Topographic and Control Surveys
Topographic surveys, to obtain ground relief data and locations of natural and constructed features, are the basis for many soil and water conservation projects. Such surveys involve both control surveys and surveys for topographic features. A relatively few points or stations are established by the control survey. They are so arranged that they can be easily
observed and measured by triangulation, traverse, or grid. Elevations of such points are
determined by leveling. These provide an accurate framework on which less accurate survey data, such as ground elevations, can be based without accumulating accidental errors or incurring high cost of making all measurements precise.
There are two general types of topographic surveys: route surveys and area surveys.
Route surveys are comprised of ribbon- or strip shaped tracts as would be required of a natural stream or a drainage or irrigation ditch. These surveys are usually open traverses whose horizontal control throughout their length is fixed by stationing (as described in the section on "Measurement of Horizontal Distances: Taping") and by offset ties that allow you to reestablish a station if it is destroyed. Such traverses may not be checked completely by calculations; however, when started and ended on points of known position, the surveys become closed traverses for which complete mathematical checks can be made. Vertical controls can be determined in conjunction with the traverse
survey or independently from a closed circuit of differential levels.
Area surveys are comprised of block-shaped tracts as for a pond or reservoir site, a surface
drainage plan, or irrigation system. An area survey requires a closed traverse with a control network of stations and bench marks, even though it is only a rudimentary one for a small tract. Several types of surveys are used in making route and area surveys.
Lesson 23
Engineer's Transit (Primary Instrument)
Setting Up the Transit The transit is the primary instrument for making route or area surveys. Ordinarily you should set the transit over some definite point, such as a tack in a
hub The plumb bob provides a means of centering the instrument over the tripod legs Adjust until the tripod head is nearly level. Then, pick up the instrument without disturbing the position of the legs, and carefully set it over the point. Press each
leg firmly into the ground, at the same time adjusting its location until the plumb bob falls close to the point and the tripod plate is nearly level. Then loosen the two adjacent leveling screws and shift the transit head until the plumb bob is over the point.
Another method used by experienced surveyors is to grasp two legs of the tripod and place the third leg on the ground at such a point with respect to the hub that when the other two legs are allowed to touch the ground, the tripod plate will be nearly level, the height of the telescope will be convenient, and the plumb bob will be nearly over the tack in the hub.
After the instrument is centered over the point, level it. First, loosen the lower clamp screw and turn the instrument about its vertical axis until one of the plate level tubes is parallel to a line through a pair of opposite leveling screws. The second plate bubble will then be parallel to a line through the other pair of leveling screws. To level the instrument,
uniformly turn a pair of opposite leveling screws. Tighten one screw by the same amount that the other is loosened. This will tilt the leveling head and at the same time maintain definite support for it on both screws. The screws should rest firmly on the tripod plate at all times but should not be allowed to bind. Center the other bubble in a similar manner, using the other pair of leveling screws. Alternate the process until both bubbles are
centered. Now, observe the position of the plumb bob. If it has moved off the point, reset it by shifting the head and releveling the instrument.
The instrument is now ready for measurement of horizontal and vertical angles. Transits most widely used in SCS have two verniers (figs. 1-28 and 1-29) for reading horizontal angles, the zeros being 180"apart. The horizontal circles of SCS transits are graduated in one of four ways: 30 minutes reading to 1 minute; 20 minutes reading to 30 seconds; 15
minutes reading to 20 seconds; or 20 minutes reading to 20 seconds. The graduations of the circle usually are numbered at intervals of 10" continuously from 0" to 360" in both directions from 0".
The inner row of numbers increases clockwise, whereas the outer row increases counterclockwise. The verniers are provided so that the angle can be read closer than the smallest circle division, In every case the graduation on the vernier dependupon the subdivision of the circle For example when the circle is graduated in half-degree (30')
spaces (fig. 1-29), the space between each line on the vernier will be 29/30of the 30'arc space on the circle; thus, an arc consisting of 29 divisions of 30'each (equivalent of 14"30ron the circle) is subdivided into 30equal parts to obtain the space between
the lines on the vernier. One division on the vernier then is 01'less in angular measurement than one division of the circle. When the horizontal angles are measured with the transit, the inner circle within the vernier moves with the telescope while the outer circle remains fixed. The zero of the vernier always points to the reading on the outer circle. 
Lesson 24