Saturday, 31 March 2018

leaflet geojson coordinate problem


I just started to play with leflet/geojson a little. But my coordinates are not rendered properly and I have no clue what is going on.


My coordinates are: 52.23943, 4.97599. They work correct with the setView function.



var map = L.map('leaflet_map').setView([52.23943, 4.97599], 15);

But using a geojasonFeature they are, hmmm, 'projected', somewhere east of Somalia.


var geojsonFeature = {
"type": "Feature",
"properties": {
"name": "Coors Field",
"amenity": "Baseball Stadium",
"popupContent": "This is where the Rockies play!"
},

"geometry": {
"type": "Point",
"coordinates": [52.23943, 4.97599]
}
};
var myLayer = L.geoJson().addTo(map);
myLayer.addData(geojsonFeature).bindPopup("I am a gjson point.");

Anyone who knows what is happening here?


EDIT



Out of pure curiosity I changed the coordinates around [4.976143930893815,52.23925499011473] and the point appears at its correct location. A known bug!?



Answer



I wouldn't call it a bug, just a matter of confusing and contradictory standards.


When talking about geographic locations, we usually use Lat-long. This has been codified in the ISO 6709 standard.


When dealing with Cartesian coordinate geometry, we generally use X-Y. Many GIS systems, work with a Geographic Location as a special case of a 2 D coordinate point, where the X represents the longitude and Y represents the Latitude. This order of coordinates, is exactly opposite that of the regular Lat-long notion.


Coming to your problem:


The map.setView takes a l.LatLong as an input, where the first cordinate is a Latitude, and the second is Longitude.


Hence when you want 52.23N, 4.97E, you pass in [52.23943, 4.97599]


The GeoJSON standard says that for any point, the first parameter is the X Coordinate (i.e. Longitude) and second parameter is the Y coordinate (i.e. Latitude);


Hence when you want 52.23N, 4.97E in GeoJSON, you need to pass [4.97599, 52.23943]



For further reading, go through this Q&A


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