为什么我不能在极坐标立体图中添加刻度？ （OpenLayers 4）

Question

I'm working on a map in OpenLayers 4 with projection EPSG:32661

This is the JavaScript, which targets a #map div on the page:

proj4.defs('EPSG:32661', '+proj=stere +lat_0=90 +lon_0=0 +k=1 +x_0=0 +y_0=0 
+a=3396190 +b=3376200 +units=m +no_defs');
var north_stereo = ol.proj.get('EPSG:32661');
north_stereo.setExtent([-2357032, -2357032, 2357032, 2357032]);
var npoleURL = "https://planetarymaps.usgs.gov/cgi-bin/mapserv? 
map=/maps/mars/mars_npole.map";

var np = new ol.layer.Tile({
    title: 'North Pole',
    source: new ol.source.TileWMS({
        url: npoleURL,
        params: {LAYERS: 'MOLA_color_north'}
    }),
    projection: north_stereo,
});

var map = new ol.Map({
    layers: [
        np
    ],
target: 'map',
view: new ol.View({
    projection: north_stereo,
    center: [0, 0],
    zoom: 4
  })
});
var graticule = new ol.Graticule({
    strokeStyle: new ol.style.Stroke({
        color: 'rgba(0,0,0,0.8)',
        width: 1,
    }),
});
graticule.setMap(map);

This CodePen demonstrates the problem I'm having. Commenting out the final line, which is the line that attaches the graticule to the map, causes the map to render normally. Running the code with that line in causes the following error:

Uncaught TypeError: Cannot read property '0' of null
at Object.ol.extent.applyTransform (ol-debug.js:4719)
at Object.ol.proj.transformExtent (ol-debug.js:6268)
at ol.Graticule.updateProjectionInfo_ (ol-debug.js:57921)
at ol.Graticule.handlePostCompose_ (ol-debug.js:57876)
at ol.Map.boundListener (ol-debug.js:7277)
at ol.Map.ol.events.EventTarget.dispatchEvent (ol-debug.js:7697)
at ol.renderer.canvas.Map.dispatchComposeEvent_ (ol-debug.js:26392)
at ol.renderer.canvas.Map.renderFrame (ol-debug.js:26480)
at ol.Map.ol.PluggableMap.renderFrame_ (ol-debug.js:18709)
at ol.Map.<anonymous> (ol-debug.js:17624)

It's my understanding that proj4 automatically generates the transformation functions. Transforming points or feature layers has been successful, so as far as I can tell, the graticule is the only thing causing trouble.

Answer 1

该错误是由于未定义worldExtent引起的，对此进行设置可以解决该问题

north_stereo.setWorldExtent([...]);