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What is Contour Interval?
A contour interval in surveying is the vertical distance or the difference in the elevation between the two contour lines in a topographical map. Usually there are different contour intervals for the different maps. Considering the size of the area to be mapped, contour intervals are assumed.
In every map, on the right-hand bottom side, the contour interval is specified. When the contour interval is not specified in the map, it can be calculated as explained in the following sections. The commonly used contour interval is 20 feet for a 1:24,000 map scale.
Topographic Map Contour Interval
A contour line is a line drawn on a topographic map to indicate ground elevation or depression. A contour interval is the vertical distance or difference in elevation between contour lines. Index contours are bold or thicker lines that appear at every fifth contour line.
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How to Calculate Contour Interval from Maps?
A contour map consists of contour lines of a given geographical region. To keep the contour map simple and easy to read, not every contour line is marked with its elevation reading. These marked or labeled lines are known or termed as Index Contour Lines.
In the above figure, the dark lines with reading are index contour lines. The calculation of the contour intervals is as below:
Step 1:
Firstly locate 2 index contour lines that are labeled with a specific elevation.
Step 2:
Now calculate the difference between the two-selected index contour line selected from a map. To take the difference, subtract the higher elevated line with the lower elevated line reading.
Step 3:
Now count the number of non-index lines contour lines between the 2 index contour lines selected for the contour interval calculating in the 1st step.
Step 4:
The number of lines obtained in the above step is taken and added with 1. For Ex: If the number of lines between 2 index lines are 5. Then add 1 to 5 that becomes 6.
Step 5:
the final step is the quotient of the difference between 2 index lines (step 2) and the number of lines in between two index lines plus 1 (step 5).
Step 6:
The final answer we get after dividing is the contour interval of the specific topographical map
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Example Calculation of Contour Intervals:
Considering the above map, the steps involved in contour interval calculation are, Let’s assume, 7000 and 7100 and calculate the interval between it. Now difference between 7100 and 7000 is 7100 - 7000 = 100 The number of contour lines in-between 7000 and 7100 are 4. Adding 1 to 4, 4 + 1 = 5 Now dividing 100 by 5, 100/5 = 20 units The Contour Interval of the above map us 20 Units.
Uses of Contour Intervals in Surveying
When a large area is to be mapped in small piece of paper contour intervals are used. A higher contour interval is used for a large area and small contour interval for small area.
In a large map, index contour lines are less to keep it simple to read the map easily. In this case, to find out the intermediate points elevation, contour intervals are used.
Earthwork estimations for any type of structure like bridges, dams or roads can be found out with the help of contour intervals in a map.
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.
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