Slope diagram
The SLOPE plate computes the steepness of the ground at each point of the elevation grid and divides it into 5 colour bands.
Overview
Contours show height; slope shows steepness. TERRAIN and SLOPE are drawn from the same elevation grid. Where contours are close together the ground is steeper, but reading spacing by eye gives different answers. SLOPE divides steepness into numeric bands and fills them as colour areas.
Most of central London is flat at the scale of this plate. The slopes that do appear in Greater London are on the higher ground to the north and south (for example Hampstead, Highgate, Crystal Palace) and along river valleys.
What the plate shows
| Item | Content |
|---|---|
| Colour areas | 5 slope bands |
| Water | Light blue, drawn over the colour areas |
| Colour | Changing the accent colour changes the hue only; the lightness of each band stays |
| Note | Always states «smoothed at 50 m», with the maximum and mean slope |
The colour areas do not say which side of a slope is higher. Aspect is read from the TERRAIN plate of the same site.
How to use
- Find the steep and flat areas around the site on the colour areas
- Check on TERRAIN which way the ground rises
- Check the side from which vehicles arrive and its slope (it sets the position and length of a basement ramp)
- Locate the steep parts inside the site (they set where the building can go)
- Compare with Sun hours for the same site
Method
| Item | Content |
|---|---|
| Elevation data | Environment Agency LiDAR Composite DTM (1 m, bare earth), the same grid as TERRAIN; Copernicus DEM GLO-30 only where LiDAR has gaps |
| Height reference | Height relative to the site centre (absolute elevation is not used) |
| Slope | Derivative of the east–west and north–south height differences at each grid point |
| Smoothing | 50 m (fixed, in metres) |
| Bands | 5 · 10 · 15 · 20° (5 bands) |
Bands
The UK edition uses degrees. In percent, 5° ≈ 8.7%, 10° ≈ 17.6%, 15° ≈ 26.8% and 20° ≈ 36.4%.
| Band | Meaning for design |
|---|---|
| Under 5° | Practically flat; slope does not shape the layout |
| 5–10° | Gentle slope; handled with split levels or a raised ground floor |
| 10–15° | Sloping ground; layout and access start to follow the ground |
| 15–20° | Steep; retaining walls, cut and fill become part of the design |
| 20° or more | Very steep |
50 m smoothing
Slope is the derivative of elevation, so small noise in the elevation data is amplified. Without enough smoothing, flat ground fills with false patches. 50 m removes most false patches on flat ground and keeps real slopes; at about 100 m, real steep slopes are smoothed too. The width is fixed in metres, not in grid cells, because the grid spacing changes with the radius (6.2 m at 300 m, 16.7 m at 800 m).
Limitations
The effective resolution of this plate is 50 m. Cuttings, retaining walls and embankments narrower than 50 m do not appear, including many railway cuttings in London.
Bare earth, not surface
The Environment Agency DTM is a bare-earth model: buildings and trees have been removed, so tall buildings do not show up as hills. Bridges, viaducts, railway embankments and retaining walls can remain as steep strips; very narrow ones are filtered out. Where LiDAR has gaps (mostly over water) the plate falls back to Copernicus DEM GLO-30, a 30 m surface model. In the Bloomsbury example the relief within 800 m is 19.1 m.
Date of the elevation data
The Environment Agency LiDAR Composite combines surveys flown mostly between 2016 and 2022. Later earthworks are not included.
Not a survey
The plate is for reading landform at concept stage. Levels for design, drainage or access gradients need a topographical survey.
Data sources
- Elevation: Environment Agency LiDAR Composite DTM 1 m (© Environment Agency copyright and/or database right 2022. All rights reserved; Open Government Licence v3.0) · where LiDAR has gaps: Copernicus DEM GLO-30 (produced using Copernicus WorldDEM-30 © DLR e.V. 2010-2014 and © Airbus Defence and Space GmbH 2014-2018 provided under COPERNICUS by the European Union and ESA; all rights reserved)
Related guides
Elevation: Environment Agency LiDAR Composite DTM 1 m (© Environment Agency copyright and/or database right 2022. All rights reserved; Open Government Licence v3.0) · where LiDAR has gaps: Copernicus DEM GLO-30 (produced using Copernicus WorldDEM-30 © DLR e.V. 2010-2014 and © Airbus Defence and Space GmbH 2014-2018 provided under COPERNICUS by the European Union and ESA; all rights reserved)