Surveying Transit

Surveying Transit

Surveying

Physical Description:

The transit is broken up into three different layers. Starting at the base, there is a opening in the bottom allowing the device to be fixed to a tripod, or legs as they were referred to by Gurley, the creator of the transit. (1) Just above the base sit three brass leveling screws which supported the brass leveling base. On top of the leveling base were the lower horizontal tangent and locking knob. A small, cylindrical support leads from the leveling base to the circular Vernier Plate. In the middle of the Vernier Plate is the compass, which has a steel interior and a brass exterior. The compass is held in place by two needles. On the outside of the Vernier Plate as N and W, there are two Plate Vials, or levels. At W and E of the compass, there are twin supports that lead up to the telescope. At the top of one of these two-legged supports is a vertical circle which runs parallel to another Vernier Plate to find relative angles. Suspended from the two-legged supports is the telescope. Below the telescope is another level.

Functional Description:

This device is used to measure very precise angles both vertically and horizontally. The surveyor would set the device on a flat surface and look through the transit to sight it in on point they wanted to know relative to them. They would then take the compass and vertical dial angle readings. This process takes a bit of patience. First, they adjust the four base knobs until the device reads level on the two levels next to the compass, assuring accurate measurements. Failing to do so would cause the surveyor to grossly miscalculate their position. They then line up the compass so the north lines up with the needle. At this point, the surveyor unlocks the transit scope from it horizontal and vertical axes and point it at the point in question. The surveyor uses the focus knob on the side of the scope to get a clear image then locks the scope in place. The surveyor makes further horizontal and vertical adjustments using the fine adjustment knobs and checking the progress through the scope. When satisfied that the scope is on point, the surveyor takes the horizontal bearing from the gauge around the compass and the vertical angle from the vertical dial on the side of the scope. These measurements can be used to help calculate distances and elevations using trigonometry.

1. (Gurley Manual of Surveying Instruments)

Timothy Judge, Xena Cortez, James Dykstra, and Conner Deur

Between 1880 and 1908

English

Physical Object

No accession number

United States of America

245mm length of barrel
75mm Diameter of stand
155mm outside compass diameter
110mm compass
263mm height

brass, steel, glass

W & L.E. Gurley, Troy, NY

On the face of the compass
W & L. E. Gurley, Troy, New York
Circiular compass scale 0 to 360 by 10s, also includes N, E, S, and W at appropriate points. There are also numbers from 30 to 0 to 30 for both the horizontal and vertical axes of rotation on verniers.

No clear history was found about this particular transit at Michigan Tech. However, some information is known about the origin of similar transits made by W & L.E. Gurley and a date of origin can be estimated from this information. Around 1852, W & L.E. Gurley was the largest manufacturer of engineering and surveying instruments. The design of Gurley instruments remained very similar for most of production including their surveyor’s transits. This particular transit dates before 1908, evident by the lack of serial number; serial numbers weren't added to the transits until 1908. It is probably from the post-1880 because of the placement of "Troy, NY" inscription on the compass (2). The transit is part of a larger collection of similar transits at Michigan Tech in the School of Technology.

2. ("How old is my Gurley")

Electrical Engineering Resource Center 231