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Surveying may be defined as the method of making measurements of the relative positions of natural & man-made features on the earth's surface and the presentation of this information either graphically or numerically. In other words, Surveying is the art of determining the relative positions of objects on the surface of the earth by taking measurements in the horizontal and vertical planes. The surveying may primarily be divided into two main categories: Plane Surveying, in which the curvature of the earth is not taken into account and Geodetic Surveying, in which the curvature of the earth is taken into consideration. Geodetic Surveying is also known as Trigonometrical Surveying. It should be noted that generally, for an area less than , Plane Surveying is employed.
Prismatic Compass Surveying is very important step in creating the ecosystem that surrounds us as humans. This technique or science does measure and determines the 3D positions of particular points and distances on a surface, as well as the angles between them. Compass Surveying is essential when it comes to measuring angles.
In the following article, we will be going to study Prismatic Compass Surveying as a technique to measure angles. This article will be designed in such a way that it will be helpful for your preparation of various examinations like GATE CE, RRB JE Civil and SSC JE CE.
Prismatic Compass surveying is a significant part of surveying that is frequently used to locate an item using both angular and linear measurements. In this case, a compass is used to measure angles, while a chain or tape is used to measure lengths.
Some priorities for prismatic compass surveying are as follows-
But there are also some limitations to prismatic compass surveying. It is not recommended for the areas that avoid public attention and have the presence of iron ore deposits, magnetic substances like metallic or steel structures, electric cables conveying current, etc.
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If the R.L of a B.M is 50 m, the back sight is 1.25 and foresight is 1.85, then what will be the R.L of the forward station? (in 'm')
Match the code of signals for direct ranging. List 1 indicates the Signals by the surveyor and List 2 indicates the Action by the assistant in the field:
|
List 1 |
|
List 2 |
M. |
Rapid sweep with the left hand |
1. |
Continue to move to the left |
N. |
Slow sweep with the left hand |
2. |
Move considerably to the left |
O. |
Left arm extended |
3. |
Plumb the rod to the left |
P. |
Left arm up and moved to left |
4. |
Move slowly to the left |
5. |
Fix the rod in position |
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The various instruments in Compass Surveying are:
There are mainly two kinds of Compass used in Surveying, namely Prismatic Compass, Level Compass and Surveyor Compass.
The prismatic compass is a magnetic compass in which there is a prism for taking observations. It consists of a circular box about 100 mm in diameter. The magnetic needle used in a prismatic compass is broad in shape. The prismatic compass is used in the calculation of whole circle bearings of the lines. Sighting of the object and reading the bearing are done simultaneously in the case of a prismatic compass.
Parts of a prismatic compass are as follows:
Fig.1. Prismatic Compass
There are two types of adjustments of a prismatic compass: temporary and permanent.
Temporary adjustments include the following:
Permanent adjustment of a prismatic compass includes the following:
Fig.2. Prismatic compass bearing
A surveyor compass is an instrument used for the measurement of horizontal angles and the bearing of a line of sight. The Surveyor compass is commonly referred to as the Circumferentor. It essentially consists of a graduated horizontal circle, a pivoted magnetic needle, and a sighting device. The size of the surveyor compass is defined by the diameter of the reading edge of the graduated ring. It generally ranges from 50 mm to 200 mm. The surveyor compass is used to determine the reduced bearings or quadrantal bearings of lines.
The parts of the surveyor compass are listed below:
Fig.3. Surveyor Compass
The readings from a surveyor's compass are expressed in terms of bearings, which are angular measurements relative to the magnetic north or south.
Fig.4. Surveyor Compass Bearing
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A compass traverse is a series of connected survey lines measured with a compass and chain/tape to plot boundaries or features. It is used when terrain prevents full triangulation. There are two main types of compass traverse:
A "level compass" typically refers to a compass that incorporates a bubble level, also known as a spirit level. The bubble level is a small, transparent tube partially filled with liquid and an air bubble.
Bearings are used to express the direction of survey lines or features. A bearing is the angle between a survey line and a reference direction, typically measured clockwise from the north direction.
Type |
Description |
Open Traverse |
Ends at a point different from the starting point. Used for roads, canals, pipelines. |
Closed Traverse |
Starts and ends at the same point. Used for boundary surveys and land plotting. |
Local attraction refers to the deflection of the magnetic needle due to nearby metallic objects, underground cables, or iron ores. This causes incorrect bearing readings, leading to errors in plotting. Surveyors must detect and correct local attraction by comparing fore bearings and back bearings of survey lines. If the difference deviates from 180°, local attraction is present.
Common Causes of Local Attraction:
Correction methods include adjusting readings using corrected bearings or avoiding the affected station altogether.
Measurements of angles and distances are made in various surveying operations. It is impossible to determine the true values of these quantities because some type of errors always creep in every measurement. The errors occur due to imperfection in instruments, due to human limitations, due to environmental changes or due to carelessness. As quality surveyors, our aim should be to minimise these errors while taking observations. Still, the errors which creep in should be eliminated, or their effects should be corrected. Errors can arise due to various reasons stated above during the process of surveying are classified as:
Personal errors, as the name suggests, arise because of the carelessness of the surveyor. Carelessness here signifies;
Natural errors are occurring due to the various natural causes which affect the working of the compass. It is not within the limit of the surveyor to minimise or eliminate them. They are:
The accuracy of a set of repeated observations is defined as the amount of closeness of
their mean to the population or distribution mean, i.e., the closeness of the mean of
observations to the true value.
For linear measurement:
The degree of accuracy of linear measurement is usually expressed as the ratio of the
probable error and the measured distance.
For angular measurement:
For angular measurements, the degree of accuracy is usually expressed as k× √N, where
N = Number of angles measured
The angular measurement taken in compass surveying has the error should not be more than 5 minutes. Hence, the limit of accuracy is not more than 5 minutes.
Unit of Measurement in a Compass
The unit of measurement typically used in compass surveying is the degree (°). A compass can measure angles in degrees, and bearings are expressed in degrees as well.
Accurate field note booking is crucial in compass surveying. It involves recording the name of lines, observed bearings, and measured distances in a tabular format for plotting and calculations. Below is a basic format of a compass field book:
</table></div>Line |
Fore Bearing (FB) |
Back Bearing (BB) |
Distance (m) |
AB |
60° |
240° |
50 |
BC |
120° |
300° |
40 |
CD |
200° |
20° |
60 |
The difference between prismatic compass and surveyor compass is discussed below.
Prismatic Compass |
Surveyor Compass |
Graduation circle is fixed to a broad type needle. Hence, it will not rotate with the line of sight. |
Graduation circle is fixed to the box. Hence, it rotates with the line of sight. |
There is a prism at the viewing end. |
At the viewing end, there is no prism. There is only a slit. |
Sighting and reading can be done simultaneously |
Sighting and viewing cannot be done simultaneously. |
The magnetic needle does not act as an index. |
Magnetic needle acts as index while reading. |
The graduations are in whole circle bearing. |
The graduations are in the quadrantal system or called Reduced Bearing. |
Graduations are marked inverted since their reflection is read through prism. |
Graduations are marked directly. They are not inverted. |
The reading is taken through a prism. |
The reading is taken by directly viewing from the top glass. |
Tripods may or may not be used. It can be held on a stretched hand also. |
Tripod is essential for using it. |
Compass Surveying comes with several advantages and disadvantages of its own. Let's look at them;
The application of compass surveying are as follows:
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