Data Sheets And Azimuth Marks

[+CharlieP]

I logged my first benchmark last week and I am still learning how to interpret the NGS datasheets. I don't understand the nature of the azimuth marks, what they are used for and how they are located. It seems that these marks are typically located some distance from the station mark and reference marks. What does "Grid Az" mean? Why is no distance to the azimuth mark listed in the Reference Objects table?

[+Black Dog Trackers]

CharlieP -

 

I'm not a professional surveyor, but I recall asking the same question in the forum a few years ago. I got various answers that were hard to interpret. What I assimilated from them and further reading is this:

 

A triangulation station establishes a physical marker with measured and adjusted coordinates of its horizontal position. The pair of triangulation station and its aziumth provides a particular bearing based on true North. So, with the pair, you get 2 things: position on the planet and an orientation to true North. From this, simpler surveys using turning angles can be done from the pair.

 

I'm responding partly to see if I've got it right and if there's anything else.

 

Since the purpose of the azimuth is just to provide a local orientation to true North, its distance from the main station isn't needed. Except to find it!!!!

[Z15]

Triangulation stations were observed from special portable towers, as much as 100+ ft high and sighted on other stations up to 25 miles and further away. The surveys were conducted on large areas of land at a time.

 

The Azimuth was for use by local surveys as it would be impossible to see the distant marks for the trees and the curvature of the earth, in most instances and well as not being feasible. Without the Azi, the station would be useless for local surveys for without a bearing, you have no idea where you are.

 

As BDT said, a local surveyor can then set up on tri station, point his telescope to the Azimuth mark and has a known bearing to orignitate his work. Note-Very few ever used these for that purpose because a great many were is such remote areas they could not justify the time needed to traverse many miles for a simple property survey that would take a day or less to accomplish. Mostly large projects like highways, pipeline, transmission lines etc made use of these station to position thier ROW and infrastructure on maps etc. GPS has made the Azimuth mark obsolete for that purpose.

 

Its not important to know the distance to the Azi, direction is all that is needed. You have lat and lon on the station itself. Also, to measure the distance would have meant taping it by hand over very rough terain (in most cases) which would not lend well to accurate measurement, the time required to do that was not justified at the time. Its just an unimportant distance for any survey work.

 

It far enough away (normally a min of 1/4 miles) to allow better sighting accuracy. The closer an object it to you the more error in sighting it.

 

Grid - A pattern of lines on a chart or map, such as those representing latitude and longitude, which helps determine absolute location.

 

A grid is a pattern of squares on your map which serve to fix your position. Coordinates will provide numbers that allow you to find a horizontal line and also a vertical line and follow them to the point of intersection, placing you at the bottom left-hand corner (south-west) of a particular grid.

Edited January 2, 2006 by Z15

[holograph]

"Grid Az." means the direction with respect to a rectangular grid coordinate system such as UTM or SPC. If you had a map projected in a grid system such as UTM or SPC, and you aligned a protractor with the vertical grid lines on the map, then the "Grid Az." is the angle of the line drawn between the station and its azimuth mark.

 

"Geod Az." is the geodetic azimuth, or the true bearing of the great circle that passes between the station and the azimuth mark.

 

Because UTM and SPC are projection systems, they suffer from "convergence of the meridians", i.e. distortion, which means that true north is not aligned with the map grid lines, except at particular meridians. The datasheet tells you the amount of convergence where it shows the station's coordinates in UTM and SPC coordinates.

 

If you add the convergence to the grid azimuth, you get the geodetic azimuth.

 

edit: Oops, I see the Z15 already answered before I hit "post". Oh well.

Edited January 2, 2006 by holograph

[+CharlieP]

Thanks for those replies, I am beginning to understand this. In 1934 the station mark (EE2010) I found last week had an azimuth mark about 250 feet away, and I had assumed that was a normal distance. But in 1972 the NGS placed another station mark when they could not "occupy" the original mark because it was close to the edge of a cliff. Apparently that means they could not erect the tower described by Z15. They also could not find the old AZ mark and set a new one. I have read the description more closely, and the new AZ mark is about a half-mile away, not a few hundred feet. I see now that all references to the AZ mark are about bearings, either geodesic or grid.

 

So now, tell me if I understand how this works. The reference marks are to help establish the location of the station mark, and the azimuth is to set a known bearing. So if you have a point within view of two station marks, a surveyor can shoot accurate bearings to that point from each station and compute the location.

 

Maybe?

[Z15]

The reference mark(s) help locate the station and verify that is has not been disturbed.

 

The Azi is not really set to a known bearing rather its azimuth is computed from Polaris Observations (the North Star). Its does not need to be in any specific place.

 

Converting Azimuth to bearing is easy, 0° is North as is 360°, measured clockwise.

 

Edited January 2, 2006 by Z15