A slope map shows how steep the ground is at every location. Each cell of the map holds the steepness of the land there, calculated from elevation data, and colours group those values into classes such as "flat", "moderate" and "steep". Planners, engineers and developers use slope maps to see at a glance where land can be built on, where grading will be expensive, and where erosion and landslides are a risk.
Slope in percent and in degrees
Slope is rise over run: how far the ground goes up for each unit you move horizontally. It can be written two ways:
- Percent slope = rise ÷ run × 100. Rising 10 m over 100 m is a 10% slope.
- Degrees = the angle of the ground from horizontal, which is the arctangent of rise ÷ run. A 10% slope is 5.7°.
The two scales are not proportional. A 100% slope is 45°, not vertical: it rises one metre for every metre across. Engineering and planning rules in the US usually use percent; geology and some software default to degrees, so always check which one a map uses.
| Percent slope | Degrees | Where you'd meet it |
|---|---|---|
| 2% | 1.1° | Typical minimum for drainage on paved surfaces |
| 5% | 2.9° | Maximum running slope for an accessible route (ADA) |
| 8.33% | 4.8° | Maximum ramp slope under the ADA (1:12) |
| 15% | 8.5° | Where many steep-slope rules begin |
| 25% | 14.0° | Often treated as very steep or undevelopable |
| 100% | 45.0° | Rises one metre per metre |
How a slope map is made
Slope is calculated from a digital elevation model (DEM), a grid where every cell stores ground height. For each cell, GIS software compares the heights of its eight neighbours to find the steepest rate of change. A widely used method (Horn's, the default in GDAL; ArcGIS uses a similar 3 × 3 window) weights the closest neighbours more heavily to reduce noise.
The result depends heavily on the elevation data:
- Resolution. A 1 m lidar DEM picks up road cuts, ditches and small banks. A 30 m DEM averages them away and tends to underestimate steep slopes.
- Ground vs surface. Use a bare-earth model. A surface model that includes trees and buildings shows the slope of rooftops and canopy, not land.
- Units. Elevation and cell size must be in the same unit. Mixing feet of height with metres of distance, or using latitude/longitude cells without conversion, gives wrong slopes.
In the US, the USGS 3D Elevation Program (3DEP) provides 1 m lidar-derived DEMs across much of the country, and 1/3 arc-second (about 10 m) data nationwide.
How to read a slope map
Start with the legend: check whether classes are in percent or degrees and where the breaks fall. Then look for patterns rather than single cells:
- Large flat areas (often 0–5%) are the easiest to build on, but check them for flooding: valley floors are flat for a reason.
- Bands of steep slope along streams and ridges usually mark land to avoid or protect.
- Scattered steep cells in flat areas are often small features such as banks and ditches, or noise in the data.
A slope map is often the first layer in a suitability analysis, where it is combined with zoning, access and other factors.
Maps that go with slope
- Aspect: which direction a slope faces. South-facing slopes in the northern hemisphere get more sun, which matters for solar and agriculture.
- Hillshade: a shaded-relief image that makes terrain easy to read.
- Contours: lines of equal elevation, familiar from survey and topographic maps.
Frequently asked questions
Is a 100% slope vertical?
No. 100% is 45°: one unit up for every unit across. A vertical cliff would be an infinite percent slope.
What resolution should I use?
For site-level work, the finest bare-earth DEM available, ideally 1 m lidar. For regional screening, 10 m is usually enough.
Can I get slope for a single parcel?
Yes. Slope can be clipped to a parcel and summarised as area in each class, which is how it is used in site plans and due diligence.
Need a slope map for a site?
We'll map the slope and total the buildable area.
Send us an address, parcel or boundary, and we'll deliver slope maps, GIS layers and area by class.