Two Common Methods of Surveying
Surveying processes are generally divided into two categories, geodetic and plane. The primary difference between geodetic and plane surveying is the reference that they use.
Geodetic surveying measures all elevations from a level surface, and because all instruments use a straight reference plane, this requires all instrument readings of elevation to be adjust for the curvature of the earth. Geodetic surveys are used when a high degree of accuracy is needed and when the survey will cover long distances or a large area.
Plane surveying assumes the earth is flat and all elevations are measured from a flat surface (plane). We know that the surface of the earth is a level surface, not a flat surface. There fore, in plane surveying every elevation measurement has a small amount of error. Plane surveying is easier to complete and provides sufficient accuracy for smaller areas.
To understand the methods and techniques of any subject it is important to understand the terms that are used. This is especially true for surveying because many terms are unique, a different meaning is used. A good understanding of the following terms and definitions is essential for understanding surveying.
Oblate Spheroid is the term used to describe the shape of the Earth. It is a solid obtained by rotating an ellipse on its shorter axis. This means the distance between the poles through the center of the earth (P) is less than the diameter at the equator (E).
The term level is used compare the relative position of an object with the horizon or the relative position of two or more objects. Objects that are level are parallel with the horizon and at the same elevation with each other. Level is usually determined by an air bubble in a small container of liquid. The container is usually either a tube or cylinder. This apparatus is commonly called a spirit level or simply a level. The spirit level by itself is not a useable tool. It is usually incorporated with additional tools, for example, a carpenters level, or a survey level. Objects are also level if they are perpendicular to a vertical line. A vertical line can be established with a plumb bob or a piece of string with a weight.
A vertical line is a line that follows the direction of gravity. At any point on the Earth’s surface, a string with an attached weight will always point toward the center of the earth forming a vertical line.
A plane is defined as a flat surface. This means that a vertical plane is a flat surface that is vertical. A vertical surface will incorporate a vertical line. The walls of a building are usually vertical planes
A horizontal line is formed when a line is established perpendicular to a vertical line, or when a line is established parallel to the horizon. At any one point, there are an unlimited number of horizontal lines.
A horizontal plane is a plane that is perpendicular to the vertical line or parallel with the horizon at a point. There is only one horizontal plane with the same elevation at a given point, but an infinite number of planes at different elevations may exist at a point.
A level surface is a continuous surface that is at all points perpendicular to the direction of gravity. A level surface is not a flat surface or a plane. A level surface follows the curvature of the Earth. A large body of still water best illustrates a level surface. For a small area, the difference between a level surface and a horizontal plane is indistinguishable. The difference between a horizontal plane and a level surface is the primary difference between the two types of surveying methods, plane surveying and geodetic surveying. Plane surveying assumes the earth is flat, plane, and as long as the distances are short this is not a bad assumption. Geodetic surveying does not use this assumption and all elevations are adjusted for the curvature of the earth.
A distance is the amount of separation between two points, lines, surfaces, or objects measured along the shortest path joining them. A horizontal distance is a distance measured on a horizontal line or plane.
A horizontal angle is an angle measured on a horizontal plane. A horizontal angle will be the angle between two vertical planes.
A vertical angle is an angle measured in a vertical plane. Two different reference points are used, horizontal zero and zenith zero. If horizontal zero is used, angles measured upward from a horizontal line or plane are referred to as plus angles, and angles measured downward from a horizontal line or plane are referred to as minus angles. When zenith zero angles are used, zero degrees is vertically overhead and 180 degrees is vertically down.
A benchmark is a point of known or assumed elevation. To be considered a benchmark the point should be identified by a permanent or semi-permanent structure that will not be affected by frost heave, traffic vibrations or environmental changes. Surveying standards have very specific guidelines on the appropriate structure for benchmarks. The United States Geological Service (USGS) has established bench marks all over the United States. These are used to establish additional local bench marks. It is common practice to establish a benchmark at a construction project. This becomes the reference point during the construction. These benchmarks may be removed when construction is complete, or if they meet the standards, they may be left in place and used for additional projects.
Elevation is the distance above or below a reference level surface. The most commonly used reference level surface in the U.S. is the National Geodetic Vertical Datum of 1929. Established by connecting 26 tidal benchmarks along the Atlantic, Gulf of Mexico and Pacific Coasts, this datum is the zero elevation reference point used for surveying. For many construction surveys the true elevation is not important, so a local benchmark is established and used as a reference point.
Difference in elevation
A difference in elevation is the vertical distance between two level surfaces or planes.
If the elevation of each of the level surfaces is known, then a difference in elevation could be calculated between the earth and elevation or either surface and also between any two surfaces.
A back sight is a rod reading taken on a point of known or assumed elevation. The elevation of a point would be known if the true elevation above sea level were known. The elevation would be assumed if it was a local benchmark where the true elevation was not known.
A foresight is a rod reading taken on a point of unknown elevation. In surveying two different types of foresights are used, intermediate, and true. An intermediate foresight is a rod reading on a point that will not be used as a turning point or benchmark. A true foresight is a rod reading on an unknown point that will be used for a turning point or for a benchmark.
A turning point (TP) is a station along a survey that is established as a temporary benchmark. The purpose of the turning point is to provide a new reference point when the instrument is moved. The turning point should be a stake or other durable structure and the elevation should not be part of the survey data. The instrument would be set up at instrument position one (IP1); a back sight would be taken on bench mark one (BM1) and foresight on the turning point. Once the back sight and foresight rod readings are known, the elevation of the turning point can be computed. The TP becomes a new point that can be used as a reference point. It is not considered a bench marks unless the appropriate marker is used. Turning points are intended to be temporary. Then the instrument would be moved to IP2, and the process repeated until the second bench mark was reached.
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