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What Are Contour Interval?
A contour interval in surveying is the vertical distance or the distinction in the elevation between the two contour traces in a topographical map. Usually there are one of a kind contour intervals for the special maps. Considering the measurement of the vicinity to be mapped, contour intervals are assumed. In each map, on the right-hand backside side, the contour interval is specified. When the contour interval is no longer specific in the map, it can be calculated as defined in the following sections. The regularly used contour interval is 20 ft for a 1:24,000 map scale.
Steps of Calculating Contour Intervals
Step 1:
At first, we want to discover two index contour traces that are given with a unique elevation.
Step 2:
Now, locate the distinction between the two chosen index contour traces from a map. To word the difference, now minus the greater multiplied line with the decrease accelerated line reading.
Step 3:
We want to calculate the variety of non-index traces contour strains between the two index contour strains chosen for the contour interval calculated in Step 1.
Step 4:
The variety of traces calculated in the above step is cited and delivered with 1. Let’s take a easy example: If the range of traces between two index strains is 5. Then add 1 to 5 which will be 6.
Step 5:
Find the quotient of the distinction between two contour index strains (step 2) and the quantity of traces between two index traces via including 1 (step 4). Thus got quotient is the contour interval of the given topographical map.
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Factor Affecting Contour Interval
The Contour Spacing Depends On The Following Factors:
1. The Scale of The Map
The contour interval is inversely proportional to the scale of the map. If the scale is small, the contour interval ought to be massive and vice versa.
2. Nature of The Ground
The customary terrain of the place determines the contour interval. For a flat area, the contour interval need to be small. For sloping or undulating terrain, the chosen contour interval must be large.
3. Purpose and Expansion of the survey work
If surveying work is to be used for unique and designated calculations, then the small contour interval need to be chosen and a massive contour interval is chosen for the following cases:

For catchment areas
To the reservoir
For place survey

4. Time on hand and Eligible Expenses for discipline and workplace work
A giant contour interval ought to be used if the time on hand for survey work is less. The small contour interval, survey work and plotting want to be extra precise. For a small contour interval, the cash wanted will additionally be greater as the area and workplace work will be larger.
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Contour Intervals Calculation
Usually, the map legend identifies the contour spacing on the map, however occasionally solely a portion of the map is available. Knowing how to calculate contour interval will become a beneficial technique. On most maps, every fifth contour, marked with a thicker or darker line, is an index line or an index contour. This index line is marked highly.
Find the elevation of the two adjoining index lines. The easiest range represents an uphill ascent. Find the distinction between the two elevations.
For Example, Considering the map, the following steps are used to calculate the contour intervals calculation as follows:

Let’s say, 6000 and 6100 and calculate the interval between them.
Now the distinction between 6100 and 6000 is 6100 – 6000 = 100.
The range of contour traces is four between 6000 and 6100.
Add 1 to 4, four + 1 = 5
Now divide one hundred by using 5,
100/5 = 20 units
The contour interval of the above map gave us 20 units.

Uses Of Contour Interval In Surveying
The makes use of of contour interval in surveying are as follows,

Interval is utilized when a giant area is to be mapped to a small piece of paper.
A greater interval is utilized for a large vicinity and smaller intervals for a smaller area.
In a large map, index contour strains are plenty much less to hold the map and are easy to learn.
To discover the peak of the intermediate point, the interval is utilized.
Earthquake estimation for the dam, bridge, or street can be discovered with the assist of contour intervals inside the map.
As intervals are for the computation of the vertical elevation of an area, a comparable way to compute the horizontal distance is termed Horizontal Equivalent.
The horizontal distance between two factors on two consecutive contour strains for a furnished slope is recognised as the horizontal equivalent.

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How to Calculate Contour Intervals?
Contour strains are spaced by using contour intervals and characterize areas on land that lie the identical distance above sea level, taken to be zero ft (0') via convention. That potential that if you had been to exactly observe the course proven by means of a contour line – which is frequently however no longer constantly a closed loop – your elevation would now not trade at all even as the surroundings round you in all likelihood different substantially in the course of your trip.
Graphically, contour strains provide a experience of the topography, or hilliness, of the vicinity the map depicts. Since every contour line represents a given elevation above sea stage (also referred to as altitude or actually elevation), mapmakers need to select how many consultant elevation traces to hire barring both crowding out different important points by using the use of too many such strains or failing to to furnish adequate simple elevation data through the use of too few.