Unit IIISurveying, Levelling & Site Planning

Contouring

Reading the shape of the land from lines of equal height.

≈ 35 min + worked example

A contour map is the architect's X-ray of the land. Each contour joins points of equal height, so the pattern of the lines tells you, at a glance, where the ground is steep or gentle, where the hills, valleys and drainage lines run, and where it is sensible to build. Learn to read them — and to turn their spacing into a gradient.

Learning objectives

By the end of this lesson, you will be able to — mapped to the course outcomes for Surveying, Levelling & Site Planning:

CO3 · Understand

Define a contour, the contour interval and the horizontal equivalent.

CO3 · Analyse

Read the characteristics of contours to identify hills, valleys and slope.

CO3 · Understand

Distinguish the direct and indirect methods of contouring.

CO6 · Apply

Compute the gradient between two contours as 1 in N and a percentage.

The shape of the land

Reading contours

With the contour interval held constant, the spacing of the lines reads as slope; their pattern reveals hills, valleys and ridges. Close together is steep, far apart gentle; across a valley the V points uphill.^{[1, 2]}

DiagramA contour map reading a hill, a valley where the V points uphill, and a ridge where it points downhill

DiagramA section showing close contours for a steep slope and widely spaced contours for a gentle slope

A line of equal height

A contour joins points of equal reduced level. The contour interval — the vertical step between contours — is kept constant on a map, so the horizontal spacing reads directly as slope. The horizontal equivalent (the spacing) varies; the interval does not.^{[1, 2]}

PhotoA contour map of undulating terrain, hills and valleys read from the line pattern.Kitwalkerr · CC BY-SA 3.0 · via Wikimedia Commons

PhotoContour lines wrapping a landform, closer where the ground is steeper.U.S. War Department · Public domain · via Wikimedia Commons

Plotting & slope

Methods and gradient

Contours are plotted directly (tracing real points) or, usually, indirectly (grid spot levels then interpolation). The spacing converts to a gradient — 1 in N, or a percentage.^[1]

DiagramA grid of spot levels with contours interpolated between them

Tracing the contour

In the direct method the contour itself is found on the ground — points actually at a target RL are located and surveyed, then joined. Accurate but slow; used for small important areas like dam and reservoir sites.^{[1, 2]}

Live calculator

Find the gradient

A 5 m fall over 100 m is a gradient of 1 in 20, or 5% — a gentle, buildable slope. Change the levels and distance and watch the gradient and its site-planning verdict.

Contour gradient · slope between two points

Reduced level — point 1105 mReduced level — point 2100 mHorizontal distance100 m

Gradient 1 in N = distance ÷ rise; slope% = rise ÷ distance × 100.

1 in 20.0

Gradient

0.0 %

Slope

0.0 m

Height difference

0.0°

Slope angle

Gentle — ideal for building

Bands vary by reference; a 5 m fall over 100 m = 1 in 20 = 5%.

At a glance

Contour quick reference

Aspect	One	The other
Contour spacing	Close: steep slope	Far apart: gentle slope
Obtained by	Direct: tracing real points	Indirect: interpolating spot levels
Accuracy / speed	Direct: accurate, slow	Indirect: faster, usual method
Valley vs ridge V	Valley: V points uphill	Ridge: V points downhill
Gradient form	1 in N (horizontal ÷ vertical)	Percent (vertical ÷ horizontal × 100)

Vocabulary

Key terms

Contour

A line joining points of equal elevation (reduced level).

Contour interval

The constant vertical distance between successive contours.

Horizontal equivalent

The horizontal distance between contours — varies with slope.

Gradient / slope

Rise over horizontal distance, as 1 in N or a percentage.

Spot level

The reduced level of a single surveyed point used for interpolation.

Interpolation

Estimating where whole-number contours fall between spot levels.

Direct method

Tracing contour points of a target RL on the ground — accurate, slow.

Indirect method

Taking grid spot levels then interpolating contours — fast, usual.

Apply it

Worked example

Two points read 105 m and 100 m, 100 m apart: the fall is 5 m, so the gradient is N = 100 ÷ 5 = 1 in 20, and the slope is 5 ÷ 100 × 100 = 5% — gentle, ideal for building. Confirm in the calculator, then test a steeper case.

Check your understanding

Self-assessment

1. Contours drawn very close together indicate —

2. Where a contour crosses a valley, the V points —

3. A 5 m fall over a horizontal distance of 100 m is a gradient of —

In a nutshell

Recap

A contour joins points of equal height; the contour interval is constant, so spacing reads as slope.

Read the characteristics: close = steep, contours never cross, valley-V points uphill, ridge-V downhill.

Contour by the direct method (trace real points, accurate) or the indirect method (spot levels + interpolation, usual).

Gradient is rise over run — 1 in N or a percentage; 5 m over 100 m is 1 in 20, or 5%.

The evidence

References & further reading

[1]B.C. Punmia, A.K. Jain & A.K. Jain, Surveying Vol. I. Laxmi Publications.
[2]S.K. Duggal, Surveying Vol. I. McGraw-Hill Education.
[3]T.P. Kanetkar & S.V. Kulkarni, Surveying and Levelling. PVG Prakashan.