There are a few of these Department of Agriculture bench marks in northeastern Nebraska, but I have never been able to find any information on when they were set or how their elevations were derived. Has anyone come across published data on these types of bench marks?
The benchmarks that are set in the face of a buildings that I have run across, have all been in southwest Tennessee, and all had USGS identification numbers. The ones I tied to checked flat against the traditional benchmarks.
Here is a page from a Department of Agriculture bulletin showing their use in Kansas as early as 1907. Another bulletin described them in Illinois in 1911.
Where on that cap or its surroundings would the elevation be? I don't see anything glaring "THIS IS THE POINT" at me. Where do I shoot it? Then we can figure out the datum.
I remember the one listed in St. Paul. In fact, I used it to compare with a couple of KDOT BM's in the mid-1990's. Seems like it checked quite well.
In response to the question below about where to shoot it, I seem to recall that the cap was actually curved a little bit such that the center would be the highest point on the cap. It was not a flat-as-a-pancake horizontal surface.
But... That one is mounted on a wall.
vern
Not to be rude, but if you read the page Mr. Penry posted it tells you...
vern
I read the page, only if it were mounted on a pipe as the page
suggests. That one in the original post is in a vertical wall.
vern
Rotate Mr. Penry's page accordingly.
No, no, no. It was in the sidewalk. The problem is they eventually tore down the bank building and the sidewalk.
The one in the initial post is shown as being in a wall.
vern
>Rotate Mr. Penry's page accordingly.
It says "the elevation of the highest point of the cap". If it is mounted on a wall that would be the top of the rim.
But in the context given, I'm not absolutely sure that statement was meant to apply to mounting on a wall. My guess without any instructions would have been to use the center bar that has the stamping as a bench and read the top of that bar. It sticks out further and just begs to have the edge of the rod shoe set on it.
The USGS vertical benchmarks had a horizontal groove to hook a pocket rod into.
Is that an arrow below the disk?
Here are images of a cap hanging on a wall in a local museum in the same area of the state where the one is located in the brick wall of the building.
As far as where to place the level rod on one set vertically instead of on a pipe set in the ground, that would be anyone's guess. I think these BM's originated between 1905-1915 as evidence by the bulletin in Kansas where they were placed in 1907. I would go with the center of the cap, but who knows what the surveyors did who placed the caps.
The bigger issue is figuring out what datum they are on. These are pre-NGVD 1929 elevations. Sure, the cap is stamped with an elevation, but what does that mean today? I actually had a surveyor tell me that he was glad the cap was stamped because he needed a bench mark. (Slap palm to forehead).
I'm surprised that the elevation is stamped to the hundredth.
Many of the older BM's in my area are stamped to the nearest foot with revised published figures to the hundredth.
Disks in water tables of buildings...
I have seen a lot of City of Pittsburgh benchmarks which were set vertical in the water tables of old buildings which had a flattened portion at the top, just barely protruding above the top of the water table, and extending back about 1/2 inch. It was obvious from looking at them that this flat spot was the datum point. I am pretty sure I have a picture somewhere, but right now I can't find one. For those who don't know, a water table is a ledge near the ground that deflects rain away from the edge of the building (water table)
Here is a rough sketch of what they look like:
