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[:triangulation station:Triangulation station]s often have ["reference mark"]s associated with them. ["NGS"] recovery reports of a triangulation station involve looking not only for it, but also any reference marks and the ["azimuth mark"]. Usually at least one of these is fairly obvious and that will give you a start on finding the others.

The measurements between the triangulation station and the reference marks are almost always given in what's called the ["box score"]. The box score gives the ["azimuth"] from the triangulation station to each reference mark in the format dddmmss.s but using just ddd part is sufficient for hunting.

It helps to have a compass that reads to the nearest degree, but the most precise thing you will be using is the tape measure, so depend mainly on it. Make arcs on the ground at the required distance if you can, so as to use the tape to the greatest advantage. The ideal is to have two out of the three marks found, and then you can use an intersection of measurements to find the third mark. (Of course, if you have two marks, you don't really need the azimuths at all, since there will only be two intersecting points.) Usually reference marks are more or less at right angles to each other with respect to the triangulation station.

If you're looking for a reference mark from the triangulation station, you use the azimuth from the box score. If you're having to look in the opposite direction, you need to convert the azimuth to the opposite direction using the following simple rule. If the azimuth is less than 180 degrees, add 180 degrees, otherwise subtract 180 degrees.

Azimuths in a box score are in terms of ["true North"]. Your compass points to magnetic North instead. To convert from one to the other, you must get the ["declination"] and use it. The declination is different everywhere on Earth and changes as time goes by, so you will need the current local declination at the approximate position of the triangulation station. In order to follow the azimuth that the box score calls for, subtract the local declination from the box score's azimuth, then follow your compass with the resulting value.

Since you will probably be using a tape measured in feet, you must either bring a calculator with you or convert the metric distances in the box score from meters to feet on your printed datasheets before going on your hunt. To convert, multiply the number of meters by 39.37 and divide the result by 12. Then, if your tape is feet-and-inches, multiply the decimal part of the last result by 12 to get the number of inches.

If you are wanting to find an azimuth mark from a triangulation station, first get the azimuth and distance from the triangulation station to the azimuth mark from the datasheet's box score. Then use the GeoCalc or ["FORWARD"] program to convert this data into the azimuth mark's coordinates. Even though a more accurate way to find the azimuth mark would be to tape along the azimuth, the amount of tape required makes it impractical for a non-surveyor.

ReferenceMain (last edited 2009-03-21 21:03:18 by localhost)