05 December 2005

Benchmark No. 20300


This survey marker is in Black Hill Regional Park, near Boyds, Maryland. I discovered it in April 2000 while exploring the meadows near the southern boundary of the park. I returned last Friday to take these pictures. The marker itself, a metal disk embedded in concrete, is less than a meter† in front of the so-called witness post.


The engraving on the marker was difficult to read in the field, because the marker had gotten soiled from being buried under a thick layer of grass. By enlarging the picture on the computer screen I was able to make out most of the pertinent information, which states that this is a W.S.S.C. (Washington Suburban Sanitary Commission) First Order Control Station, number 20300. The abbreviation "B.M." stands for "benchmark". I don't know what "T.S." and "MC" stand for.

I also measured the coordinates at the spot with my Garmin eTrex, visible in the first picture. The data sheet‡ for this mark gives its coordinates as 39° 11’ 07.27002’’ N and 77° 18’ 29.63964” W, while I measured 39° 11’ 07.3” N and 77° 18’ 29.6” W. They seem to be pretty darn close, but it is difficult to relate to latitude and longitude. I find it easier to work with UTM coordinates, which are in meters. Again, the data sheet gives the UTM coordinates (for zone 18) as 4,339,883.768 m north and 300,637.576 m east. In comparison, I measured 4339884 m north and 300,639 m east, which place my location about 1.42 m east and about 0.23 m north of the mark.

Finally, the data sheet gives the elevation as 125.8 m, while I measured 120 m.

When I am doing land snail surveys, I try to measure the GPS coordinates of every one of my collecting stations. I am hoping that the coordinates will help me or others to return to the same spots in the future. This exercise shows that my coordinate measurements are accurate enough for my purposes. In this case, what contributed to the accuracy of my measurement was that the sky was unobstructed by trees or nearby hills. Under tall trees, especially when they have leaves, or in deep and narrow canyons, the accuracy of GPS measurements goes down; sometimes I can't even get a fix on satellites.

You can download a list of all the registered survey marks in your area from the National Geodetic Survey’s database. Many of the marks around here seem to be in residential neighborhoods. There are, however, a few in more rural and isolated locations, this being one of them. Whenever time and weather permits, I will try to locate more of them.


Little Seneca Lake photographed facing north from near the survey marker.



†The data sheet gives the exact distance from the witness post to the mark as 0.65 m.
‡If you live in the Germantown area and want to find this survey marker, its data sheet provides detailed directions to the location. Alternately, you may let your GPS receiver, if you have, direct you to the spot. It would be a fun activity.

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