A chaining pin (also called a taping arrow) is a metal pin about 1 ft long. It has a circular eye at one end and a point for pushing it into the ground at the other (fig. 11-47). These pins come in sets of 11 pins, carried on a wire ring passed through the eyes in the pins or in a sheath called a quiver.

Chaining pins can be used for the temporary marking of points in a great variety of situations, but they are used most frequently to keep count of tape increments in the chaining of long distances.

A leveling rod, in essence, is a tape supported vertically and is used to measure the vertical distance (difference in elevation) between a line

of sight and a required point above or below it. This point may be a permanent elevation (bench mark), or it may be some natural or constructed surface.

There are several types of leveling rods. The most popular of all is the Philadelphia rod, as shown in figure 11-48. it is a graduated wooden rod made of two sections and can be extended from 7 to 13 ft. In view A, each foot is subdivided into hundredths of a foot. Instead of each hundredth being marked with a line or tick, the distance between alternate ones is painted black on a white background. Thus, the value for each hundredth is the distance between the colors; the TOP of the black, EVEN values, the BOTTOM of the black, ODD values. The tenths are numbered in black, the feet in red. This rod may be used with the level, transit, theodolite, and with the hand level on occasion to measure the difference in elevation.

The leveling rod may be read directly by the instrumentman sighting through the telescope, or it may be target-read. Conditions that hinder direct reading, such as poor visibility, long sights, and partially obstructed sights, as through brush or leaves, sometimes make it necessary to use targets. The target is also used to mark a rod reading when numerous points are set to the same elevation from one instrument setup.

Targets for the Philadelphia rod are usually oval, with the long axis at right angles to the rod, and the quadrants of the target painted alternately red and white. The target is held in place on the rod by a C-clamp and a thumbscrew. A lever on the face of the target is used for fine adjustment of the target to the line of sight of the level. The targets have rectangular openings approximately the width of the rod and 0.15 ft high through which the face of the rod may be seen. A linear vernier scale is mounted on the edge of the opening with the zero on the horizontal line of the target for reading to thousandths of a foot. When the target is used, the rodman takes the rod reading.

The other types of leveling rods differ from the Philadelphia rod only in details. The Frisco rod, for direct reading only, is available with two or three sliding sections. The Chicago rod is available with three or four sections that, instead of sliding, are joined at the end to each other like a fishing rod. The architects or builders rod is a two-section rod similar to the Philadelphia but is graduated in feet and inches to the nearest one-eighth in. rather than decimally. The upper section of the Lenker self-computing rod has the graduations on a continuous metal belt that can be rotated to set any desired graduation at the level of the height of the instrument (HI). To use the rod, you set the rod on the bench mark and bring the graduation that indicates the elevation of the bench mark level with the HI. As long as the level remains at that same setup, wherever you set the rod on a point, you read the elevation of the point directly. In short, the Lenker rod does away with the necessity for computing the elevations.

View B (fig. 11-48) shows the rod marked with metric measurements; the graduations of the rod are in meters, decimeters, and centimeters. The targets that are furnished with the metric rod have a vernier that permits reading the scale to the nearest millimeter. The metric rod can be extended from 2.0 to 3.7 meters.

For high-precision leveling, there are precise leveling rods as well as precise engineers levels. A Lovar rod is usually T-shaped in cross section

and has the scale inscribed on the strip of Lovar metal. A precise rod usually has a tapering, hardened steel base. Some are equipped with thermometers, so temperature correction can be applied. Precise rods generally contain built-in rod levels.

When a rod reading is made, it is accurate only if the rod is perfectly plumbed. If it is out of plumb, the reading will be greater that the actual vertical distance between the HI and the base of the rod. Therefore, to ensure a truly plumbed leveling rod, use a rod level. Two types of rod levels that are generally used with standard leveling rods are shown in figure 11-49. The one at the left is called the bulls-eye level, and one on the right is the vial level. Figure 11-50 shows the proper way of using the bulls-eye level; the vial level is attached in the same manner.

Proper care should be taken of leveling rods. The care consists of keeping them clean, free of sand and dirt, unwarped, and readable. They must be carried over the shoulder or under the arm from point to point.

Dragging them through the brush or along the ground will wear away or chip the paint. When not in use, the leveling rods should be stored in their cases to prevent warping. The cases are generally designed to support the reds either flat or on their sides. The rods are not to be leaned against a wall or to remain on damp ground for any extended period, since this can produce a curvature in the rods and result in unpredictable random and systematic errors in leveling.