Compass surveying (Clinometer Compass and Prismatic Compass)

Let’s come to know the topic of Compass surveying (Clinometer Compass and Prismatic Compass). Do you know? Compass surveying is the process of land survey where different types of the compass are used for the different objectives of the survey. Mainly Compass Survey refers to Prismatic Compass Survey be which area of small plots can be measured and area plan can be framed.

For surveying purposes we generally used main two types of Compass:

  1. Clinometer Compass
  2. Prismatic Compass

Clinometer Compass

A clinometer or inclinometer Compass measures the inclination or dip of any rock bed or slope of a plane. ‘Cline’ or ‘incline’ means dip or inclination and a ‘meter’ means the measuring instruments. Surveyors mainly use to measure the dip and direction of a rock bed or earth layer to prepare a map or other civil planning for construction purposes in Geological and Geomorphological fieldwork. Generally, clinometer compass surveying is useful for preparing the geological map.

Basic Terminology for Clinometer Compass Survey:

  • Strike line of a bed is a line representing the intersection of that feature with a horizontal plane.
  •  Dip or Angle of dip is the acute angle that a rock surface makes with a horizontal plane.
  • Dip direction is the azimuth of the direction of an inclined plane. It can be true dip direction and apparent dip direction.

Data tabulation:

Surveyors use the Clynomete for compass surveying to measure the alignment of strike and amount of dip with the direction of a particular rock bed at least three times and thereafter to eliminate the manual error they take the average value of each. The following table represents a sample average value as well as a rough scratch of the concerned field.

Prismatic Compass

Prismatic Compass is an instrument that has a prism attached with it to read the value engraved on a magnetic compass. The prism is Plano-convex in nature i.e. a 45o reflecting prism plane with the eye side face and its reading faces made slightly convex to magnify the image of reading on a circular magnetic plate. The magnetic bearing can easily visualize the presence of a prism.  It is a small instrument that is capable to hold on the hand for observing. Most of the surveyor uses it on tripod stands to view the ranging rod.

 A popularly used compass is the surveyor’s compass which gives a better and accurate reading. It is a larger and more accurate instrument and is generally uses on a stand or tripod and ranging rod.

We use a Prismatic compass or surveyor’s compass to measure the area. It also helps us to plot the data and prepare an area plan in a Geographical survey and general land survey. It is the main task of Compass surveying.

Basic Terminology for Prismatic Compass Survey:

  • Magnetic North:  A freely oscillated magnetic needle indicates a north point. It is the magnetic north.
  • Geographical North or True North:  The Earth’s geographical axis indicates the geographical north or true north.
  • Magnetic variation: It is the deflection angle between true north and magnetic north. It occurs due to the solar storm, the precessional motion of the earth and other cosmic phenomena.
  • Whole Circle Bearing (WCB): Whole Circle Bearing is the total angular distance of a line in a clockwise direction from the magnetic north.
  • Forward Bearing (FB): Forward Bearing is the total angular distance of a line in a clockwise direction from the magnetic north in the starting point.
  • Backward Bearing (BB): Backward Bearing is the total angular distance of a line in a clockwise direction from the magnetic north in the ending point.
  • Bearing Error or Error: Bearing Error or simply Error is the difference between the forward bearing (F.B.) & backward bearing(B.B.) is not equal to 1800. It is a Positive Error when the difference is more than 1800 and it is the Negative Error when the value is less than it.
  • Bearing Error Correction: We divide the total error into two. The value of half of the error we add with the grater value and deduct from the lesser value of forward bearing or backward bearing in case of a negative error. If the error is positive then it will be vice versa. It is very important to remember when anyone prepares a final book.

Collection of field data:

Surveyors collect the data from the field by preparing a field book for Compass surveying. Here is a specimen of the field book and a rough sketch of the field. Anyone can easily follow the method of field data tabulation for prismatic compass surveying.

Preparation of Final Book:

After collecting the data from the field surveyor proceeds for preparing  Final Book. It is most important to remember that there are many types of bearing errors. The most common bearing errors are manual error, instrumental error, and error for a local attraction. We must eliminate the error by the error correction method. Remember that two types of errors are positive error and negative error. We distribute the error in both forward and backward bearing so that the difference between the two bearings will be 180 degrees. The following table represents a sample Final Book of close traverse survey by Prismatic compass or compass surveying.

Plotting of Data:

To represent the drawing sheet we must consider the corrected forward bearing in the calculation table of the above final book. In the case of a close traverse survey starting point is always the ending point. Closing error is the gap between the starting point and the ending point when we plot the data of a close traverse in a compass survey. We always correct the closing error in a close traverse survey in different ways. Bowditch’s method is widely popular to correct the closing error in compass surveying. The following diagram is a sample representation of data plotting. Here error correction process will be helpful for the compass survey. Anybody can follow it to represent the work properly.

Methodology for calculating of Area:

In the compass survey, the area calculation is important. Here the Methodology of area calculation is based on the formula of area of a triangle. You first divide the total traverse area into two or more triangles. Anyone can follow the formula below, to calculate the area of a plot in compass surveying.

Sample area Calculation:

We have plotted the traverse here and based on it, we calculate the area of traverse in compass surveying. Here we must remember that the conversion of the map area to the ground area is very important. Everyone should take care of the unit of area in both cases.

Internal Angle calculation:

We can calculate the corrected bearing and internal angle of the traverse on the basis of collected data. The formula of the sum of the internal angle of a polygon is 1800 – (n-2). Always remember the drawing to calculate the internal angle. Sometimes at some angle, the value of bearing needs deduction from 3600 based on the drawing. The table below shows the sample internal angle calculation.

Conclusion:

This is all about the Compass surveying (Clinometer Compass and Prismatic Compass). All the procedures of data collection, tabulation, and plotting of data are minutely represented here. To know more about Compass surveying (Clinometer Compass and Prismatic Compass) you can comment below so that I can clear your doubts. Thank You.

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