1.16.1. Contour interval

The constant vertical distance between the successive contours is called the contour interval.

A contour interval is defined as the fixed vertical distance that separates one contour line from another on a map. Contour lines represent points of equal elevation, and the contour interval helps in understanding how steep or flat the terrain is. The smaller the contour interval, the more detailed the information about elevation changes, but it also requires more data collection.

Imagine walking along a gentle hill. If the distance between your footfalls (the contour interval) is short, you’ll notice every bump and dip in the ground. But if you’re running fast downhill and only care about the overall slope (a larger contour interval), you might miss the smaller changes in elevation.

Smaller the contour interval, greater will be the fieldwork to collect the elevation data. A contour interval sufficient to show features in a hilly region would be insufficient in flat regions.

The size of the contour interval directly affects how much fieldwork is needed to gather elevation data. A smaller contour interval means that there are more lines to draw on the map, and thus, more points need to be surveyed to represent the changes in elevation accurately. Conversely, in flat areas where elevations do not change much, a larger interval can be used effectively to represent the area.

Think of measuring a flat countertop with a ruler. If the ruler has small increments (small contour interval), you can see every little bump or scratch. If you use a ruler with larger increments, you would only notice major issues, missing out on the finer details.

A contour interval for an area/project depends upon several factors, such as the nature of ground, the purpose and extent of survey, the scale of mapping, and the time & funds available.

When determining the appropriate contour interval for a map, several key factors must be considered. These include: 1) The type of terrain - hilly regions require a different interval than flat areas; 2) The purpose of the survey - technical projects may need detailed maps, while others may not; 3) The map scale - larger scales may require smaller intervals; 4) Available resources like time and budget, which can limit how detailed the survey can be.

Just like when planning a family road trip, how detailed you need your map to be depends on your destination. If you’re just driving through a city, a basic map will do. However, if your destination is a national park with trails and lookout points, you’d want a detailed map with all the contours to make the most of your visit.

In a fairly flat area, a large map scale may be appropriate with small contour interval to depict the small ground variation, while in hilly areas, a small scale map with large contour interval will be adequate for representation of details.

In flat terrains, it's possible to have maps with a large scale and a small contour interval, which allows for the detailed illustration of minor elevation changes. Conversely, in hilly areas where elevation changes dramatically, a smaller scale map with larger contour intervals can efficiently illustrate the terrain without overwhelming the reader with excessive detail.

Consider a video game. In a racing game set in a flat city, you can see all the small streets and details. But in a mountain climbing game, seeing every tiny ledge isn't necessary; the big hills and valleys are what make it exciting. Similarly, maps need to balance detail for practical use.

As an example, the recommended values of the contour intervals are as follows: For building sites: 0.5 m; For reservoirs, town planning schemes: 1 to 2 m; For location surveys: 2 to 5 m; For topographical surveys: 5 m and above.

Different projects have specific recommended contour intervals based on their requirements. For instance, building sites typically require very detailed information (0.5 m interval) to ensure effective planning, while broader projects, like topographical surveys or reservoir planning, can use larger intervals (5 m and above) since a general overview of the land is sufficient.

Think about different maps you use for different activities – a map for a new building site has lots of details like utility lines and potential obstacles, while a hiking map might just indicate mountains and valleys without concern for all the little details. Each map serves its purpose based on the scale and interval used.

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