Setting the Rod on a Bench Mark

When setting the leveling rod on a bench mark disk, remove the footplate keeper and set the footplate directly on the disk making sure that the edges are not catching on any of the concrete around the disk. If nothing is catching, turn the rod side to side to find the high point on the disk. Once satisfied that the high point has been found, extend the brace poles and plumb the rod.

If a high point cannot be established or the footplate is catching on the concrete, it will be necessary to use the 20 mm offset plugs. These are machined magnetic plugs which are placed on the footplate of the leveling rod to lift it above an obstruction or establish a high point on a very flat disk with no definable high point. If the disk is flat, center the 20 mm plug in the center of the disk.

IMPORTANT NOTE!!! Always use the plugs in pairs. In other words, if it is necessary to use a 20 mm plug on a disk, be sure to put a plug on the other rod at the same time, otherwise, a 20 mm error will occur in the leveling. Just as important is removal of the plugs after the setup is completed. At this time, the I&M Branch has very few of these machined offset plugs, so take care not to lose them. Call the NGS I&M Branch for information about acquiring a set.

Once the plugs are in place, take the normal setup observations. Make a note on the field recording sheet whenever the 20 mm plugs are used. The plugs can also be used with the 60 CM invar strip ( See - Appendix F).