January 25, 2001

To : NGS Advisors and other interested parties

From : Orland W. Murray

National Geodetic Survey

Instrumentation & Methodologies Branch

P.O. Box 190

Corbin, VA. 22446

Ph:(540)-373-7605

Subject: Availability of 20 mm offset plugs for leveling and future availability of Temperature Gradient Thermistors Sets

20 mm plugs

Owing to a request from several geodetic advisors for 20 mm off-
set plugs used in precise leveling, I was able to locate and contact the vender, Jobmaster Corporation, who had previously supplied NGS with these plugs. Unfortunately, the plugs are not an in-stock product and require tooling up to be produced. Before the company would tool-up, they required orders of at least 50 to 100 plugs. The price quoted would be $27.60 each.

After talking to Jobmaster, I contacted the people most interested in acquiring the plugs and informed them of the minimum order requirement and asked them to contact the company of there intent to buy. Apparently, enough orders were placed for the company to decide to make at least 100 or perhaps more.

This letter is to inform you that these plugs should be available in a few weeks. Jobmaster accepts most credit cards and purchase orders. You must order them directly. Contact the company at the following:

Jobmaster Corporation

9006-9010 Liberty Rd.

P.O. Box 207

Randallstown, MD 21133

Phone: (800)642-1400 or (410)655-1400

email: path@jobmaster.com

Ask for Pat, she's the person I have been dealing with. If she's is not available, others will also be able to take your order.

The plugs are:

Part/Item Number: A8630X787

Description: ALXIII 0.630"x0.787"

The plugs are precision made magnetic cylinders, 16 mm in diameter and 20 mm tall with the top and bottom surfaces flat and parallel. See below for more explanation on their use in precise leveling.

Temperature Gradient Thermistor Sets

In a conversation with, Dixon Hoyle, Minnesota Geodetic Advisor, he informed me that the Minnesota DOT were attempting to manufacture the aspirated temperature probes previously made here at the I&M Branch. It may be possible for them to manufacture sets for other user's. He will let me know when and if this is possible and what the cost may be. I will pass this info on as soon as I hear from Dixon.

More 20 mm Plug Information (from Digital Leveling User's Guide):

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).