on the tripod head threads. Rotate the footplatecounterclockwise one-fourth turn or until theinstrument seats itself; then rotate it clockwise toengage the head nut threads to the tripod headthreads. If the threads do not engage smoothly,they are cross-threaded. Do not force the headif you encounter resistance, but back it off, squareup the instrument, and try again gently to engagethe threads. When they engage, screw the headnut up firmly but not too tightly. Setting up theinstrument too tightly causes eventual wearing ofthe threads, making unthreading difficult.After you have attached the instrument, if youare set up on stable soil, thrust the tripod legs’tips into the ground far enough to be sure of astable support, taking care to keep the footplateapproximately level. Some tripods have legsequipped with short metal stirrups. These stirrupsFigure 14-6.-Two ways of preventing tripod legs fromspreading on hardened surface.allow you to force the legs’ tips into the groundby foot pressure.If you are set upon a hardened surface, suchas concrete, make sure the tripod legs do notaccidentally spread, causing the tripod to collapse.In figure 14-6, view A, the legs’ tips are insertedin cracks in a concrete pavement. In figure 14-6,view B, they are held by an equilateral woodentriangle called a floor triangle.Leveling the Engineer’s LevelAs a rodman, you must concentrate onkeeping your rod perfectly plumb. Readings ona rod that is out of plumb are inaccurate.Similarly, as a levelman, you must constantlybear in mind that the line of sight throughthe telescope must be perfectly level in everydirection or every reading you make with theinstrument will be inaccurate. After you initiallyplace the instrument, level it carefully as follows:Train the telescope in line with a pair of levelscrews and manipulate the level screws by turningthem in opposite directions, as shown in figure 14-7,until the bubble in the level vial is in the exactcenter. It is helpful to know that the bubble in thelevel vial will move in the direction that your leftthumb moves. To put this another way: When youturn the left-hand screw clockwise, the bubblemoves to your left; when you turn the left-handscrew counterclockwise, it moves to your right.When the bubble is centered with the telescopeover one pair of screws, train the telescope overthe other pair and repeat the process. As a check,swing the telescope over each pair of screws inall four possible positions to make sure thebubble is centered in each position.Making Direct ReadingsThe instrumentman makes a direct rod readingas viewed directly on the graduation of the rod(self-reading) that is in line with the horizontalFigure 14-7.-Manipulating level screws.14-8
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