What Is Contour Interval?
A contour interval is a vertical distance or difference in elevation between contour lines. Index contours are bold or thicker lines that appear at every fifth contour line. A contour interval in the survey is the vertical distance or the difference in elevation between the two contour lines on a topographic map.
How to Calculate the Contour Interval of the Maps?
A contour map consists of contour lines for a given geographic region. To keep the contour map simple and easy to read, not all contour lines are marked with their elevation reading. These marked or labeled lines are known or called Contour lines of the index.
Calculation of Contour Intervals
A map legend usually identifies the contour interval on the map, but sometimes only part of a map is available. Knowing how to calculate the contour interval becomes a useful skill. On most maps, each fifth contour line, shown as a heavier or darker line, is an index line or index contour.
These index lines will be marked with their elevation. Find the elevations of two adjacent index lines. The highest number shows the uphill rise. Find the difference between the two elevations.
For example,
If the uphill elevation is equal to 1,000 above average sea level and the lowest elevation is equal to 800 above average sea level, the difference in elevation is equal to 200.
To Knowing how to calculate the contour interval becomes a useful skill.l, start by counting the contour lines from one index line to the next index line. Maps generally count five contour lines from one index line to the next, including the next index line.
- As with counting from one number to the next,
- like from five to 10,
- Start with the next line of the index line,
- Counting each contour line up to and including the next index line.
- To find the elevation interval between the contour lines,
- Divide the difference in elevation between the index lines by the number of contour lines from one index line to the next.
- In the example above, the distance 200 is divided by the number of lines, 5.
- The contour interval is equal to 200 / 5 = 40,
- or 40-unit contour intervals.
- If, on the other hand, the elevation difference between the index lines were 100 feet, the contour interval would be 100 / 5 = 20 or a 20-unit contour interval.
Uses of Contour Intervals in Surveying
When a large area is to be mapped onto a small piece of paper, contour intervals are used. A higher contour interval is used for a large area and a small contour interval for a small area. Earthwork estimates for any type of structure, such as bridges, dams or roads, can be found with the help of contour intervals on a map.
Since the contour intervals are for calculating the vertical elevation of an area, in the same way as for calculating the horizontal distance, it is referred to as the horizontal equivalent.
Reading Contour Lines
The contour lines show the shape of the earth. A single contour line marks an equal elevation line, which means that if the contour line measures an elevation of 1,000 feet above average sea level, all points along that line are 1,000 feet above average sea level.
The contour lines never cross, as a point on the map cannot have two different elevations at the same time. The farther the contour lines appear on the map, the smoother the slope of the earth is. The closer the contour lines appear, the more inclined the terrain will be.
Where the contour lines get very close, and almost precipice occurs. If the relief is a vertical cliff, the contour lines almost come together and can look as if they are merging. Pending cliffs can have one line crossing over the other (this is the only time those lines can cross), with one line appearing as dotted.
Be aware, however, that smaller cliffs can occur between the contour lines, even in areas with smooth slopes. A cliff 15 feet high, for example, along a flow channel or due to minor faults, would not necessarily show whether that cliff lies between two contour lines, especially if they have a longer contour gap.
What Is the Importance of Topographic Maps?
Topographic maps are an important tool because they can represent the three-dimensional landscape in two dimensions. A person who can read a map from the top can discover the location of peaks, valleys, mountain intervals, and saddles, among other land features. Topo maps can also show whether you are traveling up or down a specific road or trail.
1. Contour Lines
Elevations on an upper map are marked with contour lines, which connect equal elevation points. Imagine walking around a mountain in a circle, never going up and down the hill, but staying at the same altitude.
If you followed the path, you would have a contour line on a map. The contour lines are usually separated by 40 vertical feet, but you should check the map you are using to be sure, and each fifth contour line is usually marked with an actual elevation.
2. Land Features
The shape of the contour lines can indicate the shape of the landforms in a specific area. For example, concentric circles show a peak, with the smallest circle marking the summit.
The contour lines next to each other indicate that the land is very steep, while the scattered contour lines show that the land is relatively flat. The contour lines surrounding two peaks – or two sets of concentric circles – may indicate the presence of a saddle or space between the peaks.
3. USGS Maps
Topographic maps across the country were produced by the US Geological Survey, which began surveying land to create these maps in 1879. Today, the USGS has created more than 54,000 maps, which form the basis of most commercially available topographic maps today.
USGS topographic maps also show features that you would see on regular road maps, including roads, dirt roads, cities, and structures. The maps also show power lines, rivers, glaciers, and mines.
4. Orienting the Map
To combine a topographic map with the surrounding landscape, which will allow you to identify features such as mountains and rivers, it is important to ensure that the map is oriented correctly.
You can quickly orient the map using a compass and the “compass rose” found on the map, which will have an arrow pointing north. Align the compass needle, which points north, with the arrow on the wind rose, rotating the map if necessary.
Frequently Asked Questions (FAQ)
What is a contour interval?
A contour interval is the vertical distance or difference in elevation between two contour lines on a topographic map. It helps represent changes in elevation in a simplified manner.
How can I calculate the contour interval of a map?
To calculate the contour interval:
- Find the elevations of two adjacent index contour lines (usually marked with their elevation).
- Determine the difference in elevation between these two index lines.
- Count the number of contour lines between the two index lines, including the index lines themselves.
- Divide the elevation difference by the number of contour lines to get the contour interval.
What are index contours?
Index contours are heavier or thicker lines on a topographic map that are marked with their elevation. They appear at regular intervals, usually every fifth contour line, and help in determining the contour interval.
Why are contour intervals important in surveying?
Contour intervals help in representing vertical elevation changes on a map, making it easier to estimate earthwork quantities for construction projects, such as bridges, dams, and roads. They are essential for understanding the topography of a mapped area.
How do contour lines indicate the shape of the land?
Contour lines show the elevation of land at specific points. The shape and spacing of contour lines reveal the terrain’s steepness and form. For example, closely spaced lines indicate steep slopes, while widely spaced lines indicate gentle slopes.
What do concentric contour lines represent?
Concentric contour lines typically indicate a peak or summit. The smallest circle represents the highest point, with progressively larger circles showing lower elevations around it.
How can I read and interpret topographic maps effectively?
To read a topographic map, understand the contour lines and their intervals to gauge elevation changes. Look for patterns in the lines to identify landforms like peaks, valleys, and slopes. Use map legends and the compass rose for orientation and scale.
What are USGS topographic maps?
USGS topographic maps are produced by the U.S. Geological Survey. They provide detailed information on landforms, elevation, and features such as roads, rivers, and buildings. These maps are widely used for various applications, including navigation and planning.
How do I orient a topographic map with the landscape?
To orient a map, use a compass and align it with the north arrow on the map (the compass rose). Ensure the map is rotated so that north on the map matches north on the ground to accurately relate map features to the real world.
Can contour lines cross each other on a map?
No, contour lines cannot cross each other. If they do, it typically indicates a mistake on the map. In rare cases, contour lines may appear to merge in areas of extreme elevation change, such as vertical cliffs.
