Lot elevation, slope and contours: how the land falls on a lot

A listing photo rarely shows how much a block slopes, and a site visit on a wet day can mislead you either way. MapMyLot reads the lot's ground heights from a government elevation model and shows the contours, the highest and lowest points, and which way and how steeply the land falls.

What MapMyLot shows

Open a lot with MapMyLot Pro and the Additional Layer Data tab has an Elevation and slope line: the lowest and highest ground level in metres above sea level, the fall between them, and the average slope with the direction the land falls. It reads like this: 41.2 to 44.8 m above sea level, 3.6 m fall, 6.1% average slope falling to the north-east.

Turn on the Elevation pill in the layer row and the map draws, inside the lot boundary:

• Contour lines, each labelled with its height in metres above sea level. Every fifth line is drawn thicker so you can count them quickly.
• A red dot at the highest point on the lot and a blue dot at the lowest, each labelled with its height.
• A yellow arrow across the middle of the lot, pointing the way the land falls.

Turning Elevation on hides any other layer you had showing, so the contours stay easy to read. The same figures, with a contour map of the lot, are in the PDF report under Ground level, with a note on the grid the heights came from and the contour spacing.

Why slope matters to a buyer or builder

Two lots of the same size can cost very different amounts to build on. The fall across a lot is one of the first things a builder, designer or engineer looks at, because it affects:

• Drainage and stormwater. Water runs downhill, and roof, driveway and paving water has to go somewhere. A lot that falls away from the street can need a drainage easement through a neighbour's land or a pump to reach the street drain. Your council sets the rules.
• Cut and fill, and retaining walls. A slab needs a level pad. On a sloping lot the builder cuts into the high side, fills the low side, or both, and usually retains the edges. More fall across the house site generally means more earthworks and taller walls.
• Build cost. Builders often price site costs separately from the house, and slope drives much of them: earthworks, retaining, deeper footings, extra drainage, and sometimes a split-level or suspended floor instead of a slab.
• Driveway grade. A garage near street level is simple. A garage well above or below the street needs a driveway steep enough to make up the difference, and councils limit how steep a driveway can be.
• Overland flow. In heavy rain, water that the drains can't take runs over the ground. A lot that sits below its neighbours, or with its low point at the back, can take water from upslope. The fall direction shows which way water on the lot will move. Councils map overland flow and flooding separately, so also check whether the lot is in a mapped flood zone.

Slope is given as a percentage: metres of fall for every 100 m across. 5% is 1 in 20, 10% is 1 in 10 and 20% is 1 in 5. Words like gently sloping or steep mean different things to different builders, so compare the numbers rather than the labels.

None of this replaces a site survey or an engineer's advice. Knowing that a lot falls 4 m before you inspect it changes the questions you ask, of the agent and of the builder.

How to read the contours

• Each line joins points of the same height. Walk along one and you neither climb nor drop.
• Lines close together mean steep ground; lines far apart mean gentle ground. Lines bunched along one edge mean a bank there and flatter ground elsewhere.
• The land falls at right angles to the lines, from the higher labels to the lower ones. The yellow arrow shows the overall direction.
• The height between lines, the contour interval, changes from lot to lot. MapMyLot picks it from the lot's fall so there are roughly 4 to 15 lines: as fine as 0.1 m on a nearly flat lot, up to 5 m on a steep one. Check the labels, or the interval in the report note, before comparing two lots by eye.

To estimate the slope between two lines, divide the height between them by the distance between them on the ground. Lines 0.5 m apart in height and 10 m apart on the ground are a 5% slope. The boundary lengths on the map help you judge the distance.

The average slope comes from a flat plane fitted through every height on the lot, so it describes the lot as a whole. A lot with a steep bank at the back and a flat front can show a modest average; the contours show where the steep part is. Under 0.5% average slope the lot is treated as flat, with no fall direction and no arrow.

Where to find it

• On the map: the Elevation and slope line in the Additional Layer Data tab, and the Elevation pill in the layer row. Both are Pro features; without Pro they show a lock and what they hold.
• In the PDF report for a lot, under Ground level, with the contour map.

The first time anyone looks at a lot, its heights are fetched from the government elevation service, which can take a few seconds. After that they are stored and load straight away. If the service has no heights for a lot, the line says elevation is not available for this lot and the report leaves the section out.

Coverage and grid size

Heights come from government elevation models, mostly built from airborne LiDAR, read on a grid across the lot:

• 1 m grid: Queensland, England, Wales, New Zealand, and Canada (British Columbia, Manitoba, New Brunswick and Yukon)
• 5 m grid: New South Wales, Victoria and Tasmania

On large lots the grid is coarser, so the whole lot can be read at once. The report note states the grid used for that lot.

Not covered yet: Scotland and the ACT. Lots there show no elevation line, pill or report section.

Each country and state measures height from its own reference level, so heights are labelled above sea level. They are good for how the land falls across a lot. Don't use them as survey levels, for example to set a floor height against a flood level.

What it can't tell you

• Small features may not show. A retaining wall, house pad, kerb, swale or mound smaller than the grid can be missed or smoothed out.
• It shows the ground when the model was captured. Earthworks since then are not in it.
• It is the bare ground. These models leave out buildings and trees, so the heights are not the top of a house or a hedge.
• It is a guide to how the land falls, not a site survey. Before you design, price earthworks or set levels, get a contour and detail survey from a registered surveyor.

For where the lot's legal boundary sits, see how to find property boundaries.