specialised types of survey

also known as tape survey is one of the methods of land surveying in which sides of various triangles are measured directly in the field. It is the simplest but accurate method of land surveying. The method involves measuring a series of straight lines using chain or tape measure. The method is purposely carried out to map flat small areas or near flat terrain and the associated features like foot paths and buildings; to determine the area of surveyed land; to prepare an accurate plan of a plot of land; to restore lost boundary marks; to divide a plot of land into a number of smaller units; to update an existing large scale map, and to set out engineering structures such as roads, railways and dams when implementing engineering projects.

Chain

a chain is a tool used for measuring distances on the ground. A chain is primary equipment in chain surveying. It is a tempered steel wire with the length ranging from 20 m to 30 m long.

Tape measure

A tape measure is also a primary equipment in chain/tape surveying. It is used to measure distance. It consists of a ribbon of cloth, plastic fibre or metal strip with linear measurement markings. Surveyor's tape are normally measures in lengths from 50m to over 100m.

Cross staff

This is a metal or wooded cross fitted on a short ranging pole with eye slits at right angle . It is the simplest instrument used for setting out perpendicular lines from a chain line. Establishment of right angles in chain surveying is the easiest and quickest method though it is not very accurate. If a survey project requires data of great accuracy, survey instruments such as Theodolite or Total stations can be used.

Ranging pole and ranging rods

The two equipment have always been used interchangeably due to their visual appearance and mode of formation. Ranging rods and poles are wooden, or steel rod made round or rectangular in shape. However, there is a slight difference between ranging rod and a ranging pole. Ranging rods are used for marking temporarily positions of stations while straightening a line. They are made of well- sectioned straight grained timber of teak or deodar and are generally available in 2 m or 3 m length and 3 cm in diameter. A rod is divided into equal parts, each part measuring 0.2 m. Its lower end is provided with a cross shoe of 15 cm length. They are generally painted alternatively red and white, black and white or yellow and white throughout their length to facilitate visibility. On the contrary, ranging poles are similar to a ranging rod except that they are of heavier section of length of 4 m to 6 m long. They are used for ranging very long lines in undulating ground.

(a) Ranging pole (b) ranging rod

Arrows

These are thin steel skewers for marking points on ground temporarily. They are 30 cm to 40 cm long with colored rag tied to the circular end to make them more visible.

Pegs

Pegs are usually wooden with 40 mm square and 50 cm long. They are used to mark permanent positions of station in the traverse by driving them into the ground by a mallet at the required point. In a very hard ground iron points, nail or a bar made of cement are used to mark position permanently,

Offset rods

Offset rods are similar to ranging rods. They are usually of 3 m long, divided into equal parts of 0.2 m. Different from ranging rods, offset rods are provided with an open hook at their top to aid for pulling or pushing a chain through obstruction like bushes, rocks, and small water streams . Two narrow vertical slots passing through the centre of the section at right angles to one another are provided at the eye level. It is used for aligning the offset over traversing.

Plumb bob

Plumb bob is a metal weight, usually with a pointed tip on the bottom, suspended from a string and used for transferring points vertically from the ground to the tape or chain and vice versa.

Abney level and clinometer

While Abney level is used for insuring the chain or tape is on horizontal plane when taking measurement. Clinometer is used to measure slope angles for slope reduction.

Abney level Clinometer

Field notebook and recording materials

These are important facilities every surveyor needs for recording all measurements and other information taken in the field. It include: notebook, rubber, pen, and pencil, however, with technological development, surveyors also use laptops, and iPad for recording field data.

There are several important technical terms used in chain/tape surveying. These terms are;

1. main survey station,
2. subsidiary survey station
3. main survey lines
4. check lines
5. auxiliary or subsidiary or tie lines.

Main survey stations

are points where two sides of a main triangle meet. They are point at either end of a chain line. Main survey stations are usually inter visible from either side of observation.

Subsidiary survey station (or tie station)

refers to a station which is selected on the main survey lines for running auxiliary lines.

Main survey lines

are chain lines joining the two main survey stations. In chain surveying, the main survey line is a baseline that surveyors establish to take lateral measurements.

Auxiliary, subsidiary or tie lines

are chain lines joining two subsidiary survey stations. Usually, they are established to locate the interior details which are far away from the main lines.

Check lines

are lines established to check the accuracy of the fieldwork. If the measured length of a check line correspond with the length scaled off the plan, the survey is accurate. Each triangle is generally provided with a check line. The check lines may be laid in such a way that maximum numbers of details are intersected by it. Check lines may also be laid by joining the apex of the main triangle to any point on the opposite side or by joining two points on any two sides of the triangle.

There are number of causes of errors in the data obtained from chain surveying. Human source of errors out pace other sources including instrumental and environmental causes. Errors resulting from;

1. improper arrangement of ranging poles
2. incorrect reading
3. misplacing decimal points
4. surveyors' negligence
5. inexperience leading improper set of instruments
6. use of incorrect unit of measurement
7. use of steel tape whose length varies with changes in temperature
8. use of out dated equipment lead to incorrect data acquisition
9. loss of resources including funds and time.

However, the effects of environmental factors such as land obstacles, weather effects like wind and rainfalls and steep slopes on a land to be surveyed can be overcome.

1. Dealing with Land Obstacles (e.g., trees, buildings)
   • Use offsets: Measure around the obstacle by taking measurements at right angles to the main line.
   • Use longer chains or tapes: Go around the obstacle and extend the survey line.
   • Use triangulation: Measure from two points to locate the obstacle indirectly.
2. Managing Weather Effects (e.g., wind, rain)
   • Choose the right time: Conduct the survey early in the morning or when the weather is calm to avoid strong winds or heavy rain.
   • Use weatherproof tools: Use tools like tapes and chains made of materials that are not affected by water.
   • Protect tools: Cover equipment with plastic or waterproof materials during rain.
   • Postpone the survey: If the weather is extreme, wait for better conditions to ensure accuracy.
3. Overcoming Steep Slopes
   • Step chaining: Break the slope into small, flat steps and measure each step separately.
   • Use vertical angles: Use instruments like a clinometer to calculate the true horizontal distance.
   • Anchor properly: Secure chains and tapes to prevent slipping while measuring on slopes.
4. Improving Visibility
   • Use ranging poles: Place them at regular intervals to maintain a straight line even when obstacles block direct sight.
   • Use flags or markers: Mark points clearly to avoid errors when obstacles obscure the line.
5. Avoiding Human Errors
   • Work in teams: Assign specific roles to each team member, such as holding the chain, recording measurements, and marking points.
   • Double-check measurements: Verify every measurement to minimize errors caused by environmental factors.

The compass surveying involves fixing of an object's position in the field by measuring the angles of bearings between the line of magnetic North (0°) and the line of sight to the object. This section exposes readers into concepts related to compass survey. Geography students especially of survey class must adopt some important personalities necessary for theoretical and practical studies through learning to check for the equipment's condition, handling them with care and wearing proper field-dress. Concepts covered in this section are history of compass surveying, types of compass, compass bearing, compass traversing, local attraction, closing error adjustments, intersection method, and resection method.

Measuring angle by compass

Types of compass survey

There are two major types of compass surveys which are the prismatic and surveyor's compasses. The compasses are differentiated by key features including; their body size, the bearing system they use to designate readings, and their mode of formation.

Surveyor's compass

A surveyor's compass is an old form of compass used by surveyors which is similar to the prismatic compass but with few modifications. It is used to determine the magnetic bearing of a given line. The instrument has commonly been referred to as the Circumferentor. Surveyor's compass is larger and more accurate than prismatic compasses and designated to read bearings in quadrant bearing system (QBS).

Surveyor's compass

Prismatic compass

This is a non-magnetic metal case with a graduated ring and glass top used for determining angles, bearings and direction to objects of surveyor's interest. It is a small, hand-held device in a circular box of about 100 mm in diameter used in fixing objects in the field and from which the angle of bearing is measured between the line of magnetic north and the line of sight to the object. It is usually used for surveying works that do not require very high accuracy. For example preliminary survey for minerals exploration.

Prismatic compass

To reduce possible errors during a compass survey, the following are very crucial.

1. Check the accuracy of the compass by comparing it with an accurate compass or with location of magnetic north (MN) for the year of the survey with updated MN correction adjustment for field readings.
2. Always check every reading with a back bearing and adjustment by adding or subtracting the mean of the error.
3. Hold the compasses steady at 180° and ensure accuracy of all chain or tape measurements between ends of legs and to points of observation on legs.
4. Check that all conversions from magnetic bearing to true bearing are accurate and all scaled conversions are accurate.
5. Ensure the booker takes down reading accurately by asking him to repeat where necessary.
6. Avoid areas where there are deposits of metalliferous ores steel structures, metal gates, railway lines and others, which would influence reading.
7. Avoid wearing metal rimmed spectacles, metal bangles or steel watches when using the compass.

1. First, fairly rapid method in the field, compared to other old survey methods, like simple chain survey.
2. Secondly, a check can be made on all compass bearing, simply by calculating the forward and back bearing.
3. Thirdly, cumulative error is reduced and can be easily rectified, with the help of advanced devices used.
4. Fourthly, near and distant objects can be pin pointed with accuracy by using various instruments which are able to accommodate the distance of object.
5. Fifthly, the method can be combined with other methods such as chain levelling and plane table, and in fact the prismatic compass is sometimes needed for mapping.

1. In elementary survey, compass observations over long distance object cannot be checked by back bearings unless transport is provided.
2. Human error is involved in taking as it is difficult to hold the compass absolutely steady; taking the mean of the difference between forward and back bearing reduces error but does not entirely eliminate it.
3. The presence of ore bodies may not be known to the surveyor and this would affect readings. It is always better to consult the geological map or geologist first but the presence of iron ores in large quantity would necessitate using another method.