Lesson 2. Classification and basic principles – Linear Measurement
The practice of measuring angles and distances on the ground so that they can be accurately plotted on a map
GENERAL PRINCIPLE OF SURVEYING
The general principles of surveying are:
1. To work from the whole to the part, and
2. To locate a new station by at least two measurements (linear or angular) from fixed reference points.
According to the first principle, the whole area is first enclosed by main stations (i.e. controlling stations) and main survey lines (i.e. controlling lines). The area is then divided into a number of parts by forming well conditioned triangles. A nearly equilateral triangle is considered to be the best well-conditioned triangle. The main survey lines are measured very accurately with a standard chain. Then the sides of the triangles are measured. The purpose of this process of working is to prevent accumulation of error. During this procedure, if there is any error in the measurement of any side of a triangle, then it will not affect the whole work. The error can always be detected and eliminated.
- But, if the reverse process (i.e. from the part to the whole) is followed, then the minor errors in measurement will be magnified in the process of expansion and stage will come when these errors will become absolutely uncontrollable.
According to the second principle, the new stations should always be fixed by at least two measurements (linear or angular) from fixed reference points. Linear measurements refer to horizontal distances measured by chain or tape. Angular measurements refer to the magnetic bearing or horizontal angle taken by a prismatic compass or theodolite.
- In chain surveying, the positions of main stations and directions of main survey lines and check lines.
CLASSIFICATION OF SURVEYING
Generally, surveying is divided into two major categories: plane and geodetic surveying.
PLANE SURVEYING is a process of surveying in which the portion of the earth being surveyed is considered a plane. The term is used to designate survey work in which the distances or areas involved are small enough that the curvature of the earth can be disregarded without significant error. In general, the term of limited extent. For small areas, precise results may be obtained with plane surveying methods, but the accuracy and precision of such results will decrease as the area surveyed increases in size. To make computations in plane surveying, you will use formulas of plane trigonometry, algebra, and analytical geometry.
A great number of surveys are of the plane surveying type. Surveys for the location and construction of highways and roads, canals, landing fields, and railroads are classified under
plane surveying. When it is realized that an arc of 10 mi is only 0.04 greater that its subtended chord; that a plane surface tangent to the spherical arc has departed only about 8 in. at 1 mi from the point of tangency; and that the sum of the angles of a spherical triangle is only 1 sec greater than the sum of the angles of a plane triangle for a triangle having an area of approximately 75 sq mi on the earth’s surface, it is just reasonable that the errors caused by the earth’s curvature be considered only in precise surveys of large areas.
In this training manual, we will discuss primarily the methods used in plane surveying rather than those used in geodetic surveying.
GEODETIC SURVEYING is a process of surveying in which the shape and size of the earth are considered. This type of survey is suited for large areas and long lines and is used to find the precise location of basic points needed for establishing control for other surveys. In geodetic surveys, the stations are normally long distances apart, and more precise instruments and surveying methods are required for this type of surveying than for plane surveying. The shape of the earth is thought of as a spheroid , although in a technical sense, it is not really a spheroid. In 1924, the convention of the International Geodetic and Geophysical Union adopted 41,852,960 ft as the diameter of the earth at the equator and 41,711,940 ft as the diameter at its polar axis. The equatorial diameter was computed on the assumption that the flattening of the earth caused by gravitational at traction is exactly 1/297. Therefore, distances measured on or near the surface of the earth are not along straight lines or planes, but on a curved surface. Hence, in the computation of distances in geodetic surveys, allowances are made for the earth’s minor and major diameters from which a spheroid of reference is developed. The position of each geodetic station is related to this spheroid. The positions are expressed as latitudes (angles north or south of the Equator) and longitudes (angles east or west of a prime meridian) or as northings and castings on a rectangular grid.
Classifications of Surveying
Based on the purpose (for which surveying is being conducted), Surveying has been classified into:
• Control surveying :
To establish horizontal and vertical positions of control points.
• Land surveying :
To determine the boundaries and areas of parcels of land, also known as property survey, boundary survey or cadastral survey.
• Topographic survey :
To prepare a plan/ map of a region which includes natural as well as and man-made features including elevation.
• Engineering survey :
To collect requisite data for planning, design and execution of engineering projects. Three broad steps are
1) Reconnaissance survey :
To explore site conditions and availability of infrastructures.
2) Preliminary survey :
To collect adequate data to prepare plan/map of area to be used for planning and design.
3) Location survey :
To set out work on the ground for actual construction/execution of the project.
•Route survey :
To plan, design, and laying out of route such as highways, railways, canals,pipelines, and other linear projects.
Construction surveys :
Surveys which are required for establishment of points, lines,grades, and for staking out engineering works (after the plans have been prepared and the structural design has been done).
•Astronomic surveys :
To determine the latitude, longitude (of the observation station) and azimuth (of a line through observation station) from astronomical observation.
•Mine surveys :
To carry out surveying specific for opencast and underground mining purposes
As mentioned earlier in this chapter, SPECIAL SURVEYS are conducted for a specific purpose and with a special type of surveying equipment and methods. A brief discussion of some of the special surveys familiar to you follows.
LAND SURVEYS (sometimes called cadastral or property surveys) are conducted to
establish the exact location, boundaries, or subdivision of a tract of land in any specified area.
This type of survey requires professional registration in all states. Presently, land surveys generally consist of the following chores:
1. Establishing markers or monuments to define and thereby preserve the boundaries of land belonging to a private concern, a corporation, or the government.
2. Relocating markers or monuments legally established by original surveys. This requires examining previous survey records and retracing what was done. When some markers or monuments are missing, they are re-established following recognized procedures, using whatever information is available.
3. Rerunning old land survey lines to determine their lengths and directions. As a result of the high cost of land, old lines are re-measured to get more precise measurements.
4. Subdividing landed estates into parcels of predetermined sizes and shapes.
5. Calculating areas, distances, and directions and preparing the land map to portray the survey data so that it can be used as a permanent record.
6. Writing a technical description for deeds.
CONTROL SURVEYS provide "basic control" or horizontal and vertical positions of points to which supplementary surveys are adjusted. These types of surveys (sometimes termed and traverse stations and the elevations of bench marks. These control points are further used as References for hydrographic surveys of the coastal waters; for topographic control; and for the control of many state, city, and private surveys.