Is a geological survey the same as a geodetic survey? If not, what is it?

What does boring mean?

What does a gravimeter measure?

What does a magnetometer measure?

What does a seismograph measure?

Surveying

Before any civil engineering project can be designed, a survey of the site must be made. Surveying means measuring-and recording by means of maps-the earth's surface with the greatest degree of accuracy possible. Some engineering projects- highways, dams, or tunnels, for example-may require extensive surveying in order to determine the best and most economical location or route.

There are two kinds of surveying: plane and geodetic. Plane surveying is the measurement of the earth's surface as though it were a plane (or flat) surface without curvature. Within areas of about 20 kilometers square-meaning a square, each side of which is 20 kilometers long-the earth's curvature does not produce any significant errors in a plane survey. For larger areas, however, a geodetic survey, which takes into account the curvature of the earth, must be made.

The different kinds of measurements in a survey include distances, elevations (heights of features within the area), boundaries (both man-made and natural), and other physical characteristics of the site. Some of these measurements will be in a horizontal plane', that is, perpendicular to the force of gravity. Others will be in a vertical plane, in line with the direction of gravity. The measurement of angles in either the horizontal or vertical plane is an important aspect of surveying in order to determine precise boundaries or precise elevations. In plane surveying, the principal measuring device for distance is the steel tape. In English-speaking countries, it has replaced a rule called a chain, which was either 66 or 100 feet long. The 66-foot-long chain gave speakers of English the acre, measuring ten square chains or 43,560 square feet as a measure of land area. The men who hold the steel tape during a survey are still usually called chainmen. They generally level the tape by means of plumb bobs, which are lead weights attached to a line that give the direction of gravity. When especially accurate results are required, other means of support, such as a tripod - a stand with three legs - can be used. The indicated length of a steel tape is in fact exactly accurate only at a temperature of 20° centigrade, so temperature readings are often taken during a survey to correct distances by allowing for expansion or contraction of the~tape.

Distances between elevations are measured in a horizontal plane. When distances are being measured on a slope, a procedure called breaking chain is followed. This means that measurements are taken with less than the full length of the tape.

Lining up the tape in a straight line of sight is the responsibility of the transitman, who is equipped with a telescopic instrument called a transit. The transit has plates that can indicate both vertical and horizontal angles, as well as leveling devices that keep it in a horizontal plane. Cross hairs within the telescope permit the transitman to line up the ends of the tape when he has them in focus.

Angles are measured in degrees of arc. Two different systems are in use. One is the sexagesimal system that employs 360°, each degree consisting of 60 minutes and each minute of 60 seconds. The other is the centesimal system that employs 400 grads, each grad consisting of 100 minutes and each minute of 100 seconds. A special telescopic instrument that gives more accurate readings of angles than the transit is called a theodolite.

In addition to cross hairs, transits and theodolites have markings called stadia hairs (stadia is the plural of the Greek word stadion, a measure of distance). The stadia hairs are parallel to the horizontal cross hair. The transitman sights a rod, contour lines, the lines on a map that enclose areas of equal elevation.

 Contour maps can be made in the field by means of a plane-table alidade. The alidade is a telescope with a vertical circle and stadia hairs. It is mounted on a straight-edged metal plate that can be kept parallel to the line of sight. The surveyor can mark his readings of distances and elevations on a plane (or flat) table that serves as a drawing board. When the marks epresenting equal elevations are connected, the surveyor has made a contour map.

Heights or elevations are determined by means of a surveyor’s level, another kind of telescope with a bubble-leveling device parallel to the telescope. A bubble level, which is similar to a carpenter's level, is a tube containing a fluid that has an air bubble in it. When the bubble is centered in the middle of the tube, the device is level. The surveyor sights a rule called a level rod through the telescope. The rod is marked off to show units of measure in large, clear numbers. The spaces between the marks usually are alternately black and white in order to increase visibility. The number that the surveyor reads on the level rod, less the height of his or her instrument, is the vertical elevation.

Heights are given in relation to other heights. On maps, for example, the usual procedure is to give the elevation above sea level. Sea level, incidentally, can be determined only after averaging the tides in a given area over a definite period. A survey carried out by level and rod often gives the elevation in relation to a previously measured point that is called a bench mark. Approximate elevations can also be measured with an altimeter, which is a device that takes advantage of changes in atmospheric pressure. Readings taken with an altimeter are usually made at two, and sometimes three, different points and then averaged. The readings must be corrected for humidity and temperature, as well as the weight of the air itself.

Modern technology has been used for surveying in instruments that measure distance by means of light or sound waves. These devices direct the waves toward a target that reflects them back to a receiver at the point of origin. The length of time it takes the waves to go to the target and return can then be computed into distance. This surveying method is particularly useful when taking measurements over bodies of water.

Aerial photography is another modern method of surveying. A photograph distorts scale at its edges in proportion to the distance the subject is from being in a direct vertical line with the lens of the camera. For this reason, the photographs for an aerial survey are arranged to overlap so that the scale of one part joins the scale of the next. This arrangement is called a mosaic, after the pictures that are made from hundreds of bits of colored stone or glass.

Geodetic surveying is much more complex than plane surveying. It involves measuring a network of triangles that are based on points on the earth’s surface. The triangulation is then reconciled by mathematical calculations with the shape of the earth. This shape, incidentally, is not a perfect sphere but an imaginary surface, slightly flattened at the poles, that represents mean sea level as though it were continued even under the continental land masses.

In addition to measuring surfaces for civil engineering projects, it is often necessary to make a geological survey. This involves determining the composition of the soil and rock that underlie the surface at the construction site. The nature of the soil, the depth at which bedrock is located, and the existence of faults or underground streams are subsurface factors that help civil engineers determine the type and size of the structural foundations or the weight of the structure that can rest on them. In some areas, these can be critical factors. For example, Mexico City rests on a lakebed with no bedrock near the surface; it is also located in an earthquake zone. The height and weight of buildings must therefore be carefully calculated so that they will not exceed the limits that are imposed by the site. Geological samples are most often obtained by borings, in which hollow drills bring up cores consisting of the different layers of underground materials. Other devices that are used in geological surveys are gravimeters and magnetometers. The gravimeter measures the earth's gravitational pull; heavier rocks like granite exert a stronger pull than lighter ones like limestone. The magnetometer measures the strength of the earth's magnetic field. Again, the denser the rock, the more magnetic force it exerts. A third instrument is the seismograph, which measures vibrations, or seismic waves, within the earth. It is the same instrument that is used to detect and record earthquakes. In a geological survey, it is used by setting off small, man-made earthquakes. The waves created by a blast of dynamite buried in the ground reflect the different kinds of rock under the surface; hard or dense rocks reflect the waves more strongly than soft or porous rocks.

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