Plane Surveying

Summary

Surveying is the process of measuring distances, heights, distributions, and dimensions of features or areas that lie on, above, or below the Earth's surface. For mapping projects that cover small areas, such as the construction of roads and dams, or the surveys of mines and archaeological sites, plane surveying is used. This system is based on the assumption that the Earth's surface is a perfectly flat plane. For projects that cover larger areas, geodetic surveying is required. This system considers the curvature of the Earth's surface and is used primarily by governmental agencies in mapping states and countries.

Definition and Basic Principles

Plane surveying is a method used to describe the location of features on, above, or below the Earth's surface. These features are then shown on a map oriented toward the north and with a uniform horizontal scale. One common surveying instrument is the plane table, a rectangular drawing board with a sheet of cloth-backed paper, mounted on a three-legged tripod. A telescopic alidade placed on the plane table is used for taking sight of distant objects. Another instrument is the theodolite, a telescope attached directly to the tripod head that can be used to take two readings at a time, either in a horizontal or vertical plane. Sights are taken on a ten- to fifteen-foot-long wooden stadia rod, jointed for ease of carrying and with numbers painted on it. The rod is carried by the surveyor's assistant, known as the rodman. The job is to hold the rod upright at the various “stations” the surveyor wishes to locate on the map. Surveying has a long history, used since ancient times to establish land boundaries and to make corners of buildings square. In the twenty-first century, surveying techniques are widely used by governments, corporations, engineers, builders and foundation contractors, geologists, archaeologists, topographers, hydrographers, and even underwater treasure hunters.

Background and History

Surveying is unquestionably one of the earliest skills utilized by humans as they began to adapt to life in communities and to construct buildings. Evidence for the ancient use of surveyors is found in the Egyptian pyramids, built around 3000 BCE, which have corners that are mathematically square, and at Stonehenge, the monument built on England's Salisbury Plain around 2800 BCE, which provided accurately surveyed lines of sight to help the Druids determine the starting dates for seasons and the dates that eclipses would occur. In 200 BCE, Greek scholar Eratosthenes was able to calculate the circumference of the Earth because he knew that the surveyed distance between two Egyptian cities (Alexandria and Syene) was 500 miles. The Romans employed surveyors to lay out their city streets and determine the proper placement for building foundations, and the Incas in Peru built a mountaintop complex at Machu Picchu where the survey stones used for laying out the buildings can still be seen. In succeeding years, surveyors have been called on for the layout of city streets, the sites of bridges, the placement of building foundations, and the determination of property boundaries.

How It Works

Plane surveying is the process of constructing a map based on measurements surveyed on the ground. It is prepared either on a level drawing board, known as the plane table, using an alidade, or at the surveyor's office after the measurements have been taken in the field with the use of a theodolite. The measurements consist of lines drawn at angles to distant points, either at the same elevation as the instrument or higher and lower if the terrain is hilly. The plane table or the theodolite is mounted on a three-legged tripod, and a ten- to fifteen-foot-long stadia rod, graduated in feet and tenths of feet or meters and tenths of meters, is held by a rodman at varying distances from the surveyor, where the sights are to be taken.

Equipment Required. When the alidade is used, the map is made on a sturdy drawing board that has a basal plate that allows it to be attached to the tripod. Cloth-mounted paper is preferred, and the sheets must be “seasoned” by exposing them to the climate it will be in for a few days before surveying so that shrinkage or expansion does not result from the local atmospheric conditions. The tripod head allows the drawing board to be rotated or tilted in any direction the operator wishes. The alidade consists of two parts—a base, often called the blade, which is a straightedge beveled on one or both sides, with a telescope above it, aligned with the base and mounted on a sturdy pedestal. The other common surveying instrument is a theodolite. This is a telescope that can measure two angles simultaneously. It is mounted directly on the tripod head, so there is no plane table board. This means that when a theodolite is used, the resulting map is not created in the field as sights are taken, but subsequently back at the surveyor's office based on the field notes.

Preparations for Mapping. When the plane table is used, the first step is for the operator to level the board using a bubble level. Next, the operator must orient the board north, remembering that two north-south lines will appear on the map. One will indicate true north and be parallel to the north-south meridians of longitude on a globe. The other line will indicate the direction a compass needle points in the area where the map is being made. This direction is known as magnetic north. The operator must now select a scale for the map that will ensure that the entire area being surveyed will fall within the confines of the sheet of paper on the plane table drawing board. A baseline is also selected, between two points on the ground, and then drawn on the map. The locations to be surveyed should be visible from both ends of this baseline so that it becomes the starting point for the map. Extreme care must be taken in measuring the baseline and plotting it on the map because any errors introduced here will result in corresponding errors throughout the map. Distances on the ground are measured with a steel tape or similar device.

Mapping Techniques. The operator begins by setting the tripod at one end of the baseline. Ideally, this should be a place of known elevation, such as a benchmark, which will make it possible to determine the comparative elevations of all the points surveyed on the map. Next, the surveyor sends out the rodman with the stadia rod, which will be held vertically at places where the surveyor wishes to take sights. Distances to the rod can be measured on the ground with steel tape or determined by a process known as triangulation. Heights can be determined by comparing the known height of the surveying instrument with the numbers painted on the stadia rod. After the surveyor has located enough points to provide the “control” needed for the map, topographic, hydrographic, and cultural features can then be sketched in. These features may include topographic contours, water features, property boundaries, roads, railroads, buildings of all kinds, or features of mining and archaeological interest.

Applications and Products

Plane-surveying projects vary in size from those that map extensive areas in considerable detail (known as small-scale and intermediate-scale mapping projects), those that map smaller areas in considerable detail (known as large-scale mapping projects), and those that map either large or small areas in a sketchy fashion to meet temporary or immediate needs (known as reconnaissance mapping projects). The terms “large scale” and “small scale” when applied to maps can be confusing. The scale referred to here is the mileage scale on the base of the map. On a map of the United States, for example, the width of one mile on this scale would be tiny, and these maps are called small-scale maps. On a map of a town, on the other hand, the width of a mile on the mileage scale would be large, so these maps are referred to as large-scale maps.

Small- and Intermediate-Scale Mapping. The first step in mapping a large area, such as a county or state, is to determine the purpose of the map. Next, a search of the records for prior work in the area should be made. The surveyor needs to know what mapping has been done previously and where boundary lines and points of known elevation (benchmarks) should be located. Then, a scale should be selected that will ensure the entire land area being studied will fall within the limits of the map, and if the map is to be contoured, a contour interval must be selected that will enable the draftsman to draw well-formed contours that are not too closely spaced. When the amount of funding is limited, a less detailed map might be considered, or the most important part of the project completed first. Comparison with the costs for similar projects in other areas will help in ascertaining the cost of the new project. If it is to be carried out in a tropical region, there may be only a certain number of months when the conditions for mapping are favorable, and before the start of fieldwork, the cloth-backed paper sheet on which the map is to be drawn should be seasoned. When the sights to be taken with the alidade or theodolite are long ones, the stadia rod must be durable, portable, not too heavy, and have numbers that will be legible from far away.

Large-Scale Mapping. Large-scale mapping projects are used for towns and cities, mining operations, archaeological sites, bridges, dams, and reservoir locations. Topographic contouring is frequently needed, and because of the large scale, details such as cultural features must be shown with more care than on the small- and intermediate-scale maps. Sight lines and angles must be drawn with great accuracy, and the distances measured with pacing or even with steel tape. Stadia rods marked in one-foot intervals are available for these projects, rather than the stadia rods with ten-foot intervals used for small- and intermediate-scale projects. Exact dimensions are required for details such as roads, buildings, and streams, and not just the picture symbols used on the small- and intermediate-scale maps.

Reconnaissance Mapping. The purpose of reconnaissance mapping, which may be on a small, intermediate, or large scale, is to provide information on a temporary or immediate basis and at as low a cost as possible. For this reason, there can be a lack of uniformity in the amount of detail shown in different parts of the map, especially regarding the topographic or cultural features. Since these maps are generally made for a specific purpose, such as a geological survey or a highway location, features that have no bearing on the purpose for which the map is intended are often omitted. Distances can be approximated using a measuring wheel rolled along the ground, or clocked with an automobile speedometer, and elevations in localities with extreme height can be approximated using a barometer calibrated in thousands of feet, such as the altimeters used in airplanes.

Careers and Course Work

Traditionally, surveying was learned in an apprenticeship program. Interested young people would seek out a surveyor who needed assistants and was willing to teach them the skills involved. Years of fieldwork would be involved, and during this time, it was assumed that the apprentices would pick up the necessary background in mathematics, geometry, and trigonometry at a high school, in an adult education course, by attendance at a college or university. Eventual employment as a surveyor would result. Though apprenticeships remain a valuable learning tool, the typical path to a career as a modern-day surveyor differs.

Colleges and universities offer surveying courses leading either to a two-year associate's degree in surveying, or a four-year bachelor of science degree in surveying. Numerous career paths are open to graduates of such programs. Employment by private surveying and engineering firms is one possibility. Another is working for the highway departments of cities, counties, or states. Other governmental agencies that employ surveyors are state lands commissions, the US Forest Service, and the Bureau of Land Management, which is part of the US Department of the Interior. A typical educational program is the one offered by the surveying education department at New Mexico State University. It leads to a bachelor of science in surveying engineering and is accredited by the Engineering Accreditation Commission. Students generally go on to jobs in construction surveying or boundary surveying, but those going into boundary surveying will have to take an exam to become licensed.

Social Context and Future Prospects

Surveying offers young people interesting careers with excellent prospects. The working conditions are generally out-of-doors, and the job assignments can vary from setting up tripods in snake-infested swamps to lugging them up mountainsides to perch them on the crests of windswept peaks. Employment with private companies engaged in building construction is very common, as is surveying property boundary lines, including locating section corners and determining rights-of-way.

For land surveyors, knowledge of the legal principles of boundary surveying is essential, and one must become licensed before work as a professional can begin. Surveyors are involved in many unusual tasks, such as mapping crime and accident scenes in forensic surveying or being called to testify as expert witnesses in court. Oil prospects, historic locations, mining claims, natural parks, and archaeological sites require surveying, and a surveyor with business skills can form their own company. The demand for surveyors will not cease, and even when business downturns temporarily reduce the need for them in construction and boundary surveying, governmental agencies such as highway departments and environmental agencies will always need surveyors.

Modern surveyors primarily utilize an array of technology-aided tools. The most commonly used tools include total stations, robotic total stations, three-dimensional laser scanners, and global positioning satellites. However, each method has strengths and weaknesses, and the correct tool must be used considering the surveying task. Plane surveying is limited to relatively small areas, and calculations are based on straight rather than spherical distances between points. These limitations make measuring tapes, chains, and theodolite use more practical.

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