zwitschi
6cded45e2a
Gutters sit at -10 m south and +10..12 m north, a ~2% crown between them, and the envelope runs 0.6-1.0 m per section. Gutter spacing independently confirms the 20.9 m carriageway measured from the crossing. The sharpest breaks are 0.09-0.12 m/m, so 3DEP does not resolve a kerb upstand: kerb height stays a recorded assumption rather than a derived value.
48 lines
3.0 KiB
Markdown
48 lines
3.0 KiB
Markdown
# Cross-sections — crowned road, gutters, sidewalk shelves
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Sampled 2026-11-06 from the local 3DEP DEM (`Geo/raw/dem-3dep-1m-26911-2026-11-06.tif`) with `Geo/scripts/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.
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## What the profile shows
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Every section has the same shape:
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- a local low at about **−10 m** (south gutter),
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- a rise to a flat-ish crown around **+2 to +8 m**,
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- a second low at about **+10 to +12 m** (north gutter),
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- then the north side climbing to the building line.
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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.
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## Section table
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| Along block | Lowest point | Road crown | Cross-fall across the road | Gutter candidates |
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| ---: | ---: | ---: | ---: | --- |
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| 0.0 m | 118.78 @ −10 | ~119.41 @ 0 | ~0.6 m / 2 % | −10, +10 |
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| 40.0 m | 118.43 @ −11 | ~118.94 @ 0 | ~0.5 m / 2 % | −11, +10.5 |
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| 80.0 m | 117.84 @ −10 | ~118.33 @ 0 | ~0.5 m / 2 % | −10, +10.5–11.5 |
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| 120.0 m | 117.22 @ −10 | ~117.73 @ 0 | ~0.5 m / 2 % | −10, +11.5 |
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| 160.0 m | 116.55 @ −10 | ~117.04 @ 0 | ~0.5 m / 2 % | −10, +11.5 |
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| 200.0 m | 115.96 @ −10 | ~116.37 @ 0 | ~0.4 m / 2 % | −10, +10.5–11.5 |
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| 240.0 m | 115.63 @ −10 | ~116.02 @ 0 | ~0.4 m / 2 % | −10, +10.5–11.5 |
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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.
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## Kerbs are not in the DEM
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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.
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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.
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## What the demo ground mesh must do
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- Crown the road at roughly 2 % from each gutter to the centre, not a flat plane and not a constant tilt.
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- Step down into the gutters at about ±10.5 m from the centreline, then run the sidewalks at their own grade.
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- The north sidewalk sits higher than the road crown and keeps rising toward the building line, so the two sidewalks are not mirror images.
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- Crossing ramps must be modelled deliberately at the three crossings; nothing in the DEM supplies the local ramp geometry.
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## Open
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- Kerb height, kerb radius, and ramp slope: assumption until surveyed or measured from photographs.
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- 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.
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- Drainage inlets, driveway aprons, and pavement joints are not in this data.
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