What is Surveying? Definition and Principles

Surveying is the starting point for any construction project. Before we plan and construct any structure like buildings, dams, roads, rails, bridges, canals, or water works on land, we need to conduct surveys to collect the details of the land (field notes).

So, 

Surveying can be defined as the science of making measurements of the Earth, specifically the surface of the Earth. 

A good construction survey is essential for effective planning, design, and estimation of the quantities of the proposed project.

This is an introductory article on basic surveying in construction. We explore in detail the :

• Definition of Surveying
• Objectives of Surveying
• Applications of Surveying
• Principles of Surveying
• Procedure of Surveying
• Surveying Stations- an overview

What is Surveying and its Objectives?

In civil engineering, surveying is defined as the ‘art of determining relative positions of objects located on, above, or beneath the surface of the Earth and representing them on a paper (map or plan). 

We can measure the relative position of a point on Earth with respect to a datum or any other feature whose position is already known.

For example, if you've ever seen a map or a plan, you'll notice that every feature is depicted in relation to something else. For example, when looking at a road on a map, you might find its position marked with respect to a nearby building or a tree. Essentially, every feature on these plots is represented relative to others.

Surveying can therefore be used to determine the spatial location of the points on or near the surface of the Earth. The primary aim of field surveying is:

• To measure the horizontal distances of the points
• To measure the vertical elevation between the points
• To determine the relative direction of the line by measuring the horizontal angles with reference to an arbitrary direction
• To find the absolute direction by measuring horizontal angles with reference to a fixed direction. 

So, we can determine the relative and absolute coordinates of a point or location based on the measurements taken. 

Objectives of Civil Engineering Surveying

The primary objectives of surveying in civil engineering are:

1. To prepare plans and maps.
2. To calculate the area and volume of land. 
3. To set out field parameters at the site for further engineering works.
4. To perform the setting out operation of actual engineering works by analyzing and calculating the field parameters

Important Applications of Surveying

The main applications of surveying are:

1. To fix the national and state boundaries
2. To chart the coastlines, navigable streams and the lakes
3. To establish control points
4. To execute hydrographic and oceanographic charting and mapping
5. To prepare a topographic map of the land surface of the Earth

Principles of Surveying

The two main principles of surveying are:

1. Working from whole to part
2. Locating a point by measuring from two points of reference

1. Working From Whole to Part

The whole-to-part principle of surveying is a method that initially starts with setting or establishing control points by less precise methods, and further details are established by minor control points.

Figure 3-Surveying Principle - Working from Whole to Part

For example, if we need to survey a line AB as shown in Figure 3, we can divide this line into parts as AC, CD, DE, EF,FB. We can start the measurement with AC, then CD, DE, EF and finally FB. This is called the part-to-whole method. The issue with this method is that, as we move forward, from AC, if any error arises, it gets accumulated and would be reflected in CD, DE, EF, and AB measurements. If AC is measured right, and there comes any error in CD or any following parts, the succeeding measurements will go wrong. So, instead of AB, our measurement will be AB', which is incorrect.

So, we opt for the whole-to-part method to 

• Localise the errors
• To control the accumulation of errors

In which we start from AB to inwards. We initially measure AC, then BF, then CD, then FE, and DE. If there comes any possibility of error in any of the measurements, it is not accumulated and helps to localize the error. 

2. Locating a Point by Measuring From Two Points of Reference

This surveying principle states that any point must be measured with two points of reference, which are the points that are already determined by standard measurements, and they are fixed.

This can be explained with the following examples, where the points P and Q are already fixed by knowing the distance PQ. If so, a point R can be obtained by the following options:

Figure 4- Surveying Principle

1. The distance PR and QR are measured. The point R is located by swinging the measured arcs to the same scale to which PQ was plotted. This is commonly used in chain surveying.
2. The second option is by dropping a perpendicular from R to the reference line PQ, after which the lengths PS and SR are measured. The point R can be plotted using a set square. This is commonly used for defining the details on the way of surveying measurements. 
3. The third option is to obtain the distance QR and the angle PQR. This is used in traversing. 
4. In the next method, the angles RPQ and RQP are measured, using an angle measuring instrument. It is widely used in triangulation. 
5. Another option is to plot R by plotting RQP and PR. This is used in traversing and is used less.

What is the Procedure of Surveying?

Surveying in construction is done by taking angular measurements or linear measurements. To be more precise, the angular measurements taken can be either horizontal angle or vertical angle. At the same time, the linear measurements taken can be either horizontal distance or vertical distance. These measurements are scaled up or scaled down and represented on a plan or map.

The surveying process for a construction project can be briefed in the following steps:

• Site Inspection & Measurement – The site is visited to take measurements of the land, and a rough sketch is developed.
• Drawing the Building Plan – Based on the available land, a detailed plan is created for the intended structure, such as a single-story house.
• Setting Out the Plan – Using surveying instruments, the building plan is transferred onto the actual site to mark the positions of structural elements. This process is known as setting out.

Based on the type of the survey land, i.e., whether it's a flat terrain or terrain with large obstructions or uneven slopes, we adopt different surveying methods. Hence, we choose the method of surveying based on purpose, time, feasibility, and economy of the surveying project.

No matter what method or instrument we employ, every construction survey is performed based on two basic principles.

Survey Stations and Survey Lines in Surveying

In surveying, we start measuring distances or angles by establishing several points around the survey area. First, we start with points that connect the boundary of the area, then, as per the requirement we determine extra points to designate special features within the building. As shown in Figure 5 below:

Figure-5: Survey Stations and Survey Lines

• Survey stations are the reference points established on an area to define the boundaries of the area and form the control points for measurements. The lines connecting the survey stations are called as survey lines. 
• Main stations and Main Survey Line: The stations along the boundary line of an area forming the controlling points are called as the main stations. The lines joining the main stations are called as main survey line. 
• Tie Stations or Subsidiary Stations: The survey station points established along the main survey lines to locate fences, buildings or other features within the area are called tie stations. 
• Tie lines are survey lines joining the subsidiary stations to the main line. These lines help to locate the accurate position of details in the survey area. 
• Baseline is a carefully measured straight reference line used as the foundation for locating other points, measuring distances, or establishing the layout in various types of surveying. It serves as the starting line for taking measurements, setting out triangles, or controlling the accuracy of the entire survey. It serves different functions in different surveying methods. 

Figure 6: Baseline in Surveying 

Plane Surveying and Geodetic Surveying - Primary Division of Surveying

Plane surveying and Geodetic Surveying are two major classifications that follow two different surveying strategies in making measurements of points or distances on, below, or above the Earth's surface. The primary division of the survey is based on the shape and characteristics of the Earth. We know that the Earth is an oblate spheroid of revolution. 

Figure 7: Difference Between Plane and Geodetic Surveying

In more easy terms, plane surveying is the surveying conducted by assuming the Earth's surface is flat. Here, the spheroidal shape or curvature of the Earth's (Oblate) surface is ignored, while geodetic survey involves the survey of extensive areas that need to take into consideration the curvature of the Earth, the shape of the Earth, and the size of the Earth.

Continue Reading On: Difference Between Plane and Geodetic Surveying

Classification of Surveying

Surveying in construction can be classified into various types under various categories:

1. Classification of Surveying Based on Application
2. Classification of Surveying Based on Method
3. Classification of Surveying Based on Instrument

These surveying classifications are not distinct. When we choose a type of surveying based on application, say land surveying, we can adopt an instrument based on the land, and then we must employ a unique method to perform it. This is how we get different classifications for surveying, but they are linked somehow.

Read More:
What is Chain Surveying?
Reconnaissance Survey and Index Sketch in Chain Surveying
Linear Measurements in Surveying