thai_provinces_geo

Offline GPS reverse-geocoding for Thailand — turn a latitude/longitude into its ตำบล → อำเภอ → จังหวัด + รหัสไปรษณีย์ (subdistrict → district → province + postcode). Pure Dart, no network and no GPS plugin (you bring the coordinate), built on the thai_provinces data core.

Install

dart pub add thai_provinces_geo

Pulls in exactly one thing: the thai_provinces data core (which is re-exported, so Subdistrict/District/Province are available without a second import).

Usage

import 'package:thai_provinces_geo/thai_provinces_geo.dart';

// Bring a coordinate from any source (a GPS plugin, a map tap, a stored point).
final sub = reverseGeocode(13.7512, 100.4923); // Grand Palace, Bangkok
print('${sub?.nameTh}, ${sub?.district?.nameTh}, ${sub?.province?.nameTh} '
    '${sub?.postcode}');
// พระบรมมหาราชวัง, เขตพระนคร, กรุงเทพมหานคร 10200

// Need the distance too, or an optional cutoff?
final match = nearestSubdistrict(7.8780, 98.3980, maxKm: 50);
print('${match?.province?.nameEn} (${match?.distanceKm.toStringAsFixed(1)} km)');
// Phuket (1.2 km)

• reverseGeocode(lat, lng, {maxKm}) → the nearest Subdistrict? (or null when maxKm is given and the nearest reference point is farther).
• nearestSubdistrict(lat, lng, {maxKm}) → a GeoMatch? with the subdistrict, the great-circle distanceKm, and district / province / postcode getters.

Pairs naturally with thai_provinces_flutter: prefill an address picker from a coordinate via controller.setFromCodes(subdistrictCode: reverseGeocode(lat, lng)?.code).

Accuracy

This is a nearest-reference-point lookup: each of the 7,452 subdistricts has one representative point, and a query resolves to the subdistrict whose point is closest. That is accurate to the subdistrict area and ideal for prefilling a form (the user confirms), but it can resolve to a neighbour right at a subdistrict border. True point-in-polygon would need a much larger boundary dataset; it is out of scope here.

Data source & license

This package is MIT licensed.

The subdistrict reference points are derived from the Department of Provincial Administration (กรมการปกครอง, DOPA) subdistrict coordinate dataset — government administrative data; the factual coordinates and geocodes are not subject to copyright. Gaps are filled from kongvut/thai-province-data (MIT). Credit DOPA when redistributing the coordinate data.

The embedded table (lib/src/centroids.g.dart) is regenerated with tool/gen_centroids.py.