Dev: Lambert 93 / Latitude et Longitude

Parfois les données sur la plateforme ouverte des données publiques françaises, quand il s'agit des coordonnées géographiques, sont publiées dans le système de coordonnées Lambert 93. Pour utiliser ces données sur une carte comme Leaflet il faut convertir les coordonnées en Latitude et Longitude ou le système WGS84 - World Geodetic System 1984, utilisé pas le GPS. Pour cela le plus simple c'est d'utiliser PROJ ou un de ports existants comme https://github.com/proj4js/proj4js en JavaScript. Si les fichiers sont fournis avec le ficher prj, le mieux c'est d'utiliser cette chaine de caractères qui défini la projection en tant d'un paramètre d'entrée et le système WGS84 comme paramètre de sortie:

var fromProjection = 'PROJCS["RGF_1993_CC49",GEOGCS["GCS_RGF_1993",DATUM["D_RGF_1993",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Lambert_Conformal_Conic"],PARAMETER["False_Easting",1700000.0],PARAMETER["False_Northing",8200000.0],PARAMETER["Central_Meridian",3.0],PARAMETER["Standard_Parallel_1",48.25],PARAMETER["Standard_Parallel_2",49.75],PARAMETER["Latitude_Of_Origin",49.0],UNIT["Meter",1.0]]'
var toProjection = 'EPSG:4326' // WGS84

var lambert93Coordinates = [8103377.610158493, 4474621.729721212]
var wgs84Coordinates = proj4(fromProjection, toProjection, lambert93Coordinates) // [48.8534935, 2.3476778]

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