Page:The New International Encyclopædia 1st ed. v. 18.djvu/826

SURVEYING. 724 SURVEYING. Briefly summarized, plane surveying includes laud surveying, topographic surveying, and hy- drographic surveying (see Hydrography), min- ing surveying, railway surveying, and city sur- veying. Geological surveying is a development of topographical surveying in which the outcrops of the earth's rock formation are located and de- noted on topographical maps. Geodetic survey- ing is a class by itself, as is also photographic surveying (q.v. ). PLANE SURVEYING. Land Surveying. As at present practiced all surveys of land, properly so called, are made: ( 1 ) to establish certain monuments, corners, lines, and boundaries, so as to lay out and divide land; or (2) to identify and locate such monu- ments, lines, and boundaries after they have been established, as in all resurveys for location and area. In all cases of land surveying the boundaries and dividing lines are the traces of vertical planes on the surface of the ground, and the area is the area of the horizontal plane included between the bounding vertical planes. In other words, the area sought is not the real surface, but the horizontal projection of that surface. In laying out land, the work consists in running the bounding and dividing lines over all the irregularities of the surface, determining their bearings and horizontal distances and leav- ing temporary and permanent marks. The bear- ing of a line is the horizontal angle it makes with a meridian plane through one extremity, and it is determined by means of a surveyor's compass. The length of the line is usually de- termined by measurement with a surveyor's chain. (See Engineering Instruments.) The marks by which the line is physically denoted are various, such as stones, stakes, holes, mounds, or trees, specially marked and described. These marks, whatever "their character, are called monu- ments, and should be of distinctive and perma- nent character. The true meridian of a place may be obtained by astronomical observations and the deviation of the magnetic meridian determined. This deviation is called the declination, or variation, of the magnetic needle. The compass is usually provided with a vernier-scale (see Vernier) for turning off the variation so that the needle may indicate the true meridian if desired. An instru- ment called the solar compass, invented by Burt (1836), determines the true meridian by a single observation, the sun being on the observer's me- ridian. This instrvunent has been adopted by the United States Government for use in surveying the public domain. The declination varies from year to year and is an essential element to be recorded "in the field-notes so as to enable the surveyor at any future time to retrace the orig- inal survey. The system of laying out the United States public lands furnishes a good example of the methods of laying out land on a large scale. This system probably was devised by Gen. Rufus Put- nam, of Revolutionary War fame, and was first used in laying out the eastern portion of the State of Ohio in 1786-87. The reference lines employed in these public land surveys are in each case a principal meridian and a base line, the meridian running, of course, north and south, and the base line running east and west. From the principal meridian and its accompanying base line guide meridians are run north and south from the base line 24 miles apart, and standard parallels are run east and west from the princi])al ine- ridian 24 miles apart. These lines are run with great care, the solar compass being employed. Kvery mile is marked by a monument and is called a 'section corner,' and every sixth mile has a different mark and is called a 'township cor- ner.' From each township corner on any stand- ard parallel, auxiliary meridians are run north to the next standard parallel. Since these meridians converge somewhat toward the prin- cipal meridian, they will not be quite six miles apart when they reach the next standard parallel, and, therefore, to run the next series of auxiliary meridians north, the start is made not at the points where the first series terminate, but at the six-mile points pre- viously marked off. Where the auxiliary merid- ians have been lined out, the land is divided into a series of strips running north and south, and six miles wide. These are called 'ranges.' The next step is to cut these ranges transversely by running east and west lines at six-mile intervals. Tlie land is thus divided into squares measuring six miles on each side. These squares are called 'townships,' and each contains 36 square miles, or 23,040 acres. The next step is to run merid- ians and parallels one mile apart to divide the township into 36 sections. Monuments are set at intervals of one-half mile on the lines to di- vide each section into quarter sections. The quarter section is the smallest primary division of the public land surveys. The determination of areas between known boundaries is accomplished in several ways, but the one connnonly used consists in determining the bearings and lengths of the outside boundary lines. This is done by beginning at one corner DIAGRAM OF A SURVEY. and running around the area to the starting point, and observing the bearing and measuring the distance of each boundary line in the order in which it is encountered. To illustrate, reference will be made to the diagram, in which the lines A B C D E represent the boundary of a field of land whose area is required. Beginning