Contour Lines

Contour Interval | How To Calculate It

The contour interval is an important factor in determining the accuracy and level of detail of a topographical map. A smaller contour interval provides more detailed information about the terrain, including small changes in elevation and land features, while a larger contour interval provides a general overview of the terrain with less detail.

In addition to providing information about the terrain, contour lines can also be used to determine the slope of a surface, the location of a peak or a valley, and the direction of water flow. They are commonly used by hikers, mountaineers, and other outdoor enthusiasts to navigate and plan their routes, as well as by geologists, engineers, and land-use planners to make decisions about land use and development.

What Is Contour Interval?

Contour interval is an important concept in cartography, geography, and topography. It refers to the vertical distance or difference in elevation between two successive contour lines on a map or a topographic profile.

Contour lines are imaginary lines on a map that connect points of equal elevation and provide a two-dimensional representation of the three-dimensional terrain.
Contour Intervals

The contour interval is usually expressed in units of measurement such as feet, meters, or units of elevation relative to sea level. The contour interval is determined by the scale of the map and the degree of relief of the terrain being represented.

On large-scale maps with a lot of topographical detail, the contour interval is usually smaller, while on small-scale maps with less detail, the contour interval is larger.

In conclusion, the contour interval is an essential aspect of topographical maps and an important tool for understanding the terrain and making decisions about land use and development.

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How To Calculate Contour Interval On Maps

Contour Interval | How To Calculate It
Topographic Map Contour Lines

A map legend usually specifies the contour interval on the map, but sometimes just a portion of a map is available. Understanding how to compute the interval becomes a valuable skill.

On most of the maps, each fifth contour line, indicated as a heavier or darker line, is an index line or index contour.

Calculation Of Contour Interval

These index lines will be noted with their elevation. Discover the elevations of two adjacent index lines. The biggest number indicates the uphill rise. Locate the difference between the two elevations.

A contour map contains contour lines of a given geographical region. To maintain the contour map easy and understandable to read, not every contour line is marked with its elevation reading.

These marked or tagged lines are remembered or termed index contour Lines. In the above picture, the dark lines with reading are index contour lines.

Computation Of Intervals

The computation of the contour intervals is as below,

Step 1:-First of all, locate two index contour lines that are named with a particular elevation.

Step 2:-Now compute the difference between the two selected index contour lines established from a map. To obtain the difference, subtract the higher elevated line from the lower elevated line reading.

Step 3:-Directly count the number of non-index line contour lines between the two index contour lines specified for the contour interval computing in the 1st step.

Step 4:-The number of lines acquired in the above step is obtained and added with 1. For Example: If the number of lines between two index lines is 5. Then add 1 to 5 which becomes 6.

Step 5:-The last step is the quotient of the difference between two index lines (step 2) and the number of lines between two index lines plus 1 (step 5).

Step 6:-The last answer we get after dividing is the contour interval of the particular topographical map.

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How to Find Contour Interval

Contour Line Example
Contour Line Example

The calculation of the contour interval,

  • Considering the above map for the calculation of intervals, the steps involved in contour interval computation.
  • Let’s assume two index contour lines, 7000 and 7100, and compute the interval between them.
  • The difference between the two selected index contour lines 7100 and 7000 is 7100 – 7000 = 100
  • Now count the number of non-index lines contour lines between 7000 and 7100 as 4.
  • The number of lines acquired in the above step is 4 and added with 1.
  • Adding, 4 + 1 = 5
  • Now divide 100 by 5, 100/5 = 20 units

Uses Of Contour Interval In Surveying

The uses of contour interval in surveying are as follows,

  • Interval is utilized when a large space is to be mapped to a small piece of paper.
  • A higher interval is utilized for a larger area and smaller intervals for a smaller area.
  • In a larger map, index contour lines are much less to maintain the map and are simple to learn.
  • To find the height of the intermediate point, the interval is utilized.
  • Earthquake estimation for the dam, bridge, or road can be found with the help of contour intervals within the map.
  • As intervals are for the computation of the vertical elevation of an area, a similar way to compute the horizontal distance is termed Horizontal Equivalent.
  • The horizontal distance between two points on two consecutive contour lines for a provided slope is known as the horizontal equivalent.

How To Read Contour Line

The Contour Lines indicate the shape of the earth. A single contour line denotes an equal elevation line, which indicates that if the contour line computes an elevation of 1000 feet above average sea level, every point along that line is 1000 feet above average sea level.

The contour lines never cross, as a point on the map cannot possess two different heights at a similar time. The farther the contour lines arrive on the map, the smoother the slope of the earth is. The near the contour lines arrive, the additional inclined the terrain will be.

Reading the contour lines Where the contour lines get extremely close, an almost precipice happens. If the relief is a vertical cliff, the contour lines nearly come together and can glance as if they are merging.

Pending cliffs can have one line crossing over the additional (this is the just time those lines can cross), with the one line appearing as dotted.

Be familiar, however, that smaller cliffs can happen between the contour lines, even in areas with smooth slopes.

A cliff 15 feet elevated, for instance, along a flow channel or due to slight faults, would not certainly indicate whether that cliff lies between two contour lines, particularly if they have a higher contour gap.

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Types of Contour Lines

There are 3 types of contour lines detected on a map,

  1. Index Contour Line
  2. Intermediate Contour Line
  3. Supplementary Contour Line

1. Index Contour Line

Index Contour Lines
Index Contour Lines

Occasionally a map may have many contour lines such that it becomes tough for the cartographer to tag the elevations of each contour line.

Therefore, to create the map simpler to read and less cumbersome, every fifth (5th) contour line from the mean sea level is usually named after its elevation and sometimes bolded a little bit larger than the rest. These bolded and named contour lines are usually related to the index contour lines.

2. Intermediate Contour Line

Intermediate Contour Lines
Intermediate Contour Lines

The remaining contours are named intermediate contours. These normally fall in between the index contour lines. The Intermediate contour lines are finer than index contours and do not have their elevations named on them.

There are mainly four intermediate contour lines between any two successive index contour lines.

3. Supplementary Contour Line

Supplementary Contour Lines
Supplementary Contour Lines

In some special areas on a map, the typical interval is sometimes too wide to represent a considerable change in elevation, such as on-level terrain, and therefore, extra half-interval contours are added. These types of contours are named supplementary contours or form lines.

They usually arrive on the map as dashed or thin continuous lines with no representative value of elevation on them. They are utilized when the overall elevation change is extremely gradual.

When supplementary contours arrive on the map, the interval utilized for them is usually specified in the interval note.

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What Is the Importance Of Topographic Maps?

Topographic maps are a significant tool because they can represent the three-dimensional landscape in two dimensions. A person who can look over a map from the top can discover the location of saddles, valleys, mountain intervals, and peaks, among different land features.

Topographic maps can also indicate whether you are traveling up or down a particular road or trail.

1. Contour Lines

Elevations on an upper map are notable with contour lines, which attach comparable elevation points. Imagine stepping around a mountain in a circle, never going up and down the hill, but staying at a similar altitude.

If you pursued the path, you would have a contour line on a map. The contour lines are generally separated by 40 vertical feet, but you should examine the map you are utilizing to be sure, and each fifth contour line is usually noted with an actual elevation.

2. Land Features

The shape of the contour lines can specify the shape of the landforms in a particular area. For instance, concentric circles indicate a peak, with the smallest circle denoting the summit.

The contour lines next to each other specify that the land is extremely steep, while the scattered contour lines indicate that the land is relatively flat.

The contour lines surrounding two peaks or two sets of concentric circles may specify the existence of a saddle or space between the peaks.

3. USGS Maps

Topographic maps across the country were generated by the US Geological Survey, which started surveying land to produce these maps in 1879.

Today, the USGS has generated more than 54,000 maps, which form the basis of the highest commercially obtainable topographic maps today.

USGS Topographic Maps also indicate details that you would watch on regular road maps, containing roads, dirt roads, cities, and structures. The maps also indicate power lines, rivers, glaciers, and mines.

4. Orienting The Map

To combine a topographic map with the surrounding landscape, which will enable you to specify features such as mountains and rivers, it is significant to assure that the map is oriented correctly.

You can rapidly orient the map utilizing a compass and the “compass rose” found on the map, which will include an arrow pointing north. Align the compass needle, which points north, with the arrow on the wind rose, rotating the map if important.

Factors Affecting The Selection Of Interval

The survey leader has to define the selection of the interval before the beginning  of the mapping procedure based on ground factors as follows,

1. Scale Of The Maps

The interval is retained inversely proportional to the scale of the map. Less the scale of the map, the larger the interval.

On the other side, if the scale of the map is large, the interval should be small. If on a small-scale map, a small contour interval is obtained the horizontal distance between two consecutive contours.

For example, the horizontal equivalent is also small and when planned on the scale of the map, the two contours might join together. It necessitates enhancing the interval on small-scale maps.

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2. Purpose Of The Map

The interval on a map also relies on the objective for which the map is to be used. If the map is prepared for setting out a highway on hills slopes, a large interval might be enough.

But, if the map is needed for the construction of a university campus, a small interval will be needed for proper work.

3. Nature Of The Ground

The contour interval relies on the general topography of the terrain. In the flat ground, contours at small intervals are monitored to describe the general slope of the ground whereas high hills can simply be illustrated with contours at larger intervals.

In other words, we may explain that the interval is inversely proportional to the flatness of the ground, for example, the steeper the terrain, the larger the interval.

4. Availability Of Time And Funds

If the time available is limited, a greater interval is obtained to finish the project in a particular time.

On the other hand, if adequate time is at the disposal, a smaller interval might be determined, keeping in view all the different factors already described.

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