Guide · July 2026
How to read a contour map, what slope really tells you, and how flow paths and catchments are mapped from terrain — answering the question a buyer cannot answer on foot.
Every buyer walking a property asks some version of the same question, usually without being able to phrase it: what does this land do when it rains?
It is one of the hardest things to judge on foot. A property viewed on a dry winter morning gives almost no clue about where water collects in February, which crossing becomes impassable, which slope is quietly losing topsoil every season, or whether a proposed dam site has enough catchment above it to fill.
Contour and drainage maps answer that from the terrain itself, and they come from the same survey flight as everything else.
A contour line joins points of equal elevation. Walk along one and you stay at the same height; walk across them and you are going up or down. That single idea carries a surprising amount of information once you can read it.
Lines close together mean steep ground, and the closer they are the steeper it is. Lines far apart mean gentle or flat ground. Lines forming a V indicate a valley or watercourse, and the point of the V points upstream — which is the most useful trick for spotting drainage lines at a glance. Concentric closed loops indicate a hill, or a depression if the elevations decrease inward. And contours never cross, because a single point cannot have two elevations.
The heavier lines drawn at regular intervals are index contours, labelled with their elevation so the map can be read without counting every line.
The interval is the vertical distance between successive lines — half a metre shows fine detail on gentle ground, five metres keeps a mountainous property legible.
It should suit both the terrain and the accuracy of the survey behind it. Where fine contours genuinely matter, the answer is a better-controlled survey: flying lower, using ground control, and verifying against checkpoints.
Contours for any practical purpose are generated from the terrain model — bare earth — rather than the surface model, so they describe the ground rather than the tops of trees.
Slope is derived from the same elevation data and is often more immediately useful to a farmer than the contours themselves. It governs how fast water runs off rather than infiltrating, how much soil that runoff carries, whether machinery can work the land safely, and whether irrigation will distribute evenly.
One practical point: slope is expressed either as a percentage or in degrees, and they are not interchangeable. A 100 per cent slope is 45 degrees, not 90. South African agricultural and conservation work generally uses percentage; engineering often uses degrees. Any slope map should state its units plainly.
What the survey provides is the measurement. Interpretation for cultivation decisions belongs with an agronomist or extension officer, particularly since conservation legislation applies to certain land.
Because the elevation model records the height of every cell across the property, software can work out, cell by cell, which neighbouring cell water would move into. From that single calculation several products follow.
Flow direction gives, for every point, the direction water leaves it. Flow accumulation gives how much upslope land drains through each point — low values across open slopes, high values concentrating into lines, and those lines are the watercourses. Applying a threshold extracts the drainage network, including ephemeral channels that carry water only after heavy rain and are invisible on the ground in winter.
And for any chosen point — a proposed dam wall, a culvert, a low-lying paddock — the software can outline every square metre of land draining to it, and measure that area.
Dam siting and yield. A dam is only as good as the catchment above it. Delineating and measuring that catchment is the first step in assessing whether a site will fill, and terrain volumes give capacity for a given wall height.
Erosion risk. Erosion concentrates where flow converges on steeper ground. Overlaying flow accumulation on slope highlights exactly those places — which is where gully and donga formation begins, and where conservation works pay for themselves.
Waterlogging. Convergence zones and depressions on flat land identify where water sits after rain. These are often the paddocks that underperform for reasons nobody has satisfactorily explained.
Roads and crossings. Knowing where water crosses a track, and how much catchment feeds that crossing, informs where culverts belong and how they should be sized.
And for a buyer, drainage mapping surfaces things a seller may not mention and may not know: an erosion gully advancing on a boundary, a section that floods, a dam with an inadequate catchment.
Most of the country receives its rain in summer, frequently as short, high-intensity thunderstorms. Erosion risk here is driven more by rainfall intensity than by annual total — a property can receive modest annual rainfall and still suffer serious erosion because it arrives in a handful of violent events. That is why convergence zones on moderate slopes matter more locally than general guidance would suggest.
Gully erosion, locally called donga formation, is expensive to reverse once established. Identifying where flow concentrates before the gully forms is materially cheaper than repairing it afterwards.
One due diligence point worth raising on any farm purchase where dams carry value: the use of water, including storing it or impeding its flow in a watercourse, is regulated under the National Water Act, and constructing or enlarging a dam may require registration or a licence. A survey identifies and measures what is there; confirming authorisation is a question for the Department of Water and Sanitation or an appropriately qualified professional.
Contour and drainage mapping describes terrain and the flow paths implied by it, accurately and far faster than ground survey. It provides the terrain foundation that a hydrologist or engineer needs.
It is not itself a hydrological study — it does not estimate runoff volumes, determine flood lines or size a spillway, all of which require rainfall records, soil data and a qualified professional. It is also worth having someone who knows the site review the flow paths, because a road embankment with a culvert through it looks like a solid dam wall from the air.
Contour lines at an agreed interval as vector files for GIS or CAD, a slope map with units stated, aspect where relevant, hillshade for a legible visual, flow direction and accumulation rasters, the extracted drainage network including ephemeral channels, catchment boundaries with areas for nominated points, identified depressions and ponding areas, cross-sections along any requested line, and the terrain model it is all built on.
Want this applied to your own property, site or asset?
Book a Consult