Surveying is a significant element for developing the human environment around us. It is the process or science of measuring and determining the 3D positions of specific points and distances on the surface and also the angles between them. When it comes to measuring angles, compass surveying is a must.
What is Compass Surveying?
Compass surveying is an important branch of surveying which is usually adopted in determining the position of an object both by angular and linear measurements. Here angular measurements are taken using a compass and linear measurements are determined using chain or tape.
Some priorities for compass surveying are as following-
- If there is a large area to be surveyed such as the coastal areas or the course line areas of the river.
- If the area is crowded with many details and has many obstacles for conducting chain surveying as triangulation becomes impossible.
- If the surveyor has a specific time limit for conducting the surveying on a large and detailed area.
But there are also some limitations to compass surveying. It is not recommended for the areas that are avoiding public attention and have the presence of iron ore deposits, magnetic substances like metallic or steel structures, electric cables conveying current, etc.
Types of Compass
Following are the types of compass:
- Magnetic Compass
- Prismatic compass or Lensatic compass
- Surveyor’s compass
- Liquid compass
- Marine compass or Card compass
- Baseplate compass or Orienteering compass
- Thumb compass
- Solid-state compasses
- Qibla compass
- GPS compass
- Gyro Compass
- Astro Compass
Following are the compass types used for surveying.
- Prismatic compass
- Surveyor’s compass
- Level compass
A brief description of these types is given below.
The magnetic compass is a compass having a magnetized needle in line with the magnetic poles of the earth.
Fig-1: Magnetic Compass
The magnetic bearings of a line can be determined by a magnetic compass. The bearings can be measured either in the Whole Bearing System (WBS) or the Quadrantal Bearing System (QBS).
There are other types of compasses as following-
Prismatic compass or Lensatic compass
Prismatic or lensatic compasses are used for lining up with an object whose bearing is to be determined. A prismatic compass has a glass prism or a lens and a lid with a hairline. This type of compass often has a magnifying lens for map reading, some sort of light for low light conditions, and a ruler. It is often used for military purposes or plotting.
A prismatic compass is a surveyor’s hand compass held in the hand when in use and equipped with peep sights and a triangular glass prism so arranged that the magnetic bearing or azimuth of a line can be read through the prism from a circular graduated scale and at the same time that the line is sighted over. The name Prismatic compass is given to it because it consists of a prism which is triangular and used for taking observations more accurately (Fig-2). The compass is generally used to run a traverse line. The compass calculates bearings of lines with respect to the magnetic needle. The included angles can then easily be calculated using suitable formulas in case of clockwise and anti-clockwise traverse respectively. For each survey line in the traverse, surveyors take two bearings that are fore bearing and back bearing which should exactly vary by 180° if the local attraction is somehow negligible.
Fig-2: Prismatic Compass
It is an instrument used frequently by surveyors for measuring horizontal angles and also for determining the magnetic bearing of a line of sight. The compass consists of a pivoted magnetic needle, a graduated horizontal circle, and a sighting device. The magnetic needle swings freely over the horizontal circle which is divided into 360 degrees. The horizontal angle is measured using a pair of sights located on the north-south axis of the compass. They are usually mounted over a tripod and leveled using a ball and socket mechanism. A detailed figure of a surveyor compass is as following-
Fig-12: Surveyor Compass
It is a type of compass in which a magnetized needle or dial and the compass card rest on a pivot inside a capsule that is completely filled with liquid. Mineral oil, lamp oil, ethyl alcohol, white spirits, or purified kerosene are commonly used as the liquid.
Fig-3: Liquid Compass
Marine compass or Card compass
This type of compass has a needle that is fixed and a compass card that is mounted in fluid and rotates according to orientation. It is used on boats because the moving card absorbs much of the motion of a boat which makes it easier to read than a needle compass.
Fig-4.1: Marine Compass Fig-4.2: Mariner Compass Card
Baseplate compass or Orienteering compass
Baseplate compass or orienteering compass is a liquid-filled compass that is placed on a rectangular base made of transparent plastic so a map can be read through it. It usually a magnifying lens for map reading, some sort of light for low light conditions, and also a ruler.
Fig-5: Base Plate Compass
Thumb compass is a type of base plate compass but smaller. It is fixed on the thumb as its name says and that is which it leaves one hand free. This type of compass is commonly used in orienteering, a sport in which map reading and terrain association are paramount. The primary features to look for in a thumb compass are quick dampening and high visibility. And this thumb compass delivers on both counts.
Fig-6: Thumb Compass
Solid-state compasses are mainly found in electric devices. They often have two or three magnetic field sensors from which microprocessor reads data about the orientation of the device. They are usually used in clocks, mobile phones, and tablets.
Fig-7: Solid State Compass
Qibla compass is used by Muslims to identify the direction to Mecca, so they would know the direction to face while praying. In Islam, the direction for performing prayer is called qibla that points towards the city of Mecca and especially to the Ka’abah.
Fig-8: Qibla Compass
The full form of GPS is the Global Positioning System. The compass that uses satellites in a geosynchronous orbit over the Earth to show the exact location and direction of the movement of the bearer is called a GPS compass. The devices having GPS compass accurately determine the positions (latitudes, longitudes, and altitudes) of the antennae on the Earth, from which the cardinal directions can be calculated.
Fig-9: GPS Compass
A Gyro compass is a form of gyroscope, used widely on ships employing an electrically powered, fast-spinning gyroscope wheel and frictional forces among other factors utilizing the basic physical laws, influences of gravity and the Earth’s rotation to find the true north.
Fig-10: Gyro Compass
An astrocompass is a navigational tool that uses the positions of various astronomical bodies to determine the direction of true north. It is mainly used in polar regions where magnetic compasses and gyrocompasses are unreliable. It uses current time and geographical position in the form of latitude and longitude. According to this information, it is sighted on to any astronomical object with a known position to give an extremely accurate reading.
Fig-11: Astro Compass