
photoHT0496 (P 386)
photoP386_Reset

So P 386 was replaced by P386 RESET 1970 (HT0497) because a rod cannot be placed on benchmark due to guardrail? Whatever...more benchmarks for me!

[Photos:]
photoHT0496
photoHT0496 View

Found both benchmarks while parked nearby today for the EC. Unfortunately I had to reset my POS Sidekick 2 for the Nth time and when I did it apparently lost all of the many pics I took there today. Glad I don't need pics to log the cache, but still annoying as heck. Both BMs are in good condition.

Got it while checking out the Earthcache.

Found it accidentally while visiting the Crystal Springs earthcache today.

[Photos:]
photop386

Found it only because PDX55 stepped on it and found it first.

This was a surprise bonus while looking for the Crystal Springs Dam cache. First time I've found a benchmark without specifically looking for it.

[Photos:]
photoBonus Benchmark

No problems finding this one and the P 386 RESET less than 2' away. It's worth noting the inaccuracy of SCALED coordinates by comparing this to HT0497, the waypoint for the RESET version.

Found while on snarfari (http://www.markeroni.com). I looked for it because beckerbuns had said it was round here, and so it was.

Found while looking for a cache. Pictures to come later today.

[Photos:]
photoGPSr and benchmark.
photoBenchmark

Out for a nearby hike, see bthomas's entry.

N 37° 31.777 W 122° 21.752

A two-fer BM find. Located as described, on the concrete highway wall at the north end of the highway causeway over the 1890 dam.

[Photos:]
photo2 for 1
photoDam !
photoEarthquake proof, 1890 achievement
photoIf you seek... look about you

Visited station and recovered disk. It was found as described.

[Photos:]
photoDisk
photoBoth disks

USCGS Bm reads - P386 / 1936 - New BM (HT0497) is reset of this one, as barrier rail added decades after bridge built made it impossible to put a survey rod on BM.

This bridge is built atop the Crystal Springs Dam, which when completed in 1890 was (and still is!) the largest concrete-block dam in the world. Originally 120 ft high, in 1911, it was raised to 149 ft. The lake in front of you is but one in a string of four in the San Andreas Valley, containing a collective total of 29 billion gallons. The dam rode out the '06 quake, but if it ever failed, it would flood San Mateo from Coyote Point to Foster City.

Documented History (by the NGS)

01/01/1936 by CGS (MONUMENTED)
01/01/1973 by NGS (GOOD)
DESCRIBED BY NATIONAL GEODETIC SURVEY 1973 4 MI SW FROM SAN MATEO. ABOUT 0.7 MILE SOUTHWEST ALONG THIRD AVENUE FROM THE SOUTHERN PACIFIC COMPANY RAILROAD STATION AT SAN MATEO, THENCE 3.2 MILES SOUTHWEST ALONG CRYSTAL SPRINGS ROAD, THENCE 0.1 MILE SOUTH ALONG SKYLINE BOULEVARD, IN THE TOP OF THE NORTH END OF THE WEST CONCRETE WHEEL GUARD OF BRIDGE 35-05 OVER CRYSTAL SPRINGS DAM, 15.6 FEET WEST OF THE CENTER LINE OF THE ROAD, 2.7 FEET SOUTH OF THE NORTH END OF THE WEST BARRIER RAIL AND 1/2 FOOT HIGHER THAN THE ROAD. NOTE-- A ROD CAN NOT BE HELD ON THE MARK AS THE BARRIER RAIL IS DIRECTLY OVER IT.

Control Text

  • The horizontal coordinates were scaled from a topographic map and have an estimated accuracy of +/- 6 seconds.
  • The NAVD 88 height was computed by applying the VERTCON shift value to the NGVD 29 height (displayed under SUPERSEDED SURVEY CONTROL.) The vertical order pertains to the superseded datum.
  • The geoid height was determined by GEOID99.

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