DISTANCE MEASUREMENT
Units

  2-1.   The law prescribes the chain as the unit of linear measure for the survey of the public lands. All returns of measurements in the rectangular system are made in the true horizontal distance in miles, chains, and links. (Exceptions are special requirements for measurement in feet in townsite surveys, chapter VII, and mineral surveys, chapter X.)

Units of Linear Measure
1 chain = 100 links
= 66 feet
1 mile = 80 chain
= 5,280 feet

Units of Area
1 acre = 10 square chains
= 43,560 square feet
  1 square mile = 640 acres

The chain unit, devised in the seventeenth century by Edmund Gunter, an English astronomer, is so designed that 10 square chains are equivalent to one acre. In the English colonial area of the United States the boundaries of land were usually measured in the chain unit, but lengths of lines were frequently expressed in poles. One pole is equal to 25 links, and four poles equal one chain. The field notes of some early rectangular surveys in the southern States show the distance in "perches," equivalent to poles. The term now commonly used for the same distance is the rod.

Land grants by the French crown were made in arpents. The arpent is a unit of area, but the side of a square arpent came to be used for linear description. The Spanish crown and the Mexican Government granted lands which were usually described in linear varas. Both the arpent and the vara have slightly different values in different States. The conversions most often needed are shown in the Standard Field Tables.

Tapes

  2-2. Use of the steel tape is the commonly accepted method of measurement. The tapes used vary in length from one to eight chains, the appropriate length depending upon the topography and the nature of the survey. Graduation is to chains and links, and in some instances to tenths of links. For measurements on the slope the vertical angles are determined by use of a clinometer or a transit. The measured slope distances are then reduced to horizontal equivalents by reference to tables or by multiplying the slope distance by the cosine of the vertical angle.

Each tape should be compared with a standard steel tape before being using in the field.

Stadia

  2-3.    The stadia method is a fast way of making reconnaissance surveys for such purposes as obtaining topography or searching for original corners. Its use is not permitted for measurement of lines. Most transits used by the Bureau of Land Management have a stadia interval with a ratio of 1:132 for use with the chain unit, rather than the standard ratio of 1:100. Data for the reduction of stadia measurements are found in the Standard Field Tables.

Subtense Bar

  2-4.    The subtense bar may be used provided that no measurement is over ten chains and that the instrument used in connection with it is capable of measuring in single seconds.

Traversing

  2-5.   Traverses may be run where the terrain is too precipitous for chaining and the intervisible points required for triangulation cannot be practicably obtained. Traversing should be kept to a minimum.

Triangulation

  2-6.   Triangulation may be used in measuring distances across water or over precipitous slopes. The measured base should be laid out so as to adopt the best possible geometric proportions of the slides and angles of the triangle. If it is necessary to determine the value of an angle with a precision of less than the least reading of the vernier, the method of repetition should be employed.

A complete record of the measurement of the base, the determination of the angles, the location and direction of the sides, and other essential details is entered in the field tablets, together with a small diagram to represent the triangulation.

In the longer and more important triangulations all of the stations should be occupied, if possible, and the angles should be repeated and checked to a satisfactory closure; the latter may be kept within 0' 20" by careful use of the one-minute transit.

In line practice the chainman are frequently sent through for taped measurement over extremely difficult terrain, but with the length of the interval verified by triangulation. This is done to ensure the most exact determination of the length of the line while also noting the intervening topographic data.

Electronic Telemetry

  2-7.   The measurement of lines by use of electronic telemetry fully meets the requirements for accuracy. Determining factors in its use are the terrain, ground cover, and availability of the proper instruments. Some types are adapted to the measurement of long distances, others to measurement of intermediate distances. Transport and maintenance must be considered in determining whether the use of telemetry will expedite a particular survey. Provision must be made for measuring distances to important items of topography.

The variety of electronic distance-measuring devices, the rapid development of combinations with optical theodolites, and modifications of the instruments make it impracticable to describe the methods of use in this manual. The surveyor should consult the manufacturer's operating manual for calibration, use, care, and adjustments.

A special kind of triangulation is used when it is desired to locate on the ground a point for which the geographic position has been determined in advance. Two intervisible triangulation stations are occupied simultaneously with optical theodolites and electronic measuring devices. A mobile party sets a temporary point at the approximate position of the desired point by reference to a topographic map or aerial photographs. The position of the temporary point is then determined by triangulation or trilateration and the true point is monumented.

The system is made more adaptable by use of the hoversight developed by the United States Geological Survey. This instrument is fixed in a helicopter. The airborne observer is able to identify a point on the ground perpendicularly beneath a flashing target mounted outside the helicopter. The position of the flashing target is then determined by triangulation. Full utilization of the system requires ready contact with computers by telephone or radio. The Airborne Control Survey, as it is called, is carefully planned to coordinate ground crews, helicopter, and computers. Its use has been successful in surveys over extensive areas in Alaska, and experiment is being made in resurveys and in connection with photogrammetric surveys at the present time.