Chain surveying, It is a method of Surveying in which no angles are measured but the only linear measurement is taken in the field by using a chain or tape measure.
It measures a series of straight lines on the ground with a chain or tape measure and all fixed points relative to the line of traverse either by right angles (offsets) or tie lines.
Equipment used in chain surveying.
- The chain is made up of steel wire which is divided into links and togs (rings) to facilitate folding.
- It is sometimes used as a unit of measurement
- It has brass handles at both ends for easy handling. The link is 0.2m or 200mm in diameter.
- The length is 20m or 30m.
A tape is made from fiberglass or a steel strip and is 10m, 20m or 30m in length graduated in 10mm divisions and numbered at each 100mm (10) divisions It is used for measuring short distances
are made up of wood or light metal and measure about 2m long at the top. The equipment has steel shades on its legs so it can be stuck into the ground.
Ranging poles are painted red and white so that they can be easily seen even from a distance. They are used for making stations.
Arrows are made of steel wire of diameter 4mm and their ends are bent into a circle where red cloth is tied to facilitate visibility.
They are used for showing points on the ground.
They are also used for counting the number of chains while measuring a chain line.
Pegs are made of wood 40 mm square by 50 cm long and are used for permanently marking positions during the survey
The surveyor’s band is made of a steel strip which is rolled into a metal frame with a winding handle. It is 30m, 50m or 100m long.
Is used in projects where more accurate measurement is required.
The cross-staff is made of metal or wood with eye slips at right angles and is used to measure right angles from the line of traverse
Notebooks are used during fieldwork to record data obtained. The notebook should be of good quality and 150 mm x100 mm in size
A hard pencil and a rubber
Hard pencils are used for drawing in the field and rubber is used to erase mistakes or errors which are made. A pencil should be HB or HHB.
Methods and procedures involved in chain survey
- A survey team involves three people, the leading chainman or leader, the follower and the booker.
- The chain is thrown to extend it and disentangle any knots
- The leader takes ten arrows and a ranging rod, and the follower takes a ranging rod
- The follower erects his ranging rod/pole at the first base point and places a brass handle of the chain against the ranging rod.
- A leader straightens the chain and inserts an arrow at end of the brass handle. Offsets and tie lines can now be taken.
- The leader drags the chain so that the follower’s end is on the leader’s arrow; the follower moves to another point and places his ranging pole behind the arrow. This procedure is then repeated.
The importance and usefulness of chain surveying
- It is suitable for small areas of fairly open ground.
- It is used to fill in details on a map whose large features have been surveyed by other methods.
- It is used in mapping small areas of flat or near-flat ground and associated objects, for example paths, roads and railways.
- It is used in adding detail to existing plans or large maps.
NECESSARY PRECAUTIONS IN USING CHAINSURVEYING INSTRUMENTS
- After use in wet weather, chains should be cleaned, and steel tapes should be dried and wiped with an oily rag.
- A piece of colored cloth should be tied to arrow (or ribbon – attached) to enable them to be seen clearly on the field.
- Ranging rods should be erected as vertical as possible at the exact station point.
- The operating tension and temperature for which steel bands/tapes are graduated should be indicated.
- Linen tapes should be frequently tested for length (standardized) and always after repairs.
- Always keep tapes reeled up when not in use
GENERAL PROCEDURE IN MAKING A CHAIN SURVEY
- Reconnaissance: Walk over the area to be surveyed and note the general layout, the position of features, and the shape of the area.
- Choice of Stations: Decide upon the framework to be used and drive in the station pegs to mark the stations selected.
- Station Marking: Station marks, where possible should be tied – in to a permanent objects so that they may be easily replaced if moved or easily found during the survey. In soft ground wooden pegs may be used while rails may be used on roads or hard surfaces.
- Witnessing: This consists of making a sketch of the immediate area around the station showing existing permanent features, the position of the stations and its description and designation. Measurements are then made from at least three surrounding features to the station point and recorded on the sketch. The aim of witnessing is to re-locate a station again at much later date even by others after a long interval.
- Offsetting:- Offsets are usually taken perpendicular to chain lines in order to dodge obstacles on the chain line.
- Sketching the layout on the last page of the chain book, together with the date and the name of the surveyor, the longest line of the survey is usually taken as the base line and is measured first.
CRITERIA FOR SELECTING A SURVEY LINES OR OFFSETS
During reconnaissance, the following points must be borne in mind as
the criteria to provide the best arrangement of survey lines,
a. Few survey lines: the number of survey lines should be kept to a minimum but must be sufficient for the survey to be plotted and checked.
b. Long baseline: A long line should be positioned right across the site to form a base on which to build the triangles.
c. Well-conditioned triangle with angles greater than 30o and not exceeding 150o: It is preferable that the arcs used for plotting should intersect as close as 90o in order to provide a sharp definition of the station’s point.
d. Check lines: Every part of the survey should be provided with check lines that are positioned in such a way that they can be used for off- setting too, in order to save any unnecessary duplication of lines.
e. Obstacles such as steep slopes and rough ground should be avoided as far as possible.
f. Short offsets to survey lines (close feature preferably 2m) should be selected: So that measuring operated by one person can be used instead of tape which needs two people.
g. Stations should be positioned on the extension of a check line or triangle. Such points can be plotted without the need for intersecting arcs. Ranging:
Ranging involves placing ranging poles along the route to be measured so as to get a straight line. The poles are used to mark the stations and in between the stations.