Choosing a contour interval is a genuine engineering decision. Too large and the map misses the shape of the ground. Too small and the contours crowd together illegibly — and the survey costs far more than it needed to.
Five factors decide the answer, and they pull in different directions.
What Contour Interval Means
The vertical distance between consecutive contours is termed the contour interval.
Note the word vertical. The contour interval is a difference in height, not a distance measured across the map. The horizontal spacing between contours on the paper varies with the slope — that is precisely what makes contours informative — but the vertical interval between them stays fixed.
The Usual Range
Generally contour intervals are taken as 1 to 15 m.
Constant or Variable?
- It is desirable to have a constant contour interval throughout the map.
- In special cases, a variable contour interval may also be provided.
- A variable contour interval is, as far as possible, avoided.
Why Variable Intervals Are Avoided
A variable interval gives a false impression of the relative steepness of the ground in different parts of the map.
This is the crucial reason, and it is worth working through.
The way anyone reads a contour map is by spacing — contours close together mean steep ground, contours far apart mean gentle ground. That reading works only if every contour represents the same rise in height.
Suppose one part of the map used a 2 m interval and another used 10 m. A steep slope drawn at 10 m intervals would show contours widely spaced, while gentle ground drawn at 2 m would show them crowded. The map would suggest exactly the opposite of the truth.
There is a second, quieter consequence. Volume calculations — earthwork quantities and reservoir capacities — work by multiplying contour areas by the interval. A varying interval makes that computation far more error-prone.
The Effect of a Smaller Interval
The smaller the contour interval, the more precisely the terrain relief is depicted on the plan.
A smaller interval means more contours, and more contours mean more detail. A hollow 3 m deep will be missed entirely by a 5 m interval but shown clearly by a 1 m one.
So accuracy always favours a small interval. What opposes it is legibility and cost, which is where the five factors come in.
The Five Factors
- Scale of the map
- Purpose of the map
- Nature of the country
- Time
- Funds
1. Scale of the Map
If the scale is small, the contour interval is kept large, so that there is no overcrowding of the contours. If the scale of the map is large, the contour interval can be kept small.
The Inverse Relationship
Recall from the scale topic that a small scale means a large area squeezed onto the sheet, while a large scale means a small area drawn big.
On a small-scale map, a given hillside occupies very little paper. Draw many contours across it and they merge into an unreadable band of ink. Enlarging the interval thins them out.
On a large-scale map the same hillside spreads across much more paper, so there is room for contours at close vertical intervals without crowding.
Small scale → large interval. Large scale → small interval. The two run oppositely.
2. Purpose of the Map
- The interval should be small enough that the map serves its intended purpose.
- But it should not be too small, otherwise the cost of the work would be too much.
- The interval should be kept small when the plan is required for detailed design.
Matching Detail to Need
This factor is explicitly a balance — small enough to be useful, not so small as to be wasteful.
A reconnaissance map used to choose between two broad routes needs only the general shape of the country. A plan used to design the road — setting out gradients, computing cut and fill — needs to show every metre of rise. Hence the rule that detailed design demands a small interval.
The cost warning matters because effort rises sharply as the interval shrinks. Halving the interval roughly doubles the number of contours, and the fieldwork behind them.
3. Nature of the Ground
| Ground | Contour Interval |
|---|---|
| Flat | Small |
| Steep slope | Large |
| Broken | Large, so that contours do not come too close to each other |
Why Flat Ground Needs a Smaller Interval
This is the factor students most often get backwards, so it is worth reasoning through.
On flat ground, elevations change very little across a wide area. With a large interval — say 10 m — an entire field might contain no contour at all, and the map would say nothing about its shape. A small interval is needed to reveal the gentle undulations that do exist.
On a steep slope, the ground rises rapidly over a short horizontal distance. A small interval would pack contours almost on top of one another, producing an illegible mass of lines. A large interval spaces them sensibly.
The steeper the ground, the larger the interval — precisely so the contours do not crowd together.
Note that this is the same legibility argument as the scale factor. Both are really asking: how much paper is available per metre of rise?
4. Time
The contour interval is kept large when time is less.
5. Funds
The contour interval is kept large when funds are less.
The Two Practical Constraints
These last two factors are of a different character from the first three. Scale, purpose and nature of ground are technical considerations about what the map needs to show. Time and funds are practical limits on what can be achieved.
Both point the same way, and for the same reason — a smaller interval means more contours, which means more field observations, which takes longer and costs more. When either resource is short, the interval must grow.
It is worth noting the honesty of including them. The ideal contour interval and the affordable one are not always the same, and a surveyor has to work within real constraints.
All Five Factors Summarised
| Factor | Interval Kept Large When | Interval Kept Small When |
|---|---|---|
| Scale of map | Scale is small | Scale is large |
| Purpose | General or reconnaissance use | Detailed design |
| Nature of ground | Steep or broken | Flat |
| Time | Time is less | Time is available |
| Funds | Funds are less | Funds are available |
The Two Competing Pressures
Read the table and every factor resolves into one of two arguments:
- Legibility and economy push the interval up — small scale, steep ground, little time, little money.
- Detail and accuracy push it down — large scale, flat ground, design work.
The chosen interval is wherever those two pressures balance for the job in hand. This is why no single “correct” interval exists, and why the range is as wide as 1 to 15 m.
Quick Revision Notes
- Contour interval = the vertical distance between consecutive contours.
- Generally 1 to 15 m.
- A constant interval is desirable; a variable interval is avoided because it gives a false impression of relative steepness.
- The smaller the interval, the more precisely the relief is depicted.
- Five factors: scale of map, purpose of map, nature of the country, time, funds.
- Small scale → large interval, to avoid overcrowding. Large scale → small interval.
- Interval should be small for detailed design, but not so small that cost becomes excessive.
- Flat ground → small interval. Steep or broken ground → large interval.
- Interval is kept large when time is less and large when funds are less.
Mistakes Students Commonly Make
- Reversing the nature of ground rule. Flat ground needs a small interval; steep ground needs a large one.
- Reversing the scale rule. Small scale takes a large interval, because there is little paper per unit of ground.
- Thinking the contour interval is a horizontal distance. It is vertical.
- Saying a variable interval is preferred for varied terrain. It is avoided, because it misrepresents relative steepness.
- Listing only three factors and omitting time and funds.
- Assuming the smallest possible interval is always best. It is limited by legibility, time and cost.
Conclusion
The contour interval is the vertical rise between consecutive contours, usually between 1 and 15 m, and it should stay constant across a map so that spacing reliably indicates steepness. Choosing it means balancing two opposing pressures. Detail argues for a small interval, especially on flat ground, at large scale, and where the plan will be used for design. Legibility and economy argue for a large one, on steep or broken ground, at small scale, and whenever time or money is short.
Frequently Asked Questions
What is contour interval?
The vertical distance between consecutive contours.
What is the usual range of contour intervals?
Generally 1 to 15 m.
Why is a constant contour interval preferred?
Because a variable interval gives a false impression of the relative steepness of the ground in different parts of the map, since readers judge steepness by how closely spaced the contours appear.
What are the five factors affecting contour interval?
The scale of the map, the purpose of the map, the nature of the country, time, and funds.
How does the scale of the map affect the interval?
If the scale is small the interval is kept large so the contours do not overcrowd, while if the scale is large the interval can be kept small.
Should flat ground have a large or small contour interval?
Small. On flat ground elevations change little, so a large interval might produce almost no contours and reveal nothing about the shape of the land.
Why is a large interval used on steep ground?
Because the ground rises rapidly over a short horizontal distance, so a small interval would pack the contours too close together to read.
When should the interval be kept small for the purpose of the map?
When the plan is required for detailed design, though it should not be so small that the cost of the work becomes excessive.
How do time and funds affect the contour interval?
The interval is kept large when either is limited, because a smaller interval requires more contours and therefore more fieldwork, taking more time and costing more.
Does a smaller contour interval improve the map?
Yes in terms of accuracy, since the smaller the interval the more precisely the terrain relief is depicted. However it is limited by legibility, time and cost.