What exactly is Contour Interval?

A contour interval is the vertical distance or difference in elevation between two contour lines of the topographical map. There are usually different contour intervals on various maps.

When considering the size of the space to be mapped the contour intervals are considered. On every maps, in the right bottom the contour interval is indicated.

If the contour range is not stated on the maps, it could be calculated using the methods described in the sections below. The most commonly used range of contours is around 20 feet on the 1:24,000 scale map.

Also Read: Floating Slab

How can I calculate Contour Interval using Maps?

A contour map is made up of contour lines within an area of a particular geographical. To make the contour map easy to comprehend it is not necessary that every contour line is labeled with its elevation reading. The lines that are marked or labeled are called Index Contour Lines.

In the above image the dark lines that have readings are contour lines that are indexes. The method of calculating the contour intervals can be done as follows:

Step 1:

The first step is to locate 2 index contour lines which are marked with a specific elevation.

Step 2:

Then calculate what is the difference in the two index contour line from the map. To calculate this difference subtracting the upper elevated line from the reading of the lower elevated line.

Step 3:

Now , you can count the number non-index lines that contour lines are between the two contour lines used for the contour interval calculation in the initial step.

Step 4:

The line count obtained during the previous step is added and rounded to 1. For instance, if the lines that lie between two index lines is five. Then , add 1 to 5, and it becomes 6.

Step 5:

The last step is the ratio from the differences between two index lines (step 2) and the number of lines between the two index lines plus one (step five).

Step 6:

The final result we get after division the contour interval from the topographic map that is specific to it.

Also Read: Spread Footing

An Example of Calculation for Contour Intervals:

Based on the above map and the steps to follow in calculating the contour interval is to take the following example: 7100 and 7000, and determine the interval in between. The difference between 7100 the 7100 value is 7100 + 700 = 100. The total numbers of contour lines between 7100 and 7000 is 4. The addition of 1 to 4 is 4 + 1 = 5. Then divide 100 by 5 100/5 = 20 units A contour Interval on the previous map is 20 units.

The characteristics of Contour in Surveying

The most important aspect of the contour could be summarized as in the following manner:

1. The horizontal distance between two contour lines is a measure of the slope of the line and changes in inverse proportion to the slope.

2. Two contours with different elevations do not meet, except when there is the cliff's overhang.

3. Contours with different elevations do not join to create a single contour, except for the vertical cliff.

4. Contours that lie close to each other show steep slopes, while contours that are drawn separately represent a gentle slope.

5. Equally spaced contours show an even slope. If you draw the contours parallel to each other, parallel, and straight these contours represent sloped plane surfaces.

6. A contour line should be able to close to the map, but does not have to be within the boundaries of the map.

7. Any point of contour is parallel to the slope of the slope that is steepest at that point.

8. Irregular contours indicate uneven surfaces.

9. The same contours are required on both sides of a valley or a valley.

10. Contours don't have sharp turns.

11. Closed contours that are concentric and with lower values towards the middle are indicative of a pond.

12. Closed contours that are approximately concentric with higher values towards the middle indicate hills.

13. Contour lines in U-shape that show convexity toward the ground mark the ridge.

14. Contour lines that are V-shaped and have convexity toward higher ground are signs of valleys.

15. Contours at different elevations can't cross one another. If contour lines cross this indicates the existence of cliffs that hang overhanging or caves.

16. Contours don't travel through permanent structures, like buildings.

Also Read: Tributary Area