What is Surveying ?

     Hi Everyone, Welcome to "thecivilengineer18.com". I am a Civil Engineer with 9+ years of experience in civil engineering. On this website, I wrote articles about civil engineering like concepts, and the latest trends and also apart from civil engineering I wrote about general things too regularly. In this article, I explained surveying.    

    Surveying is the art of determining the relative position of the points on, above or on the surface of the earth by means of direct or indirect measurement of distance, direction and elevation.

    The Main aim of surveying is to plot the area on the earth's surface on a horizontal plane-like map. In Some cases, it was made along with the vertical distance with the reference points as contour maps.

    Surveying plays an important role in the life cycle of a project from the very beginning itself. Before any project is stated the boundaries of the land are very important for preparing the layout plans that can be done only with surveying. After the plans were made then the budget is prepared with the help of estimation.

    In huge projects like the Canal construction, and Highway project the topography or contour became more essential for the execution of the projects. And in Building construction the Boundaries of the plot through the layout preparation became essential for the land registration process and the topography survey or contour mapping became essential for the levelling of the ground for construction and setting up of drainage and sewage systems within the boundaries of the buildings.

Types of Surveying

 Generally, Surveying is Classified based on the scale of the survey whether the distance is small or large. We know the Earth is not a flat surface it is geoid in shape. Hence if you draw a line on the surface of the earth over a long distance it is going to curve a little bit hence the line is not a straight line. So based on this the general classification survey are 

1. Plane Survey
2. Geodetic Survey

Plane Survey

    In Plane survey, the distance between the points was below 12 kilometres and the land is considered a flat surface by neglecting the geoid shape of the earth. Hence these surveys are taken for small areas. If you form a triangle then it is called a plane triangle for calculating the area.

Geodetic Survey

    In the Geodetic survey, the geoid shape of the is considered and if drawn a line it is considered as an arc and the triangle formed by the intersection of the arc is known as the spherical triangle. This gives a very precise area and location than a plane survey but it is time consuming than Plane survey.

Both of the above methods are used in their suitable realms. But the more specific classification of the survey is listed below,

1. Classification based on the nature of the field
2. Classification based on the Object of survey
3. Classification Based on the Instrument used.

Classification based on the nature of the field

1. Land Surveying - If the surveying is used for the measurement of lands such as the topographical survey, Cadastral Survey and City Survey.
2. Marine or Hydrological Survey - Surveys that are carried out for navigation in the oceans, streams, water supply and harbour construction etc.
3. Astronomical Survey - Survey of Astronomical objects like the sun and moon with respect to Earth.

Classification based on the object of the survey

1. Engineering survey - Survey that was carried out for engineering purposes designing structural members for reservoirs and tunnel construction and road construction etc.,
2. Military Survey -Survey is carried out for the location of strategic positions.
3. Mine Survey -Survey took to locate the resources in the mines.
4. Geological Survey - A survey that was done to find the different strata of the earth below.
5. Archaeological Survey - This survey is done to unearth ancient relics below the earth's surface.

Classification based on the instrument used

    Based on the instruments and the methods used for taking the survey they are classified as follows,

1. Chain Survey
2. Plane table survey
3. Theodolite survey
4. Total Station Survey
5. Triangulation survey
6. Traverse survey
7. Tacheometric survey
8. Photogrammetric survey 
9. Aerial survey

Principle of Surveying

The Principle of Surveying can be briefly said in two things, there are many but these two forms are the basis of all the types of surveying they are

1. Location of  a point by measurement from two points of reference
2. Working from whole to part

Location of  a point by measurement from two points of reference

     Anything that needs to be surveyed can be measured from standard reference points. From this standard point of reference, we say how much the point that we need to measure is from or how much the point is elevated or below is given.

    These reference points have two purposes one it is used as a starting point for the survey and second it helps to reduce the error of the geoid shape of the earth as we have seen in the types of surveying post the geodetic survey has less error that the plane table survey.

    Hence most of the geodetic survey was done by the government of the country and they provide a reference point for a particular interval, the people of the country use this reference point and make the plane table surveying for smaller areas that they want by doing so the measurement errors were minimized.

    Let us take an example, A & B be the known reference points with this we can measure the distance of point C.

Case 1: By Using Arch in Plane Table

    By using the plane table on the sheet of paper we know the distance between points A and B let's scale the distance and draw it in the chart. Use a protector to draw arches from both points, both arches meet at a point that is point C. By Connecting Points A to C and B to C you can get the distance of point C from the respective points A and B. From that, we can calculate the angle. This method is greatly used in Chain Surveying by making arches in the ground.

Case 2: By Drawing a perpendicular line to line AB.

   In this case, if we know the exact point C that we need to measure the distance then we can make a perpendicular line to the reference line AB that joins the point C. Then the other measurements can be measured with the set square. This Principle is used for finding more details.

Case 3:  Using angles

    Here there are many sub-cases or types in which we can calculate the distances and plot the points.

1. In this Sub-Case 1, let us assume the angle ABC is known and the distance of C from B is Known then Point C can be plotted by making a line from point B in the angle ABC, then make an arch in the line with the distance between B and C. By this manner the Point C can be easily Plotted. This kind of method is mostly used in Transverse Surveying.
2. In this Sub-Case 2, the angles CAB and CBA were known and by using these angles we can plot point C by using triangulation. This type of measurement is extensively used in the triangulation survey and for very extensive works.

Working for Whole to Part

    

    The next important point in surveying both in-plane and geodetic is to establish or create control points with high precision with no or minimum errors. Then the minor control points are established. The measurement further below can be carried out with fewer precision methods.

    The Main aim is to reduce the error accumulation or to minimize the errors as a whole.

Units of Linear Measurement

The units for measurement are most important not only in engineering but also very important in our day-to-day life. The history of measurements goes way back to the colonial era when the British Empire controls the majority of the land on earth. The system of measurement created by the British empire is known as the British Imperial system or Imperial Units.

    But after independence, every country has their own set of measurements which became confusing when globalization occurs. So a standard system of measurement was adopted all over the world that system which is called the International System of Units (SI) or Metric System (Metric units).

Basic Units of Length

Imperial Units

Length

12 Inches = 1 foot

3 feet = 1 yard

5.5 yards = 1 pole, rod or perch

4 poles = 1 chain (66 feet)

10 chains = 1 Furlong

8 Furlong = 1 mile

6 feet = 1 fathom

6080feet = 1 nautical mile

Area

144 sq.inches = 1 sq.foot

43560 sq.feet = 1 acre

2.471 acre = 1 hectare

9 sq.feet = 1 sq.yard

30.25 sq. yard = 1 sq. rod, pole or perch

Volume

1728 cu.inches = 1 cu.foot

27 cu.feet = 1 cu.yard

Metric Units

Length

10 Millimetres = 1 Centimetre

10 Centimetres = 1 Decimetre

10 Decimetres = 1 Metre

10 Metres = 1 Decametre

10 Decametres = 1 Hectometre

10 Hectometres = 1 Kilometre

1852 Metres = 1 nautical Mile.

Area 

100 sq.millimetres = 1 sq.centimetre

100 sq.centimetres = 1 sq.decimetre

100 sq.decimetres = 1 sq.metre

100 sq. metres = 1 are or 1 sq. decametre

100 ares = 1 hectare or 1 sq. hectometre

100 hectares = 1sq.kilometre.

Volume

1000 cu.millimetres = 1 cu.centimetre

1000 cu.centimeters = 1 cu.decimetre

1000 cu.decimetres = 1 cu.metre

Conversions

1Metre = 1.0936 Yards = 3.2808 Feet = 39.3701 Inches

1 Kilometre = 0.53996 Nautical miles = 0.6214 miles

1 sq.metre = 1.196 sq.yards = 10.7639 sq.feet = 1550 sq.inches

1 hectare = 2.471 acre = 1,07,636.76 sq.feet ( 1 acre = 43560 sq.ft)

1 cu.metre = 1.308 yards = 219.969 Gallons = 0.999 Kilolitres,

Units of Angular Measurement

  The Word angle comes from the Latin word "angulus" meaning corner. Angles became important and we use them in all walks of engineering be it from setting right angles in walls to satellite placement in space. Angles play a major role.

    An  Angle is a difference between the direction of two intersecting lines. The Radian is the unit of plane angle. There are three systems of angle measurement used in the globe. They are,

1. Sexagesimal system
2. Centesimal System
3. Hours System

Sexagesimal System

    This System is made in such a way its base number is 60. This system was first used by Sumerians and passed down to Babylonians. Their calculation is slightly different from that of the modern one. But the modern sexagesimal system has its roots in there. This system is widely used in the US, England, India and most parts of the world.

    

    Here the Circle is divided into a combination of  60 acres with 60-degree lengths each i.e with a 360-degree circle. So,

1 Circumference (Circle) = 360° (degrees of arc)

1 degree = 60' (minutes of arc)

1 minute = 60" (Seconds of arc )

Centesimal system

    This system is most prevalent in the European nation and this system was actually discovered in the Frech revolution in France. Here the Circle is divided or made up of 400 grades (Grads(g) are the unit in the centesimal system ). This system is very popular in Europe because of the french revolution's impact all over Europe and it's easy to interpolate when compare to other systems of angular measurement. So,

1 Circumference(Circle) = 400^g (grads)

1 grad = 100^c (centigrads)

1 centigrad = 100^cc (cento-centigrads)

Hours System

    The Hours system was mostly used in astronomy and navigation. The Time that we use comes under the Hours system, the clock, watch everything. So,

1 Circumference(Circle) = 24^h (Hours)

1 Hour = 60^m (minutes of time)

1 Minute = 60^s (seconds of time)

Use of Scale in Surveying

 Before the scale let me explain to you about maps and plans. The Plan is the representation of a small area in a horizontal plane on paper. The Maps are the same as the plans but the area is so high or huge like the world map. But you can't plot the actual distance of the globe or big things on a plain sheet of paper. That's where the part of Scale comes in. 

Scale

    The Scale is nothing but the ratio in which the length or dimension on the plan or map needs to be expanded on the ground to get the actual measurement over the land. The ratio of the map distance to the corresponding ground distance independent of units of measurement is known as Representative Fraction(R.F) .The scale is useful in many ways.

  It helps us to plot a large area on a very small sheet of paper. And there is a possibility that after a period of time, the paper may shrink but since we have the scale that is already marked on the sheet we don't need to worry about the Shrinkage.

  Let's say there is a scale of 1cm = 1km, this means that 1cm in the plan or map is equal to 1km on the real ground. So we can easily plot the large area into very small pieces of paper.

Types of scale

  The scale was classified as follows,

Plain Scale

Diagonal Scale

Vernier Scale 

Plain Scale

    In-Plane Scale we can measure two dimensions only i.e let's take an example the scale is represented as 1 cm = 1 metre, in this case, the two dimensions are the centimetre and metre.

Diagonal Scale

    In Diagonal Scale, we can even use 3 dimensions i.e 3 units for measurement. or even more precise decimal measurement. 

Vernier Scale

    The Vernier scale was invented by Pierre Vernier in 1631. It consists of a small auxiliary scale which slides along the side of the main scale. The principle of the vernier scale is that the human eye can perceive without strain and with considerable precision when two graduations coincide to form one continuous straight line.

Errors in Scale

 Human errors are possible in the execution while taking measurements or while reading the scale we may use a different scale. So corrective action became necessary to rectify the error instead of taking the whole measurement again. There are two types of errors,

1. Error Due to the use of the Wrong Scale
2. Error due to  shrinkage

Error due to the use of the Wrong Scale

    How could you correct an error, if you calculated a measurement with a scale that is different from the actual scale on the map or Plan? Here you go,

    If it is, length then uses the formula below to correct it.

Correct Length = (R.F of the Wrong scale / R.F of the Correct Scale) x measured Length

Where R.F means Representative factor (The ratio of the map distance or plan distance to the corresponding ground distance)

Let's take an example,

A surveyor calculated the measurement from a plan with a scale of 1cm = 5 meters and calculated the length as 500 meters. But later he found out that the scale was 1cm = 7 meters. So, this is how he could rectify it.

Step 1: Find the R.F of both the scale,

R.F of the wrong scale = 1/(5x100) = 1/500

R.F of the  correct Scale = 1/(7x100) = 1/700

Step 2: Apply the correction formula for length, so we get

Correct Length = [ (1/500) / (1/700)] x 500 = 1.4*500 = 700 meters, this is the actual correct length.

Alternate method

    Alternatively, you can also do this,

Map the distance between two points in the wrong scale 1cm = 5 meters,  500/5 = 100cm

Actual scale 1cm = 7metres,

So, the true distance will be given by, 100*7 = 700 metres.

Error Due to Shrinkage

    The Paper that we use was obtained from Tree Pulp. Hence they were organic and when time passed it starts to shrink, so when we were reading old pages of the map or plan we need to consider the shrinkage error. So how we can rectify the shrinkage error. Let the Scale in the plan be denoted as 1cm = 15 meters. Let's take two points in the plan and plot them on the ground. Now measure the distance between the points in the ground. Let's say the plan says the distance between the two points is 15 metres. But the actual length in the ground is 16 metres.

    So, the shrinkage factor for that can be given by 15/16. The Shrinkage factor or Shrinkage ratio is the ratio of the shrunk length to the original or actual length.

    From the scale, we get the representative factor as 1/1500.

    Now, the Shrunk scale is given by,

Shrunk scale = Shrinkage Factor x original factor

Shrunk Scale = (15/16) * (1/1500) = 1/1600 i.e 1cm = 16m