Source and limitations

This tool has no data of its own. Base map from OpenFreeMap, flood potential from the Water Resources Agency, township boundaries from the Ministry of the Interior; this site only grids, aggregates and displays.

Flood potential maps

• Source: Water Resources Agency "Flood potential maps" (open-data platform dataset 25766, Open Government Data License v1). This site uses the national, per-scenario shapefiles built on 12 August 2022 (served by the WRA GIS platform): 10 scenarios — 6 h 150/250/350 mm, 12 h 200/300/400 mm, 24 h 200/350/500/650 mm.
• TWD97 TM2 coordinates; fields include depth class and township code. 1,204–2,356 polygons per scenario, up to 3.75 million vertices.
• The WRA's description: simulated flooding under design rainfall scenarios, given hydrological and terrain conditions and a hydraulic model, assuming flood-control works operate normally; "results carry uncertainty"; valid for five years.
• Regulation: Flood Potential Data Disclosure Regulations, art. 8 — data are for disaster prevention only; land-use control or restrictions follow each competent authority's own law; art. 4 — must note that the maps are drawn from design rainfall, specific terrain data and hydraulic computation and should be used with care; art. 9 — reviewed every five years.
• Production conditions (from the WRA's own map sheets): SOBEK model; Horner design hyetograph; average July–October spring-tide levels; DEM from 2010–2012; land-use survey 2008; major hydraulic structures completed before June 2016; all flood-control and drainage works assumed intact and operated by rule; urban buildings not considered as obstructions.

Township boundaries

Ministry of the Interior, National Land Surveying and Mapping Center, "Township boundaries (TWD97 lon/lat)", March 2025 edition, 368 townships (including Dongsha and Nansha). Used for township area denominators and to identify the township of a tapped point.

What this site does

• Each scenario's polygons are rasterised onto an equirectangular grid of 0.0004° longitude by 0.00036° latitude (about 41×40 m in Taiwan), the same order as the original 40 m grid. Each cell stores the depth class for all ten scenarios; only cells flooded in at least one scenario are kept — 1,276,400 cells.
• Zoomed out, 8×8 and 64×64 merged cells carry the maximum class, so the national view looks slightly "fatter" than reality; full resolution from zoom 11.
• Township and flooded areas are cell sums. Measured against the original polygons, the national cell sums exceed polygon areas by 0.3–0.5% in every scenario (max 0.53%; boundary cells count whole), and the relative error grows for small townships. Township assignment uses the MOI boundaries rather than the township field in the shapefile so that numerator and denominator match.
• Not done: depths under 0.3 m (absent from the source), historical floods, live water levels, address geocoding (no free, licensed nationwide service; search by road or township instead).

Version

Potential maps 2022-08-12; township boundaries 2025-03-18; built 2026-08-21. Rebuilt when the WRA releases a new edition.

Back to the map · What a potential map is · FAQ

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