How Are Vertical Benchmarks Used?

[myozone]

Ok Simple question, how are vertical (on walls) benchmarks used ?. I live in France and have found several of the and most seem to be mounted on walls i.e vertical surface, so how are they used ? (guessing at a distance with theodilite?). Ones on the ground obvious!

 

Or are they 'just' used for leveling ?

 

Dave

[evenfall]

Hi Dave,

 

Generally we would not seek to "occupy" the vertically mounted Station with a Theodolite, or a more modern, computerized version which we call a Total Station. There are a number of ways we could use it.

 

A Bench Mark in the strictest engineering definition is a reference mark for vertical measurement, and are said to be a known elevation. It is not uncommon to see a Bench mounted vertically. I have never come across a Triangulation station mounted vertically myself but I can't rule out that it may have happened. Latitude and longitude do not measure the vertical axis. A survey tech rodman may measure from the crosshatch on the BM to the ground by direct measurement with a tape to determine the elevation on the ground then take the shot to the prism pole and prism from the ground and use the difference measured in the calculation. They could add that length (from BM to ground) to the prism pole to compensate for the ground level, They could hold the tip of the prism pole at the same level as the BM in the air and take the shot, or they could radio the measurement to the survey tech instrument-man or party chief and they can compensate with the measurement entered to the total station. All these methods will get basically the same result.

 

The distance to the BM Station in the purest sense is not all too important as these Stations are most commonly considered elevation only, but with modern instruments we can take a laser ranging measurement with the Total Station or robotic derivative thereof and it will know and record the distance to the prism pole on any shot we choose.

 

Rob

[Z15]

To come off a bench mark set vertically in a building all you need is a knife to hold on the point and rest a level rod on that, machetee works best.

 

Our digital level rods came in 2 piece sections and were of light weight fiberglass. Hold a knife on the cross-hatch and plumb the rod. Takes less the 5-10 seconds to get a reading with a digital level. Reads bar codes on one side and for normal manual reading on other side. (see image below)

 

I agree, a mark set vertically is only useful for an elevation.

 

Edited October 11, 2004 by elcamino

[CallawayMT]

elcamino is right on the money, that is how we use these vertical benches when we are running our levels.

I have talked with our NGS advisor and he stated that most of the time the NGS crews will set up low enough to site a zero at the mark.

Edited October 11, 2004 by CallawayMT

[Prof. Y. Lupardi]

Here in the Netherlands the benchmarks come in 3 kinds:

1. old triangulation points, mostly well-defined tops of towers of churches

2. the new GPS Kernnet points, on the ground with clear vieuw of the sky, easy accessiblity with car, and plenty of room to put your GPS antenna nicely above the mark. These marks are simple bald nails (with a very small driiled hole in it ) and set in stable concrete stuctures. Position is given in mm.

3. nails in walls of structures. Many are 2 meters or more above ground.

 

Only the GPS points have a confirmed precise height.

For height measurements we have a different set of marks: NAP.

These have no associated precise position and when in a wall then about 20-30 cm above ground.

 

How they use those nails high up on walls to get position: I do not know.