What is Contour Surveying? Advantages and Disadvantages of Contour Surveying

What is Contour Surveying?

Contour surveying is the branch of surveying that uses contour lines to identify variations in the height of the earth’s surface and their relative places.

The goal of contour surveying is to determine the altitude or height of any surface.

A contour survey, in its most basic form, depicts the elevation changes over your property at regular intervals, from the lowest point to the highest one. Contour lines connect places that are at the same height. The steeper the part, the closer the contour lines are together.

These contour lines are critical to people who want to buy land or a piece of property and are often referred to as the “survey” for the property.

In other words, the contour survey shows the elevation of a piece of land or property as outlined by curved lines – this is why it is also known as ‘the form plan’.

Even though contour lines help determine if a piece of land is level or not, this does not mean that topography has nothing to do with it.

If your property is at the same level as a neighboring piece of land, it will have the same topography. If it is not, it will have its own. If it sits on a hill, it will have its own elevation.

It is through this process of experiencing and understanding changes in elevation that you start to see how topography plays an important role in determining the shape of your piece of land. That is why contour lines are so important.

Contour Surveying methods

There are two methods of contour surveying:

Direct method

You will be shown the contour lines on a topographical map.

• Method of squares-The method of squares gives you an accurate and reliable result because it removes any errors that may be caused by topography.

An example of this kind of work can be seen on a golf course where slopes are leveled off to create a uniform surface for playing golf.

• Method of cross-section-Topographical maps show the contour lines as measured from a distance. You must be aware that these lines are only as accurate as the measuring instrument used to make them.

Another method of checking for accuracy is to take a piece of sandpaper and rub it over the topographic map, which will show you any marks that are not smooth.

• Radial line method-The radial line method is done by using a ruler and string to measure the verges of the contour lines. You must be extremely careful when doing this type of work or you will end up measuring a curve that has no gradients.

Indirect method

You will be shown the contour lines on a topographical map and you will get to mark out the contour lines yourself.

The direct method is simple and accurate but it takes a lot of time to complete. The indirect method does not have quite that much accuracy and it takes less time, but it is not as simple as the direct one.

Characteristics of Contour Lines in Surveying.

The Characteristics of Contour Lines are:

1. The topographical map is a true representation of the contour lines.
2. The contour lines are at a fixed interval, which is easy to work with.
3. At higher elevations, you get steeper lines on the map and these may not be the correct ones.
4. The contour lines are going up vertically from point to point which means that rounded corners do not show up in these maps
5. The topography of the land is not shown accurately with contour lines. This can be prevented by using a topographic map with contour lines given at intervals of 10 feet rather than 20 feet.
6. Contours are affected by aerial photographs and maps, which can cause the lines to appear as if they are distorted, even though they are not.
7. The length of the contour interval is from point to point and this means that radii do not show up on these maps.

 Types of Contour Lines in Surveying and their Importance.

The different types of Contour Surveying and their importance are:

• Isopleth-On a contour map, an isopleth connects locations that have the same value of a measurable quantity of geographical or meteorological events.
• Isohyet-Isohyet denotes the points of equal rainfall in a particular region over a specific time period.
• Isobar-Isobars on contour maps represent the point at which atmospheric pressure is equal or constant for a specific period.
• This sort of contour is used to forecast future weather patterns. Isobars are frequently used in weather reporting on television.
• Isobath-It is a form of imaginary contour line on a map or chart that links all places with the same depth under the water’s surface, such as the ocean, sea, and lake.
• Isohaline-It is a form of contour line on a map that connects points of equal salinity in a body of water (majorly contour).
• Isotherm -It is a form of contour line on a map or chart that links points of equal temperature in a certain area. They are further classified into two categories.
• Isocheim: This is a line with the same mean winter temperature.
• Isothere: This is a line of equal summer temperature.

The freezing level is an isotherm at 0 degrees Celsius

• Isohel-It is a form of contour line on a map that links locations with similar amounts of sunlight for a certain period.
• Isohume-It is a form of contour line on a map that links all of the points in a specified area with equal relative humidity.
• Isoneph-It is a straight line that links all places with an equal level of cloud cover.
• Isopectic-Isopectic lines are imaginary lines drawn on a contour map or chart that link all of the sites where ice begins to develop at the same moment.

Advantages of Contour Surveying

Contour Surveying has different advantages. These are:

1. It is an easy way of measuring and mapping the surface of land.
2. They are useful in topographical engineering, geology, geography, hydrology and civil engineering etc.
3. It is possible to measure distances on sloping land with utmost accuracy by using contour lines.
4. Contour lines are used as a reference in construction and it becomes easier for builders to make precise calculations when building on sloping land or ground because they have something to refer from.
5. Contour lines are being used in mapping the areas for national security, for the purpose of defense.
6. Contour lines are used in setting up boundary limits, to define the boundary of land ownership.
7. They are used as a reference and there is no need to have any instruments to measure heights and distances when setting up contours on land because distance can be measured by a single line which can go over different elevations without having any course changes or relocations.
8. They can be used in recording the heights of man-made objects and also vehicles on an area of land.

Disadvantages of Contour Surveying

Some of the disadvantages of Contour Surveying are:

1. The contour lines are going up vertically which means that it is not possible to measure the slope without doing a lot of calculations.
2. They do not show the relationship between different places accurately, and this can result in an inaccurate representation when it comes to topography.
3. It is not possible to calculate the exact position of a point.
4. The accuracy of the contour lines is affected by aerial photographs and maps, which can result in them appearing as if they are distorted even though they are not.
5. The length of the contour interval is from point to point and this can affect the area covered on the map and this means that distances are not measured on a map accurately, even with a single line going all over different altitudes without having any changes in its course.
6. They are affected by other contour lines on the map and the terrain, for example; if there is a single line going through two contour lines then it will be difficult to differentiate which one is higher or lower.
7. They do not show surface features well and this makes it difficult to get a full representation of a topographical map.
8. They are affected by terrain faults; for example, they can be affected by landslides, which can result in their appearance as if they have been distorted even though they have not.

Uses of Contour Surveying

Contour Surveying is known to have a variety of uses. They include:

Construction of roads and highways

Contour lines are used in the construction of roads and highways, for the purpose of studying the sloping terrain, drainage, and other important things related to building a road.

Construction of railways

Contour lines are used in railway engineering because they provide information to builders on different sorts of obstacles they may encounter while constructing railways on areas with slopes.

Construction of dams

Contour lines are used in the construction of dams, to map and study the terrain of the area that is going to be flooded. In other words, contour lines are used for flood control.

Study of relief on land

Contour lines are used in studying relief on land to get information about slope, altitude, drainage slope etc. This information can help in better understanding of a landscape’s surface and make it easier to construct roads, water supply etc.

Study of vegetation

Contour lines are used in studying the trees and other plants that are present on a landscape. They are used for setting up boundary limits between properties and for legal purposes.

Study of watercourses

Contour lines are used in studying the watercourses in an area and can provide information on the body of water that is situated there, its depth, level etc.

Study of canals

Contour lines are used in the study of canals, to identify different sorts of trenches and provide information about them. The information provided by using contour lines can also help in controlling floods and providing proper drainage for any particular canal.

Study of crops

Contour lines are used in studying the crops grown in a certain area and how they grow, how effective they are when it comes to production etc.

Contour Surveying procedure

The steps involved in conducting Contour Surveying are as discussed below.

1. The initial stage is to set up a contour line on topographical maps or on a digital map device, this can be done visually or by using an instrument.
2. Then you measure the slope of the land and draw it on the contour map.
3. You check if there are any gradients in that area and make necessary adjustments to the lines if required.
4. You check the location of objects on the ground, also take note of their heights, if need be.
5. You check the height of the land by measuring the distance between contour lines.
6. You measure a straight line that goes over the altitude difference and makes it as a base line to continue your contour lines on different elevations.
7. You measure height of objects and also check if they can be seen from certain places on the land.
8. You check the contour lines on a digital map and if there are any problems then you make necessary adjustments to them.
9. You conclude your measurement of the topography by drawing the contour lines on a digital map using digital mapping software.
10. You draw the topography on a large scale wall paper and check if it is accurate or not, with an instrument, this can also be done with a computer mouse or by using software tools for maps.
11. You measure the length of the contour lines; this should be done at different levels using a measuring device and is known as contouring.
12. You plan and identify various types of surfaces like slopes, dips, terraces etc. on the landscape by using these contour lines.
13. You make drawings to show complex topography accurately and clearly by combining all those contour lines and other data that you have collected throughout your survey.
14. You check and make necessary amendments if there are any errors.
15. You prepare a report with all the details you have collected and submit it to your appointed authority.
16. You store the survey data in a file where you can access it any time required and share it with the other surveyors, government agencies etc.

FAQs

How do I find the area of a map?

While you measure the area of a map, you need to take into consideration that in most cases land formations are considered as being in square-like shapes.

For instance, if you have identified that a particular area is shaped like a rectangle and its length and breadth is equal then its area will be found by multiplying its length by its breadth. In case the resulting number is more than 10, then you can round it off to 10 but only after calculating it first.

You can get the area of a map using any subject you know that needs units of measurement to work out (metric system is mostly used for measuring areas on maps).

What is the difference between contours and heights?

Concords are the lines that are drawn at intervals on topographical map, above the contours. Con-tour lines are drawn to represent changes in elevation or slope of land or water lakes.

Height is simply the vertical measurement from a point on a map at a given moment of time. An instrument is used to measure the elevation.

How do I know when to use contour lines?

Contour lines should be used when you have to measure heights, gradients, and also if you need data on drainage direction. They are also used in defining boundaries, legal matters etc.

Once you have done your survey what is the final stage of the contour survey?

You should not only collect data on the land but also store it in a file or database so that you can recall it any time.

What are 3 types of contour lines?

Index lines are the thickest contour lines, and they are often identified with a number at one point along the line.

The thinner, more frequent lines between the index lines are known as intermediate lines. They normally don’t have a label with a number on them. In most cases, one index line appears for every five intermediate lines.

Supplementary lines, which indicate gentler terrain, show as dotted lines.

How do you calculate contour surveys?

Divide the elevation difference between the index lines by the number of contour lines connecting one index line to the next. In the above example, the distance of 200 is divided by the number of lines, which is 5. The contour interval is equivalent to 200 / 5 = 40, or contour intervals of 40 units.

What is a contour map used for?

Contours allow you to see the height and shape of mountains, the depths of the ocean floor, and the steepness of slopes.

Roads, railways, rivers, streams, lakes, borders, place or feature names, mountains, and other geographic features are also depicted on USGS topographic maps.

What is the contour interval?

A contour interval is the vertical distance or elevation difference between two contour lines. Index contours are bigger or bolder lines that emerge every fifth contour line.

When the numbers connected with certain contour lines increase, so does the height of the landscape.

What is contour line surveying?

Contour lines are lines drawn on a map with equal height points, so the elevation would be consistent if you physically followed the contour line.

It is useful because it depicts the shape of the land surface–its topography–on a map. Furthermore, Edmond topographical surveyors understand how to perform it.

How can I determine if a slope is steep or gentle?

A slope is considered to be steep if it has an angle greater than 3O degrees

It is steep if you can walk or bicycle on it and still see the next contour lines.

How do I plan an accurate topography?

A topography is the measurement of land features, elevation, slopes and other features such as drainage directions. The purpose of a surveyor is to calculate heights, gradients and drainage direction as well.

Why should I be careful when measuring contours?

The contours on a topographical map are drawn with equal elevation points so that the elevation of both sides is the same.

If you measure the contour lines located on the map, it is important that you mark them so that they are at equal intervals and then calculate the area of each contour line.

When you have done your survey, should I make a map or file with my data?

It is necessary to make a file or database of your data so that you can retrieve it anytime.

What is a unit of measurement in a contour survey?

The basic unit of contour measurement is the contour interval. The number of contour line that connects top and bottom points is divided by the contour line interval to get the area. The first step in calculating an area is to mark one index line on the map at any given section.

You can measure heights, gradients and drainage direction at your convenience.