Magnetic Compass
A magnetic compass is an instrument used for surveying in order to measure angles and direction. It utilizes the earth’s magnetic field to determine direction and usually consists of a needle that points toward the magnetic north pole. There are two main types of magnetic compasses: the prismatic compass and the surveyor’s compass.
Prismatic Compass
The prismatic compass is a specialized surveying tool used for accurate measurements of angles and directions. It comprises a prismatic compass with a small telescope, divided circle, and a graduated scale. This is used to measure the direction of survey lines, and the angles between them.
Surveyor’s Compass
The surveyor’s compass is a more basic tool used for surveying. It consists of a needle, a compass box, and a graduated scale on the box’s lid. It is used to measure the direction of survey lines, and the angles between them. The surveyor’s compass is less accurate than the prismatic compass, but is easier to use and more portable.
Principle of Magnetic Compass
How to Determine Magnetic Bearings of a Line
Using a magnetic compass, the magnetic bearings of a line can be determined. Depending on the type of compass used, these bearings can be measured in either the Whole Bearing System (WBS) or Quadrantal Bearing System (QBS).
Working Principle of a Magnetic Compass
The working principle of a magnetic compass is based on the idea of suspending a narrow strip of steel or iron around its center, so that it can freely oscillate about the vertical axis. When this happens, the strip will align itself with the magnetic meridian at the place of observation.
Features of a Magnetic Compass
Magnetic Needle
The magnetic needle is used to establish the magnetic meridian, allowing the direction of survey lines to be read.
Line of Sight
The line of sight feature allows the surveyor to sight the other end of the survey line through the compass.
Graduated Circle
The graduated circle is used to read the directions of the lines, and can be attached either to the box or to the needle.
Compass Box
The compass box is used to house the components of the compass, and is supported by either a tripod or a suitable stand.
Types of Compass Used in Surveying
The main types of compasses that are used in compass surveying are:
- Prismatic Compass
- Surveyor’s Compass
1. Prismatic Compass
What is a Prismatic Compass?
A prismatic compass is a type of magnetic compass that is small and lightweight, making it highly portable. The compass can be held in hand or placed in a tripod stand to take measurements. It is one of the most convenient forms of a magnetic compass.
Parts of a Prismatic Compass
Figure-2 below illustrates the different parts of a prismatic compass. It consists of a compass needle, a sighting arm, a sighting mirror, a protractor, and a graduated metal plate. The needle is magnetized and points to the magnetic north, while the sighting arm and sighting mirror help to make readings. The protractor is used to measure angles and the graduated metal plate is used to measure distances.
What is the Line of Sight?
The line of sight is an imaginary line connecting two points of reference, usually the object vane and the eye vane. This line is used to determine the direction of an object from the observer’s point of view. A prismatic compass is often used to help with both sighting and reading.
How Does a Prismatic Compass Help?
A prismatic compass is a specialized tool that is used to measure the angle of a line of sight between two points. It helps to conduct both sightings and readings simultaneously and accurately. The prismatic compass is fitted with a system of graduation which is used to measure the angle of the line of sight.
What is the System of Graduation?
The system of graduation is a system of markings found on the prismatic compass. These markings help to accurately measure the angle of the line of sight. By reading the markings, the observer can easily identify the direction of the object in relation to the observer.
Setting Up the Compass
The first step in taking survey measurements with a compass is to set up the compass at the starting station of the survey line. This involves taking the necessary adjustments to ensure that the magnetic meridian is obtained.
Taking Measurements
Once the compass is set up, the measurements are taken by sighting to the next station. The readings increase in a clockwise direction, starting with south (0 degrees) and moving to west (90 degrees), north (180 degrees), and east (270 degrees).
Temporary Adjustments for Prismatic Compass
Centering:
Adjustment of the Prismatic Compass for Proper Centering
Making the necessary adjustments to the prismatic compass to ensure proper centering is an important part of the process. This is done by making sure the compass is level and adjusting the prism so that the reflection of the needle is centered in the field of view.
Levelling:
Ensuring the Prismatic Compass is Level for Accurate Readings
A prismatic compass must be levelled in order to ensure accurate readings. This is done by adjusting the bubble on the compass until it is level. If the bubble is not level, the readings from the compass will not be accurate.
Focusing the Prism:
Focusing the Prismatic Compass for Clear View of the Needle
The last step in adjusting a prismatic compass is to focus the prism. This is done by adjusting the focus rings on the prism until the needle is clear and in focus. Once the focus is set, the prismatic compass is ready for use.
- Centering: Achieving Precise Instrument Position
Centering is an important step in surveying to ensure the instrument is correctly positioned over the station point. This can be done by adjusting the tripod stands or by suspending a plumb-bob from the instrument. To verify the centering, a pebble can be dropped from the instrument’s center to the bottom. This helps to ensure the instrument is precisely positioned before taking measurements.
- Setting Up the Theodolite:
Setting up the theodolite is an important step in any surveying project. To ensure the instrument is set up correctly, it must be placed such that the graduated disc is level from the top-edge. The theodolite should be held in the hand or it can be placed on a tripod for levelling, depending on the situation. Additionally, the instrument must be placed in a secure location to prevent any potential damage.
- Levelling the Theodolite:
Ensuring Accuracy with Proper Levelling
For accuracy in the measurements taken with the theodolite, the instrument must be levelled. To do this, the graduated disc must swing freely when viewed from the top edge, and should appear level. If the theodolite is not being used as a hand instrument, a tripod should be used to support it for levelling.
- Preparing the Refractometer:
How to Set Up a Refractometer
A refractometer is a device used to measure the refractive index of a material, which is how much light is bent when it passes through the material. To use a refractometer, you must first prepare the device for testing. This involves assembling the components, filling the sample well, and setting the prism attachment.
- Filling the Sample Well:
How to Fill the Sample Well of a Refractometer
The sample well of a refractometer needs to be filled with the material being tested. This can be a liquid, solid, or gas. To fill the sample well, the material must first be prepared for testing. Depending on the type of sample, this can involve diluting the material, dissolving it in a solvent, or grinding it into a powder. Once the sample is ready, it is placed into the sample well and allowed to settle before the readings are taken.
- Focusing the Prism:
How to Focus the Prism of a Refractometer
After the sample well has been filled, the prism attachment of the refractometer must be focused before the readings can be taken. This is done by sliding the prism attachment up and down until the readings are observed to be sharp and clear. Once the readings are in focus, the test can be completed.
2. The Surveyor’s Compass
Sectional View of a Surveyor’s Compass
The figure-4 below illustrates a sectional view of a surveyor’s compass. This device is composed of a circular brass box which holds a magnetic needle. The needle has the capability to move around the bass circle which has been divided into 360 degrees.
What is a Surveyor’s Compass?
A surveyor’s compass is a device used to measure horizontal angles in the north-south axis. It consists of a pair of sights which are mounted on a tripod and leveled using a ball and socket mechanism. It is commonly used by surveyors, engineers, and architects to measure and plot distances on a map.
How Does a Surveyor’s Compass Work?
The surveyor’s compass is used to measure angle and distance on a map. It consists of two sights that are aligned on the north-south axis. The sight is then leveled using a ball and socket mechanism which is mounted on a tripod. The surveyor is then able to measure the angle between two points and plot the distance between them on a map.
What are the Benefits of a Surveyor’s Compass?
The surveyor’s compass provides a fast and accurate way to measure and plot distances on a map. It is also a useful tool for surveyors, engineers, and architects who need to accurately measure and map out land or buildings. Additionally, the surveyor’s compass is easy to use and requires minimal setup and maintenance.
Temporary Adjustments for the Surveyor’s Compass
The temporary adjustments for the surveyor’s compass are the same as that of the prismatic compass. These adjustments are necessary to ensure accuracy and precision when taking measurements. The adjustments include: adjusting the levels, adjusting the needle, adjusting the sight vanes, and adjusting the pivot.
Adjusting the Levels
The levels of the surveyor’s compass must be adjusted in order to ensure that the readings are accurate. This involves setting the bubble in the level to the center of the vial and adjusting the screws to ensure that the bubble stays there.
Adjusting the Needle
The needle of the surveyor’s compass must be adjusted in order to ensure that it is accurately aligned with the magnetic north pole. This is done by adjusting the needle’s tension and ensuring that it is pointing in the correct direction.
Adjusting the Sight Vane
The sight vane of the surveyor’s compass must be adjusted in order to ensure that it is pointing in the correct direction. This is done by adjusting the screws on the vane and making sure that it is pointing in the direction of magnetic north.
Adjusting the Pivot
The pivot of the surveyor’s compass must be adjusted in order to ensure that the readings are accurate. This is done by adjusting the screws and making sure that the pivot is in the correct position.