Azimuth Orientation Of Benchmark Disks?

[Bill93]

Stupid question time. I realized that I haven't been paying enough attention as I find elevation benchmark disks (as opposed to reference marks, etc). In the future, I'll try to remember to check their rotational (azimuth) orientation. Meanwhile, any knowledge from the group would be welcomed.

 

They have a long horizontal line (as the lettering reads) and a short cross vertical. I would expect that when placed vertically the long line would be level, but haven't actually seen any of those yet. When placed horizontally, are they oriented in azimuth by any rule and if so how consistent are they? I don't think they all are aligned with the lettering to read while you face north. What have you seen?

[evenfall]

Hi Bill,

 

Azimuth Marks are just work that has been done once and made permanently part of the Station, so it won’t need be repeated. Common on stations that were used for many observations, and often, higher order Stations were ones that most often had them. Meant to make life in the Survey Party's quicker easier and more accurate.

 

It aids a quicker set up. On an instrument set up you need an initial bearing to observe to set the horizontal vernier to a zeroed setting so you can start measuring angles clockwise from there. It is no particular cardinal direction necessarily, but can be set up that way. This way a measurement can be taken of a third location quickly and the trigonometry could begin. Either way, the first shot the instrument man is going to take is to establish the Azimuth for the instrument. Thanks to the permanent Azimuth Mark, you will know the angle of and the bearing to the first reading you take after you calibrate. The Azimuth just created a line of known tangency to read the first angle from... Since you already know it, you don't have to compute the first bearing. It is close by and handy to put a target on so the calibration stays tight. You can use any station as another Azimuth too, but it is not always as handy, nor has it necessarily been pre computed. Just remember Old school. Paper and pencil, Mechanical instruments, Brain power and Time. Lots of time. That’s the short version anyhow. Setting up and getting started took longer without a known azimuth.

 

The reason GPS can put a go to between two points is because all the Satellites know where they are as well as all the others are and the GPS does too, so azimuths are all known, and the Bearing just pops right up and stays up despite the coming and going of the constellation of Satellites on that GPS Horizon.

 

I hope that made some sense, unless I misinterpreted your question.

 

Rob

 

Re Edited after MORE Coffee...

Edited September 11, 2004 by evenfall

[Bill93]

I see I wasn't clear enough in my question. This has nothing to do with the Azimuth reference mark at a triangulation station, or the relation between any two disks.

 

I'm asking specifically about elevation benchmarks. An example would be LE0252 A 114. When they placed the disk and its concrete post, did they twist it to point its line in some specific direction, and if so how accurately?

[Z15]

NO, its placed at random.

 

fyi - Those marks (set prior to the 1940's) that are in 8-inch sq concrete posts are pre-cast with the disk and then lower into a dug hole in ground. The are 5 ft long, 8 in at top and 12 in at base. I found one washed out and intact.

 

Edited September 11, 2004 by elcamino

[Wild T2]

The triangulation stations and elevation bench marks were frequently set to "read from the road" if they were along a road or railroad. That is, if you walked to the mark from the road, the disk would be oriented for easier reading. If in the middle of nowhere, they were frequently set to "read from the south". I don't think that this was documented in their rules anywhere, just a common practice. At least it seems to be the general practice for NGS marks in my area.