 1. Introduction

A contour interval is a vertical distance or the difference in the elevation between the two contour lines in a topographical map.

Generally, there are various contour intervals for the various maps. Taking the area to be mapped, contour intervals are considered. In every map, on the right-hand bottom side, the contour interval is described. The commonly utilized contour interval is 20 feet for a 1:24,000 map scale.

To calculate contour interval, we need to divide the difference in elevation between the index contour lines by the number of contour lines from one index contour line to the next.

For example, if the distance 200 is divided by the number of lines, where the number of lines is 5. The contour interval is equal to 200 / 5 = 40, or 40-unit contour intervals.

2. Steps of Calculating Contour Intervals

Step 1:

At first, we need to locate two index contour lines that are given with a specific elevation.

Step 2:

Now find the difference between the 2 chosen index contour lines from a map. To note the difference, now minus the higher elevated line with the lower elevated line reading.

Step 3:

We need to calculate the number of non-index lines contour lines between the 2 index contour lines selected for the contour interval calculated in step 1.

Step 4:

The number of lines calculated in the above step is noted and added with 1. Let take a simple example: If the number of lines between two index lines is 5. Then add 1 to 5 that will be 6.

Step 5:

The last and final step is the quotient of the difference between two index lines (step 2) and the number of lines in between two index lines adds 1 (step 5).

Step 6:

The result we calculate after dividing is the contour interval of the given topographical map.

3. Example Calculation of Contour Intervals Taking the given maps, the steps needed in contour interval calculation are,

Let us take, 7100 and 7200 and find the interval between them. Now the difference between 7200 and 7100 is 7200 – 7100 = 100 The number of contour lines in-between 7200 and 7100 is 4. Adding 1 we get, 4 + 1 = 5. Now we need to divide 100 by 5, 100/5 = 20 units.

The Contour Interval of the given map is 20 Units.

4. Rules of Contour Intervals

a. Every point of a contour line has a similar elevation.

b. Contour lines distinct uphill from downhill.

c. Contour lines never touch or cross each other than at a cliff.

d. Every 5th contour line is darker in color than other contour lines. This is an INDEX contour line.

e. Contour lines are nearer together in steep terrain and farther apart in flat areas.

5. Factors Affecting Contour Intervals

a. Scale.

The larger scale is the smaller the contour interval.

b. Importance and purpose for which the plan is to be used.

for more details, a small contour interval is used.

c. Accuracy, time, and cost of the contour plan.

For higher accuracy, a smaller interval is used.

d. Topographic variation of the area

For steep ground, a large contour interval is utilized, but for flat ground, a small contour interval is utilized.

e. Size of the area

For larger areas, a large contour interval is utilized. 6. Characteristics of Contour in Surveying

The main characteristic of the contour can be listed as follows:

1. The horizontal distance between any two contour lines indicates the amount of slope and varies inversely on the amount of slope.

2. Two contours of different elevations never cross each other except in the case of an overhanging cliff.

3. Contours of varied elevations never unite to form a single contour except in the case of a vertical cliff.

4. Contours that are close to each other depict a steep slope whereas the contours drawn apart depict a gentle slope.

5. The equally spaced contours depict a uniform slope. When the contours are drawn parallel, equidistant, and straight, such contours indicate inclined plane surfaces.

6. A contour line must close itself but may not necessarily be within the limits of the map itself.

7. Contour at any point is perpendicular to the line of the steepest slope at that point.

8. Irregular contours indicate uneven surfaces.

9. The same contours must appear on both the sides of a ridge or a valley.

10. Contours do not have sharp turns.

11. Approximately concentric closed contours with decreasing values towards the center indicate a pond.

12. Approximately concentric closed contours with increasing values towards the center indicate hills.

13. Contour lines with U-shape with convexity towards lower ground indicate ridge.

14. Contour lines with V-shaped with convexity towards higher ground indicate valley.

15.  Contours of different elevations cannot cross each other. If contour lines cross each other, it shows the existence of overhanging cliffs or a cave.

16. Contours do not pass through permanent structures such as buildings.

7. Uses of Contour in Surveying

The uses of contour in surveying are as follows:

a.It shows the slope and size of various landforms on the map.

b. It gives a full and clear image of the land and the surrounding area.

c. With the help of contour intervals it is easy to find out the different elevations of the landscape.

d. It gives the basis for the coloring method.

a. It shows the slope and size of different landforms on the map.

b. By looking at the contour intervals it is easy to calculate the various elevations of the landscape.

c. It can be utilized in drawing cross-sections of the specific features on the mapped area.

a. Contour fail to represent a few of the heights due to the limitation of the vertical interval.

b. Some of the landforms cannot be represented by utilizing contours. For example coral reef, outcrop rock, and craters.

c. The contour method is not utilized to represent relief on a small-scale map as it may unclear a few details.

10. FAQ

1. How to find contour interval?

To find contour interval divide the difference in elevation between the index lines by the number of contour lines from one index line to the next.

For example, if the distance 200 is divided by the number of lines, where the number of lines is 5. The contour interval is equal to 200 / 5 = 40, or 40-unit contour intervals.

2. What is the contour interval of the map?

A contour interval of the map is a vertical distance or the difference in the elevation between the two contour lines in that topographical map.

3. What Color Is an Index Contour Line?

The index contour is a darker or wider brown line if we compare it to other regular contour lines.

4. Why Are Contour Lines Useful?

Contour lines are very much helpful because they permit us to depictures the shape of the land surface (topography) on a map.

5. What Is the Shape of Contour Lines?

The shape of contour lines are a closed loop.