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Details for Benchmark: HV2647

N 38° 47.350 W 077° 21.750 (NAD 83)

Altitude: 252.27

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

Location:
In FAIRFAX county, VA View Original Datasheet
Designation:
M 45
Marker Type:
bench mark disk
Setting:
massive structures
Stability:
Probably hold position/elevation well.

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3:33 PM Sunday 22 December 2002. There was no trace of the signal tower or concrete foundation mentioned in the 1934 recovery description. But while looking, I noticed the raised, level residential driveway heading east, about 200 feet south of the underpass. It looked like a railroad bed. Bill was exploring a trail that headed west from about the same area. When I gave up the benchmark hunt and went to catch up with him, I immediately recognized the trail was a continuation of a railroad grade from the driveway. I checked the topo map to confirm. Yes! We stumbled upon the original right-of-way of the Orange & Alexandria Railroad. Wow! So we headed west (toward Clifton) on the old grade to explore and see if we could find the remains of the bridge on the old (low) grade at the river about a half a mile west. We also expected to see where the current (high) grade crosses over as well. We got to what was left of the old grade, one bridge abutment on the east side of the river but nothing on the west side. Even the old grade disappeared. We were confused, and speculated about Civil War and/or flood effects on the line for awhile then continued. Next we went to find the bridge that carries the current, high grade across the river (it's a small river about the size of Rock Creek, but I don't want to call it a creek). Anyway once we got on the high grade we couldn't find its bridge. We went further west and still couldn't find it. We studied the topo map again. We shoulda seen the bridge by now. Then we got it! The Railroad had cleverly diverted the river such that instead of building two bridges across it (like the original low grade) they didn't need any! Fascinating! We then hiked onward to where the second bridge would have been and confirmed the theory. That is why we could not see any remains of the low grade, it had been scooped up to make way for the new river, and the spoils used to build up the high grade line as well as divert the river.

If you want to see this graphically, go to Geocaching.com waypoint GC9A00 The Bunny Man Cometh by Billcall or benchmark HV2647 and click on Topozone map and print it using Topozone print feature. Locate the Southern Railroad on the printout. That is the current high grade. Reference the Meter Scale on the bottom of your printout. So if you look at the 1000 meter mark on the scale & then move up to that point on the railroad line you will see the location of the Geocache, Benchmark and underpass. Now look just below the railroad referencing from 700 to 900 on the meter scale. The area with few contour lines is the flood plain of the river but here you can see a few contour lines that indicate the low grade rail line. On the low grade at 650 meters is where the old bridge was. On the high grade at 600 meter is where it looks like the river goes under the railroad, loops around on the north side then comes back out at the 250 meter mark. In reality the river stays south of the railroad from 600 to 250 on its new, man-made path.
By the way, at the 450 meter mark on the high grade we found (I think) the concrete foundations of the signal tower mentioned in the description of the benchmark but there were no markers.
PKPaul.

[last edit: 12/22/2002 7:38:49 PM PST]

Documented History (by the NGS)

01/01/1934 by CGS (MONUMENTED)
DESCRIBED BY COAST AND GEODETIC SURVEY 1934 1.6 MI E FROM CLIFTON. 1.6 MILES EAST ALONG THE SOUTHERN RAILWAY FROM THE STATION AT CLIFTON, FAIRFAX COUNTY, 0.2 MILE WEST OF MILEPOST W 25, 150 FEET WEST OF AN UNDERPASS, AT SIGNAL TOWER 25.2, IN THE TOP OF THE NORTHWEST CORNER OF THE CONCRETE FOUNDATION, AND 6 FEET SOUTH OF THE SOUTH TRACK. A STANDARD DISK, STAMPED 252.824 M 45 1934.
01/01/1941 by CGS (GOOD)
RECOVERY NOTE BY COAST AND GEODETIC SURVEY 1941 RECOVERED IN GOOD CONDITION.

Control Text

  • The horizontal coordinates were scaled from a topographic map and have an estimated accuracy of +/- 6 seconds.
  • The orthometric height was determined by differential leveling and adjusted by the National Geodetic Survey in June 1991.
  • The geoid height was determined by GEOID99.
  • The dynamic height is computed by dividing the NAVD 88 geopotential number by the normal gravity value computed on the Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45 degrees latitude (g = 980.6199 gals.).
  • The modeled gravity was interpolated from observed gravity values.

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