Guelph Water Layer

Turns the City of Guelph's Stormwater Management Pond layer (93 basins kept of 152 rows) and its Watercourse Polygons (71 creek corridors, 13 named) into map-tile layers that OpenStreetMap mappers can add to the JOSM and iD editors as a reference overlay, plus a weekly page of what OSM is missing.

Live layer and how to add it: https://guelph-maps.github.io/guelph-water-layer/

Part of the guelph-maps organisation, which indexes every Guelph project. It was cloned from guelph-parks-layer: the same download -> slim -> compare -> vector -> raster -> site -> publish pipeline, pointed at two layers instead of one.

The audit behind it is guelph-osm-import-audit (private), findings/recalibration.md, section Water, which tiered the basins 4 / conflate and folded the creek names in as one status rather than a project of their own.

What it produces

• Vector tiles (MVT), z10–19, in two layers: basins (click one in iD to read kind, type, area, fenced, installyear, ownedby, locdesc, plan, objectid, swmpondid) and watercourses (name where the City has one).

• Raster tiles (PNG), z13–17: a wet pond is solid translucent blue; a dry or infiltration basin is paler blue under diagonal hatching — a grass hollow that holds water only after a storm; a constructed wetland is teal. Creek corridors are a faint grey-blue wash, labelled with the City's creek name. Basins carry no label (see What is kept).

• A landing page with copy-paste "add this layer" instructions for both editors, and a box on which tag suits which kind of basin.

• A gap-review page (/gaps/) with an interactive map and a filterable table, in five statuses:

  • missing — no OSM water, wetland or basin area matches the basin: draw it;
  • plain water (unclassified) — OSM has it as a bare natural=water, water=pond, water=reservoir or a natural wetland: retag it;
  • creek unnamed (creek_unnamed) — a creek the City names whose OSM waterway lines carry no name: add name=;
  • wetland — a City constructed wetland that OSM has something on. Kept apart: how a stormwater wetland should be tagged is not settled, so nothing is suggested. Off by default;
  • tagged as basin (classified) — done, in either scheme. Off by default.

  Each row links to OSM, iD and JOSM; the JOSM link needs JOSM running with Preferences → Remote Control enabled.

  Its history page charts missing, plain water and creeks unnamed, one point per weekly run.

All of it is published to GitHub Pages and rebuilt weekly.

What is kept

Basins. The 152 rows include things that are not basins. Dropped (DROP_TYPES, MIN_AREA_M2 in src/config.py): 37 Greenway swales (grassed ditches along lot lines), one buried Rock Trench Storage, 7 rows with no TYPE, and 14 basins under 500 m² — infiltration galleries too small to see on imagery. 93 remain. Each gets a kind from its TYPE: Wet and Wet with Infiltration Basin → wet; Dry, Dry - Online, Dry with Infiltration → dry; Infiltration, *Infiltration

• Online* → infiltration; Wetland → wetland. A TYPE the City adds later stops the slim step until someone decides which it is.

The City's NAME is not a name. It is the subdivision plan the basin was built under (Plan 680 - Hanlon Industrial Park - Phase 2). It is kept, as plan, because it helps find the basin in City documents — but never as name, never as a tile label, and the gap page never suggests it.

Creeks are the opposite case: COMMON_NAME on a watercourse polygon is a real name (Torrance Creek), and it is exactly what the gap page suggests. Every watercourse polygon is drawn; the 13 named ones (12 names — Guelph Lake is two polygons) are compared.

How the gap page matches

One Overpass query pulls every natural=water|wetland, landuse=basin|reservoir and basin=* area and every waterway=river|stream|canal|ditch|drain line in the Guelph bbox. Geometry is compared in UTM 17N with shapely.

Basins. An OSM area matches a City basin when it overlaps at least 10% of the smaller of the two, or either one's representative point lies inside the other. Measured on 2026-10-09: 10%, 30% and 50% all give the same 33 missing, so the rule is not on a knife edge. Two things are left out on purpose:

• water=river is never a candidate (nor the old waterway=riverbank). Creek-side basins touch the river multipolygon; with any-intersection that alone turns 33 missing into 17.
• Both basin schemes count as classified. Guelph's mapped wet ponds use natural=water + water=basin; the usual tag for a dry basin is landuse=basin + basin=detention|infiltration. A test that only looked for water would report a correctly mapped dry basin as missing.

When several OSM areas match, a classified one wins, then the biggest overlap.

City kind	Suggested tags
wet	natural=water + water=basin (+ basin=retention)
dry	landuse=basin + basin=detention
infiltration	landuse=basin + basin=infiltration
wetland	not settled — leave what is there

This is the beginning of the README. Read the rest at the original. Read the full README ↗