Surveying is the process of determining relative positions of different objects on the surface of the earth by measuring horizontal distances between them & preparing a map to any suitable scale.
Methods of Surveying in Civil Engineering & Lao Surveying
- Plane Surveying
- Geodetic Surveying
As we know, the earth is spherical in shape but it’s diameter is big enough to consider plane in small dimensions. It is that type of surveying in which the mean surface of the earth is considered as a plane. All triangles formed by survey lines are considered as plane triangles. The level line is considered as straight & plumb lines are considered parallel. Plane surveying is done if the area of survey is less than 250 km2.
This is the type of surveying in which the curved shape of the earth is taken into account. The object of geodetic surveying is to determine the precise position on the surface of the earth. Lines joining two points are considered as curved lines & angles are assumed as spherical angles. It is carried out if the focus area exceeds 250 km2.
Secondary classification of Surveying
Surveys in Lao surveying may be classified based on the nature of the field of survey, object of survey & instruments used.
Lao Surveying based on Nature of Survey
Topographical Surveys: carried out to determine the position of natural features of a region such as rivers, streams, hills, etc. & artificial features such as roads & canals. The purpose of such surveys is to prepare maps, often referred to as topo-sheets.
Hydrographic Surveys: carried out to determine Mean Sea Level (MSL), water spread area, depth of water bodies, velocity of flow in streams, cross-section area of flow, etc.
Astronomical Surveys: carried out to determine the absolute location of any point on the surface of the earth. The survey consists of making observations to heavenly bodies such as stars.
Engineering Surveys: undertaken whenever sufficient data is to be collected for planning & designing engineering works such as roads, bridges & reservoirs.
Archaeological Surveys: carried out to gather information about sites that are important from archaeological considerations & for unearthing relics of antiquity.
Photographic Surveys: information is collected from this type of survey by taking photographs from selected points by using cameras.
Aerial Surveys: data about large tracts of land is collected by photographs taken by airplanes.
Reconnaissance Surveys: data is collected by making physical observations & measurements using simple survey instruments.
Lao Surveying Based on Types of Instrument
Chain Surveying is the oldest & most basic form of surveying. The principle is triangulation. The area to be surveyed is divided into a smaller number of triangles. The angles of the triangles cannot be less than 30 degrees or greater than 120 degrees. Equilateral triangles are considered to be ideal triangles. No angular measurements are taken: tie lines & check lines control accuracy of the work.
Compass Surveying uses the principle of traversing. This method doesn’t require the use of triangles. It uses a prismatic compass for measuring the magnetic bearing of lines & distances are measured by chains. A series of connecting lines is prepared using a compass & measuring distances by chains. This method is well suited to large areas crowded with details, such as a river course.
Plane Table Surveying: the principle here is parallelism. Points are plotted directly on paper with their relative positions. The table is placed at each of the successive stations parallel to the position of the last station.
Theodolite Surveying: A theodolite is a precision instrument used for accurate measurement of horizontal & vertical angles. Nowadays, theodolites are being replaced by a Total Station, which can outperform similar tasks with greater ease & more accurate results.
Tacheometric Surveying: the branch of surveying in which horizontal & vertical distances are determined by taking angular observations with an instrument known as a tacheometer. It is simply a transit theodolite fitted with a stadia diaphragm & an anallatic lens. There is no need for chaining in such a survey. The principle of tacheometry is based on the properties of the isosceles triangle, where ratio of the distance of the base of the apex & the length of the base is always constant.
Photographic Surveying is based on the technique of taking photographs from different angles to prepare topographic details with relative high speed. In terrestrial or ground photogrammetry, maps are prepared from ground photographs taken from different points on the earth’s surface for measurement purposes. In aerial photogrammetry, maps are produced from the air by fixed or rotary wing aircraft.
Photogrammetry encompasses two major areas of specialization,
- Metrical Photogrammetry
- Interpretive Photogrammetry
Metrical photogrammetry is of principal interest to surveyors as it is applied to determine distances, elevations, areas, volume, etc. to compile topographic maps made from measurements from photographs.
Interpretive photogrammetry involves objects from their photographic images & their significance. Critical factors considered in identifying objects are shape, size, pattern & shadow.
Enjoy knowing more about what Lao surveying is by clicking the links to these excellent videos:
How Does Land Surveying Work?
Spotlight on Surveying