I am working on an openlayers map to add a vector layer with source from local geojson file in a create-react-app project, but the vector layer cannot be shown.
Here is my code:
import React,{ Component } from 'react'
import './BaseMap.scss'
import 'ol/ol.css'
import {Map, View} from 'ol'
import TileLayer from 'ol/layer/Tile'
import {fromLonLat} from 'ol/proj'
import XYZ from 'ol/source/XYZ'
import {defaults, ScaleLine } from 'ol/control'
import GeoJSON from 'ol/format/GeoJSON';
import VectorLayer from 'ol/layer/Vector';
import VectorSource from 'ol/source/Vector';
const vectorLayer = new VectorLayer({
source: new VectorSource({
url: './shanghang_lj.json',
format: new GeoJSON()
}),
style: function(feature) {
style.getText().setText(feature.get('name'));
return style;
}
});
const tian_di_tu_road_layer = new TileLayer({
title: "road_layer",
source: new XYZ({
url: "http://t4.tianditu.com/DataServer?T=vec_w&x={x}&y={y}&l={z}"
})
});
const tian_di_tu_annotation = new TileLayer({
title: "annotation",
source: new XYZ({
url: 'http://t4.tianditu.com/DataServer?T=cva_w&x={x}&y={y}&l={z}'
})
});
export class BaseMap extends Component{
constructor(props){
super(props);
};
componentDidMount(){
let map = new Map({
controls: new defaults({
attributionOptions: {
collapsible: false
}
}).extend([
new ScaleLine()
]),
target:'basemap',
layers:[
tian_di_tu_road_layer,//can be displayed
tian_di_tu_annotation,//can be displayed
vectorLayer//can not be displayed
],
view:new View({
center:fromLonLat([116.4,25.0]),
zoom:10
})
})
}
render(){
return(
<div id="basemap" className='basemap'></div>
)
}
}
And I got the error from console:
Uncaught SyntaxError: Unexpected token < in JSON at position 0
at JSON.parse (<anonymous>)
at getObject (JSONFeature.js:188)
at GeoJSON.readFeatures (JSONFeature.js:56)
at VectorSource.<anonymous> (featureloader.js:87)
Then I tried another way to pass geojson data:
import border_shanghang from './shanghang_lj.json';
const vectorLayer = new VectorLayer({
source: new VectorSource({
features: (new GeoJSON()).readFeatures(border_shanghang)
}),
});
No error appears in the console, but still no vector layer displayed.
I've been struggled with this problem for long. Any help will be appreciated.
BTW, the code works if I run it without react.
I tried implementing this with a sample geojson file and what worked for me was:
import border_shanghang from './shanghang_lj.geojson'; // I got a sample geojson from the web
const style = // ...;
const vectorLayer = new VectorLayer({
source: new VectorSource({
url: border_shanghang,
format: new GeoJSON()
}),
style: function (feature) {
style.getText().setText(feature.get('name'));
return style;
}
});
// ...
everything else the same. The only difference was to put the import straight into the url
field of the VectorSource, and rename your file from a .json
to .geojson