PICTURES OF TYPICAL DUTIES OF SURVEY CREW MEMBERS Chainman marking a slope stake. Points that have been established by the survey crew are described on reference stakes. The chainman is marking this slope stake from notes previously prepared by the party chief and the instrument man, to show the grading, such as cut or fill, to be done at that point. To do this the chainman must know the standard symbols and abbreviations, such as a circled number means an offset, a C followed by a number means a cut of that number of feet, an F followed by a number means that amount of fill, etc. From the nature of the terrain and the marking on the adjacent stake, the chainman is expected to be able to detect any obvious errors in the notes. Many different kinds of reference stakes are used during construction of a highway project, and each has its own standard markings which all members of the survey crew are required to know and use. Setting up theodolite for a reading on the subtense bar. The two-meter invar subtense bar and theodolite are used in measuring horizontal distances where, because of rugged terrain or other circumstances chain measurement is impracticable. The principle is that if the bar is held horizontally and at right angles to the line of sight the distance may be computed by use of the angle between the two ends of the bar. The instrument man is carefully setting up the theodolite, which must be capable of giving horizontal readings to one second. In this instance, as in all such cases, the contractor's laborers and equipment have cut trees and brush in the line of sight. Picture No. 18 Rodman at Subtense bar. The subtense bar is set up by the instrument man or by a rodman who has been specially trained. The rodman is sighting the theodolite through an optical gunsight on the subtense bar, thereby making sure that the bar is perpendicular to the theodolite's line of sight. With an optical plummet, the bar is centered over the point to which the horizontal distance from the theodolite is to be measured. The bar is kept level by manipulating the adjustment screws on the base plate. Sighting the targets on the subtense bar. The instrument man takes readings on each target on the subtense bar. The difference between the readings is the parallactic angle. The party chief records the readings and determines the angle. This operation is continued with both the direct and inverted telescope until a true mean angle is determined, a most important matter as the accuracy of the computation of distance depends on correct measurement of the parallactic angle. In normal practice, the party chief checks the instrument man's readings of the horizontal angle. After the correct angle is determined, the party chief and instrument man may obtain a figure for the horizontal distance by reference to a table of computations for various angles. Field Office Work. During inclement weather, the survey crew may work in the field construction office under supervision of the party chief. In this picture the instrument man is assisting the party chief in obtaining pertinent data from notes taken in the field. Other field-office work of the instrument man includes making calculations of paving and earth grades from the project plans, making sketches of reference point locations, assisting in the preparation of notes for horizontal and vertical curves, and plotting and planimetering borrow-pit cross sections. Field-office work of the rodman and chainman includes taking inventory of engineering supplies, seeing that the equipment is in its proper place and that the survey trucks are properly equipped, assisting the party chief and instrument man in simple calculations, and learning to plot cross sections. |