reproject-geojson.html

<script src="https://cdnjs.cloudflare.com/ajax/libs/proj4js/2.15.0/proj4.min.js" integrity="sha512-4cx6CH/aJi50kdbQ5dj6bDJM1WS0AOFBGgBfgSP2Ux2eK/5JhkL2pjUDIHwd8z6rfGuGr/aptDqs0EzbeRndvQ==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>

<script>

proj4.defs("EPSG:2154","+proj=lcc +lat_0=46.5 +lon_0=3 +lat_1=49 +lat_2=44 +x_0=700000 +y_0=6600000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs +type=crs");

function geometryPoint(entry, transformer) {
  return transformer(entry)
}

function geometryLineString(entry, transformer) {
  return entry.map(transformer)
}

function geometryPolygon(entry, transformer) {
  return entry.map(point => point.map(coordPair => transformer(coordPair)))
}

geometryMultiPoint = geometryLineString
geometryMultiLineString = geometryPolygon

function geometryMultiPolygon(entry, transformer) {
  return entry.map(part => part.map(point => point.map(coordPair => transformer(coordPair))))
}

const convert_info = {
  'Point': geometryPoint,
  'LineString': geometryLineString,
  'Polygon': geometryPolygon,
  'MultiPoint': geometryMultiPoint,
  'MultiLineString': geometryMultiLineString,
  'MultiPolygon': geometryMultiPolygon
}


let url = 'https://static.data.gouv.fr/resources/quartiers-prioritaires-de-la-politique-de-la-ville-qpv/20240822-094641/qp-2024-epsg2154-20240820.geojson'
// url = 'https://static.data.gouv.fr/resources/base-nationale-consolidee-des-zones-a-faibles-emissions/20241216-141149/aires.geojson'

fetch(url).then(res=>res.json()).then(geojson => {
  const crs_urn = geojson.crs?.properties?.name
  if (crs_urn) {
    console.log('reproject !', crs_urn)
    const transformer = proj4(crs_urn.replace('urn:ogc:def:crs:', '').replace('::', ':'), "EPSG:4326").forward
    console.log(transformer([355608.74, 6690851.32]));
    geojson_output = {
      "type": "FeatureCollection",
      "features": geojson.features.map(feature => ({
        'type': feature.type,
        'properties': feature.properties,
        'geometry': {
          'type': feature.geometry.type,
          'coordinates': convert_info[feature.geometry.type](feature.geometry.coordinates, transformer)
        }
      }))
    }
    console.log(JSON.stringify(geojson_output))
  } else {
    console.log('keep it!')
  }
})

const transformerOther = proj4("EPSG:4326", "EPSG:3857").forward

const point = {
  "type": "Point",
  "coordinates": [0.4, 45]
}
const line = {
  "type": "LineString",
  "coordinates": [
    [10.0, 0.0],
    [11.0, 1.0]
  ]
}
const poly_simple = {
  "type": "Polygon",
  "coordinates": [
    [
      [10.0, 0.0],
      [11.0, 0.0],
      [11.0, 1.0],
      [10.0, 1.0],
      [10.0, 0.0]
    ]
  ]
}

const poly_hole = {
  "type": "Polygon",
  "coordinates": [
    [
      [10.0, 0.0],
      [11.0, 0.0],
      [11.0, 1.0],
      [10.0, 1.0],
      [10.0, 0.0]
    ],
    [
      [10.8, 0.8],
      [10.8, 0.2],
      [10.2, 0.2],
      [10.2, 0.8],
      [10.8, 0.8]
    ]
  ]
}

const multipoint = {
  "type": "MultiPoint",
  "coordinates": [
    [10.0, 0.0],
    [11.0, 1.0]
  ]
}

const multiline = {
  "type": "MultiLineString",
  "coordinates": [
    [
      [10.0, 0.0],
      [11.0, 1.0]
    ],
    [
      [12.0, 2.0],
      [13.0, 3.0]
    ]
  ]
}

const multipoly = {
  "type": "MultiPolygon",
  "coordinates": [
    [
      [
        [12.0, 2.0],
        [13.0, 2.0],
        [13.0, 3.0],
        [12.0, 3.0],
        [12.0, 2.0]
      ]
    ],
    [
      [
        [10.0, 0.0],
        [11.0, 0.0],
        [11.0, 1.0],
        [10.0, 1.0],
        [10.0, 0.0]
      ],
      [
        [10.2, 0.2],
        [10.2, 0.8],
        [10.8, 0.8],
        [10.8, 0.2],
        [10.2, 0.2]
      ]
    ]
  ]
}

all_samples = [
  point,
  line,
  poly_simple,
  poly_hole,
  multipoint,
  multiline,
  multipoly
]

all_samples.map(sample => {
  console.log(convert_info[sample.type](sample.coordinates, transformerOther))
})

</script>