Unit – 3
Surveying & Levelling
BASIC PRINCIPLES IN SURVEYING
PRINCIPLE OF WORKING FROM WHOLE TO PART
It is a fundamental rule to always work from the whole to the part. This implies a precise control surveying as the first consideration followed by subsidiary detail surveying.
This surveying principle involves laying down an overall system of stations whose positions are fixed to a fairly high degree of accuracy as control, and then the survey of details between the control points may be added on the frame by less elaborate methods.
Once the overall size has been determined, the smaller areas can be surveyed in the knowledge that they must (and will if care is taken) put into the confines of the main overall frame.
Errors which may inevitably arise are then contained within the framework of the control points and can be adjusted to it.
Surveying is based on simple fundamental principles which should be taken into consideration to enable one get good results:
(a) Working from the whole to the part is achieved by covering the area to be surveyed with a number of spaced out control point called primary control points called primary control points whose pointing have been determined with a high level of precision using sophisticated equipments. Based on these points as theoretic, a number of large triangles are drawn. Secondary control points are then established to fill the gaps with lesser precision than the primary control points. At a more detailed and less precise level, tertiary control points at closer intervals are finally established to fill in the smaller gaps. The main purpose of surveying from the whole to the part is to localize the errors as working the other way round would magnify the errors and introduce distortions in the survey. In partial terms, this principle involve covering the area to be surveyed with large triangles. These are further divided into smaller triangles and the process continues until the area has been sufficiently covered with small triangles to a level that allows detailed surveys to be made in a local level. Error is in the whole operation as the vertices of the large triangles are fixed using higher precision instruments.
(b) Using measurements from two control parts to fix other points. Given two points whose length and bearings have been accurately determined, a line can be drawn to join them hence surveying has control reference points. The locations of various other points and the lines joining them can be fixed by measurements made from these two points and the lines joining them.
Principle of Levelling
Simple levelling
When the levelling instrument is properly levelled, the bubble tube axis and the line of sight will be horizontal and the vertical axis of the instrument will be vertical. The bubble must be central and traverse. The line of sight will remain in a horizontal plane when the telescope is rotated. Thus if the telescope is sighted towards a staff kept on a point of known elevation, the height or elevation of the line of sight can be determined. If the telescope is now directed to staff kept on points of unknown elevation, the staff readings can be read. From which the reduced levels of the unknown points can be determined.
Technical terms
Bench Mark:
A survey point on a fixed object, the altitude of which has been surveyed in relation to mean sea level. Many of these are now being replaced with GPS stations. (see below)
Cadastral:
A type of surveying that determines the boundaries of land parcels.
Cartography:
The organisation and communication of geographically related information in either graphic or digital form. This usually means the drawing of maps and charts.
Contour:
A line drawn on a map joining locations of equal height. Many of these at different heights indicate the shape of the landscape.
DEM:
Sometimes it is known as DTM, this is a series of points which depict the shape of the topography of the land, or of an object being measured.
GPS:
Global Positioning System using a network of satellites and receivers to identify a point in its real world location. Car Sat Nav systems and some mobile phones use this.
Laser Scanner:
A piece of equipment containing a laser which records many thousands of 3D points of a subject. Many visual fly-through and computer games are generated by this type of instrument.
Lidar:
Airborne laser scanner system which accurately measures the shape of the ground. The data may be used for flood monitoring and prediction, building detection and building heighting, tree height measurement and general DEM production.
Photogrammetry:
The science of obtaining reliable measurements from photographs.
Spot height:
A point on the earth’s surface for which the height above a reference datum is known.
Surveying:
The determination of the absolute and relative positions of points, on or near the earth’s surface , by means of measurement in the three elements of space; distance, direction and elevation.
Total Station:
Equipment used for measuring distances and angles.
Terms used in levelling
Some of the commonly used terms in levelling are defined as follows:
Station
Station is a point whose elevation is to be determined or it is thepoint which is to be marked at a given elevation. It is the point where the staff is held for taking observations from a levelling instrument.
Height of the instrument (HI)
It is the elevation or reduced level of the line of sight with respect to the datum. Height of the instrument is not the height of the line of sight above the ground where the levelling instrument is set up.
Height of the instrument= elevation of BM+ back sight
Balancing of sight
To reduce the effect of instrumental and other errors, this method is adopted. The distance of the point where the back sight is taken and the distance of the point where a foresight is taken, as measured from the instrument station, has to be approximately same. This is known as balancing of sights. The intermediate sights do not comply with the condition of equal length of sights.
Differential levelling
Differential levelling is also called as compound levelling or continuous levelling. It is adopted when the points whose elevation is to be determined are too far apart or the difference in elevation is too large. In this levelling, the instrument has to be setup at several positions and at each set up the principle of simple levelling is used.
Backsight (BS) or Backsight reading
It is a staff reading taken on a point of known elevation, as on a benchmark or a change point. This is also called as a plus sight. The backsight is the first staff reading taken after the level is set up and leveled at the point.
Foresight (FS) or Foresight reading
It is a staff reading taken on a point whose elevation has to be determined through levelling process. It is also known as minus sight. The foresight is also taken towards a change point. It is the last reading taken before the instrument is shifted.
Elevation of station= height of instrument (HI) – foresight (FS) or intermediate sight (IS)
Intermediate sight (IS)
It is the staff readings taken on staff held at other points whose elevations are to be determined before the foresight is taken. Intermediate sights are all staff readings between the B.S and F.S
Change point or Turning point (TP)
It is a point denoting the shifting of the level. It is a station at which both BS and FS readings are taken. Stable and well defined objects must be chosen as change points for accurate results.
Calculation of reduced level by Height of instrument and Rise & Fall method
Reduced level of the line of sight, RL= 0 + h1 =h1
Reduced level of the point B = h1-h2
With a levelling instrument, it is possible to establish only a horizontal line not a level line. However, for the small distances involved in ordinary levelling, the distinction between level line and horizontal line is negligible. The errors due to instrumental faults can be eliminated if the points are equidistant from the levelling instrument. If the point is lower, then the staff reading will be higher.
Arithmetic Check
(i) For rise and fall method
∑B.S - ∑F.S = ∑Rise - ∑Fall = Last R.L – First R.L
(ii) Height of instrument method
∑ B.S - ∑ F.S = Last R.L – First R.L
Reference Books
- A Text Book of Building Materials, by C.J. Kulkarrni
- Building Materials, by P. C. Varghese
- Building Construction, by P. C. Varghese
