Civil Engineering Surveying Principles and Methods

 Civil Engineering Surveying Principles and Methods

Surveying is the process of determining the relative positions of various objects on the earth's surface by measuring horizontal distances between them and preparing a map to any suitable scale. Measurements are only taken in the horizontal plane. Surveying is the art of determining the relative positions of points on, above, or beneath the earth's surface using direct or indirect measurements of distance, direction, and elevation. Leveling is a branch of surveying whose goal is to determine the elevations of points with respect to a given or assumed datum and to establish points at a given elevation or at different elevations with respect to a given or assumed datum. A survey is carried out in order to create a map of relative positions on the earth's surface. It depicts natural features of a country such as towns, villages, roads, railways, and so on. They may also include information about various engineering projects, such as a bridge, canal, or dam. Before beginning any major engineering project, a survey is conducted to determine the location of the site and its surroundings. To comprehend the site's topography and the availability of natural resources. A survey is carried out in order to create a cadastral map that depicts property boundaries.

Surveying Guidelines

The fundamental principle upon which the various plane surveying methods are based can be stated in two ways.

Measurement from two points of reference to locate a point

According to this principle, the relative position of a point to be surveyed should be determined by measuring it from at least two fixed points of reference.

If P and Q are the two reference points on the ground, then any other point, such as R, can be found using any of the direct methods depicted in the figures above. However, while a single method is sufficient to locate the relative position of 'R' with respect to reference points P and Q, at least two methods must be used to fix the position of point 'R'. While measurements taken in either the first or second methods will aid in locating the point 'R,' measurements taken in the other method will serve as a check.

Working from the whole to the part

According to this principle, it is always preferable to conduct survey work from beginning to end. This means that before surveying an area, a system of control points covering the entire area with high precision must be established. Minor details are then located using less precise methods. The goal of working this way is to prevent errors from accumulating and to control and localise minor errors that would otherwise grow to greater magnitudes if the reverse process is followed, leaving the work uncontrolled at the end.

Civil Engineering Surveying Method

The following are the primary types of surveying:

• Surveying by plane
• Surveying geodetic

1. Surveying by plane

Plane surveying is done by both government and private companies. Although the Earth is spherical, its diameter is large enough to consider planes in small dimensions. It is a type of surveying in which the earth's mean surface is treated as a plane and the spheroidal shape is ignored. Plane triangles are any triangles formed by survey lines. Level lines are considered straight, while plumb lines are considered parallel. Plane surveying is performed when the survey area is less than 250 km2.

2. Geological surveying

The country's survey department conducts geodetic surveys. It is the type of surveying that takes into account the earth's curved shape. The goal of a geodetic survey is to determine the precise position on the earth's surface of a network of widely separated points that serve as control stations for less precise surveys. A curved line connects two points, and angles are assumed to be spherical angles. If the area exceeds 250 km2, it is carried out.

Surveying's secondary classification

Surveys can be classified based on the nature of the survey field, the object of the survey, and the instruments used.

1) Surveying based on Survey Type

a) Surveys of Topography

They are carried out to determine the position of a region's natural features such as rivers, streams, hills, and so on, as well as artificial features such as roads and canals. The goal of such surveys is to create maps, which are known as topo-sheets.

b) Hydrographic Investigation

The purpose of a hydrographic survey is to determine M.S.L. (Mean Sea Level), water spread area, depth of water bodies, velocity of flow in streams, cross-section area of flow, and so on.

c) Survey of Astronomy

The Astronomical Survey is conducted to determine the absolute location of any point on the earth's surface. The survey entails making observations on celestial bodies such as stars.

d) Engineering Evaluation

This type of survey is carried out whenever sufficient data is required for the planning and design of engineering works such as roads, bridges, and reservoirs.

e) Archaeological Investigation

This type of survey is carried out to gather information about archaeologically significant sites and to unearth antiquity relics.

f) Photographic Research

In this type of survey, information is gathered by photographing specific points with a camera.

g) Airborne Survey

This type of survey collects data about large areas of land by photographing them from an aeroplane.

h) Reconnaissance Investigation

Data is collected in this type of survey by marking physical observations and taking measurements with simple survey instruments.

2) Surveying based on Instrument Type

a) Chain Mapping

Chain surveying is the most fundamental and oldest type of surveying. The triangulation principle is used in chain surveys. The survey area is divided into a number of small triangles. Triangle angles cannot be less than 30 degrees or greater than 120 degrees. Equilateral triangles are regarded as ideal triangles. There are no angular measurements taken, and the work is controlled by tie lines and check lines. This method is suitable on level ground with little undulations and area to be survey is small.

b) Surveying by Compass

The traversing principle is used in compass surveying. This method eliminates the need to draw triangles. The magnetic bearing of the line is measured using a prismatic compass, and the distance is measured using chain. A series of connecting lines is created with a compass and distances are measured with chain. Interior details are located by deviating from the main survey lines. They are appropriate for large-area surveying with many details. It can be used to map the course of a river.

c) Surveying with a Plane Table

Parallelism is the guiding principle of plane table survey. They are directly plotted on paper with their relative position. Rays are drawn from the station to an object on the ground. The table is placed at each successive station, parallel to the last station's position. They are primarily suitable for interior detailing and are recommended when precision is not required.

d) Surveying with theodolites

The theodolite is primarily used to measure horizontal and vertical angles accurately. They can measure angles up to 10" or 20" in length. Theodolite can be used to measure the following:

• Angles on the horizontal
• Angles that are vertical
• Angle of deflection
• Bearings that are magnetic
• Distance between two points on the horizontal plane
• Vertical distance between two points
• Elevation disparity

Nowadays, theodolites have been eclipsed and replaced by Total Stations, which can perform the same task with greater ease and accuracy.

Tacheometric Surveying e)

Tacheometry is a branch of surveying in which horizontal and vertical distances are calculated using angular observations made with a tacheometer. Tacheometers are nothing more than transit theodolites with stadia diaphragms and anallatic lenses. In such a survey, chaining is unnecessary. The Tacheometer principle is based on the isosceles triangle property, which states that the ratio of the distance of the base from the apex to the length of the base is always constant. There are several types of stadia diaphragms that are commonly used:

f) Surveying Photographically

Photographic survey is based on the technique of taking photographs from various angles in order to prepare topographic details quickly. There are two kinds of photography surveys.

i) Photogrammetry on the ground or on the ground.

Terrestrial photogrammetry creates maps from ground photographs taken at various points on the earth's surface for measurement purposes.

ii) Photogrammetry from the air

Aerial photogrammetry creates maps from the air of an aeroplane or helicopter. Photogrammetry is divided into two major areas of specialisation.

• Photogrammetry in metric units
• Photogrammetry with interpretation

Surveyors are particularly interested in metric photogrammetry because it is used to calculate distances, elevations, areas, volumes, and so on in order to compile topographic maps based on measurements taken from photographs. Intuitive photogrammetry involves objects and their significance from their photographic image. Shape, sizes, patterns, and shadow are critical factors in identifying an object.