Slope Diagram — Reading Percent Grade in a Site Analysis
Contours tell you how high the ground is. A slope map tells you how steep it is. Both come from the same elevation grid, but they are read differently. Everyone knows that tightly spaced contours mean steep ground; if that is the only cue, every reader arrives at a different answer. The SLOPE panel replaces the eyeballing with numeric bands rendered as flat color.
This is an explanation of what the panel computes, why the bands sit where they do, and what the panel cannot tell you. The last part matters most — do not read a grade off this diagram and put it in a code-compliance argument.
What is computed
The panel takes the same elevation grid the TERRAIN panel uses and differentiates it: the elevation difference east–west and north–south at every grid point gives a gradient, and the gradient's magnitude is the slope. Elevation comes from AWS Terrain Tiles, which over the United States are built from USGS 3DEP. We use elevation relative to the site center only, not absolute; the reasons are on the Data sources page.
For US sites the panel reports percent grade, with breaks at 5, 10, 15 and 25 percent. Five bands result.
| Band | How to read it |
|---|---|
| under 5% | Effectively flat. This is also the ADA threshold below which a walking surface is not treated as a ramp (1:20). |
| 5–10% | Gentle. A ramp with handrails (ADA maximum 1:12, 8.33%) lives in this band. Grade can be absorbed by steps and a split level. |
| 10–15% | Sloping. Building placement and the driveway begin to be dictated by the ground. |
| 15–25% | Steep. Retaining walls, cut and fill become premises of the scheme rather than details. |
| 25% and up | Very steep. Many jurisdictions restrict or forbid development on slopes in this range (hillside ordinances). |
The Korean edition of the same panel uses degrees (5·10·15·20°), because Korean development-permit rules are written in degrees. Same map, different unit — the same reason contours are labeled in meters there and feet here.
164 ft smoothing — the effective resolution of this panel
The first version came out looking like leopard skin. Unmistakably flat ground was covered in blotches averaging five degrees. The cause is noise in the elevation data: slope is the derivative of elevation, so small noise in the heights is amplified in the gradient. The light smoothing we use for contours (1.5 grid cells) was fine for contours and hopeless for a derivative.
Rather than guess a smoothing width we measured it, on three flat sites and three hilly ones in Seoul, watching the mean slope as the width changed.
| Smoothing | Flat 1 | Flat 2 | Flat 3 | Hill | Mountain | Hillside suburb |
|---|---|---|---|---|---|---|
| 52 ft | 6.2° | 5.6° | 5.2° | 23.0° | 37.2° | 17.5° |
| 171 ft (adopted) | 2.5° | 2.5° | 2.6° | 20.2° | 33.5° | 12.6° |
| 341 ft | 1.4° | 1.2° | 1.6° | 13.4° | 23.6° | 8.5° |
Around 164 ft is the hinge. The floor noise on flat ground halves; the real slopes on hilly ground survive. Widen to 330 ft and the flat sites get cleaner still, but the mountain drops from 37° to 24° — that is erasing slope that exists. So the smoothing width is fixed at 164 ft.
One technical decision worth recording: the width is fixed in meters, not grid cells. Grid spacing changes with the analysis radius (about 20 ft at a 1,000 ft radius, 55 ft at ½ mi). Fixed in cells, the same smoothing would mean different things at different radii, and the same site would change when you only changed the radius.
Data quality — what we measured and what we did not
In Korea the same elevation source carries artificial errors in dense city centers: compared with an independent DEM at random points, downtown Seoul ran a median 33 ft high (up to 157 ft), and a non-existent 394 ft hill appeared in the middle of a flat district. Tall buildings appear to have leaked into the terrain. Hilly ground was fine.
Over the United States the tiles are derived from 3DEP, which is largely lidar-based, so we expect fewer of these artifacts. We have not measured this in US cities. Treat a sudden steep blotch in a flat downtown as a data artifact first and terrain second, the same way you would in Seoul.
This is not a survey
Hillside and grading ordinances define slope in specific ways — average slope over a parcel computed by a prescribed formula, or slope measured between contours at a set interval on a survey. This panel is a point-wise slope smoothed over 164 ft from a public DEM. Where a parcel has a steep part and a flat part, the panel shows both; an ordinance's average shows one number. The two are not interchangeable.
- What it is for — reading, at concept stage, where around the site the ground is steep and where it is flat; which frontage a driveway should come from; where cut and fill are unavoidable.
- What it is not for — "this lot averages under 15% so the hillside rule does not apply." That determination needs a survey and the jurisdiction's own method, and this panel is not evidence for it.
What is on the panel
Contours are drawn thinly over the five color bands. Without them a slope map does not tell you which way is up — a 15% slope facing south and one facing north mean opposite things for a building. If you pick an accent color, the ramp keeps its lightness and saturation and only the hue follows, so the bands stay distinct.
There is no aspect (facing direction) panel yet. "South-facing slopes only, in the accent color" would be one more plate; whether to add it is undecided.
How to read it
On a sloping site this panel is the first button of the layout. Which edge the access road touches and how steep that edge is decides where a parking ramp can go and how long it must be; where the steep ground lies decides where a building can stand at all. Make those judgements from color bands here, then confirm direction with the contours — rather than the other way round.
Overlay it with sun hours. South-facing slopes cast short shadows and north-facing ones long, and the two plates explain each other.