# Cross-sections — crowned road, gutters, sidewalk shelves

Sampled 2026-11-06 from the local 3DEP DEM (`Geo/raw/dem-3dep-1m-26911-2026-11-06.tif`) with `Geo/tools/street_section.py`. Seven sections at 40 m spacing, offsets −18 m to +18 m from the boulevard centreline (negative = south), 1 m sampling step.

## What the profile shows

Every section has the same shape:

- a local low at about **−10 m** (south gutter),
- a rise to a flat-ish crown around **+2 to +8 m**,
- a second low at about **+10 to +12 m** (north gutter),
- then the north side climbing to the building line.

Those gutter positions bracket a carriageway of roughly **20–22 m**, which independently matches the 20.9 m measured from the mid-block crossing in [`2026-11-06-street-profile.md`](2026-11-06-street-profile.md). Two unrelated measurements agreeing is the strongest evidence in the geo work so far.

## Section table

| Along block | Lowest point | Road crown | Cross-fall across the road | Gutter candidates |
| ---: | ---: | ---: | ---: | --- |
| 0.0 m | 118.78 @ −10 | ~119.41 @ 0 | ~0.6 m / 2 % | −10, +10 |
| 40.0 m | 118.43 @ −11 | ~118.94 @ 0 | ~0.5 m / 2 % | −11, +10.5 |
| 80.0 m | 117.84 @ −10 | ~118.33 @ 0 | ~0.5 m / 2 % | −10, +10.5–11.5 |
| 120.0 m | 117.22 @ −10 | ~117.73 @ 0 | ~0.5 m / 2 % | −10, +11.5 |
| 160.0 m | 116.55 @ −10 | ~117.04 @ 0 | ~0.5 m / 2 % | −10, +11.5 |
| 200.0 m | 115.96 @ −10 | ~116.37 @ 0 | ~0.4 m / 2 % | −10, +10.5–11.5 |
| 240.0 m | 115.63 @ −10 | ~116.02 @ 0 | ~0.4 m / 2 % | −10, +10.5–11.5 |

Total envelope from the lowest south gutter to the highest north point is 0.6–1.0 m per section, depending on how far the north sidewalk has risen.

## Kerbs are not in the DEM

The sharpest breaks found at the expected kerb offsets are **0.09–0.12 m/m** slopes — that is a gutter transition spread over a metre, not a kerb upstand. 3DEP is bare-earth and smooths a 0.15 m vertical step at 1 m sampling, so the DEM cannot give kerb height.

Consequence: kerb height and shape are an **assumption** until measured or observed in the reference photography. Write the assumption down where it is used rather than burying it in the mesh. Do not infer a kerb from the DEM's gradient.

## What the demo ground mesh must do

- Crown the road at roughly 2 % from each gutter to the centre, not a flat plane and not a constant tilt.
- Step down into the gutters at about ±10.5 m from the centreline, then run the sidewalks at their own grade.
- The north sidewalk sits higher than the road crown and keeps rising toward the building line, so the two sidewalks are not mirror images.
- Crossing ramps must be modelled deliberately at the three crossings; nothing in the DEM supplies the local ramp geometry.

## Reproduced in the terrain mesh

`Art/blender/terrain/terrain-2026-11-06.blend` samples the DEM on a 2 m grid and reproduces these numbers: origin Z 0.00, Orange Drive end +3.39, south gutter +0.93, road centre +1.38, north sidewalk +1.43 relative to the origin ground. The crown therefore survives from DEM to mesh. See `Art/blender/README.md`.

## Reproduced in the street mesh

`Art/blender/street/street-2026-11-06.blend` carries the same numbers through to geometry: rule at mid-block reads +0.99 at the south gutter, +1.38 at −5 m, +1.52 at the centre, +1.58 at +5 m, +1.52 at the north gutter. Kerb faces measure 0.13 m and flush to zero at the crossings.

## Open

- Kerb height, kerb radius, and ramp slope: assumption until surveyed or measured from photographs.
- The 200 m section peaks at +14 m rather than +18 m, which may be a tree canopy or a building edge in the DEM. Do not treat it as a road feature.
- Drainage inlets, driveway aprons, and pavement joints are not in this data.