The slope is how much the terrain drops every hundred meters it advances. 3% means that in one hundred meters the property drops three. It is measured in percentage and not in degrees because that is how it is read directly: 3% is three meters out of every hundred.
It is the most important piece of information that decides the most, and it decides three different things at the same time: how much land the water carries away when it rains heavily, if the large machinery can work comfortably, and if it can be irrigated by gravity or if it has to be pumped.
How to read
| Slope | Terrain | What this means |
|---|---|---|
| Less than 1% | Flat | The water does not drain by itself. The problem is not erosion but waterlogging. |
| 1 to 3% | Gentle | The most sought after condition for agriculture: it drains without eroding. |
| 3 to 8% | Rolling | Water erosion is starting to count. It is advisable to plant on contour lines. |
| 8 to 15% | Strongly rolling | Big machinery gets complicated. It is usually allocated to sown pasture. |
| More than 15% | Broken | Outside of agriculture. Extensive livestock farming or native woodland. |
A property core zone with more than 3% slope is worth less than a flat one, even if it has the same soil: the soil is lost with each heavy rain and the yield falls over the years if it is not managed.
Where does it come from?
No one publishes it already calculated. We derive it from the IGN Digital Elevation Model by comparing each point with its neighbors, which is the standard way to obtain it.
What it doesn't say
- Each data point covers 30 by 30 meters, so it is the average of that surface: a five-meter-wide gully does not appear.
- It is the slope of the land, not the direction in which it falls. Two properties with the same slope can drain to different places.
- Above 50% the data is saturated. They are slopes of more than 27 degrees, where there is no agriculture or livestock.
- The model measures the surface: on a closed native woodland it calculates the slope of the cups and not that of soil.