OpenLayers 3 - WMS Transform should be defined

Question

I load wms from "geoportal" and it's good to time when I want show wms. Problems:

• Hidden default map - I want that WMS overlays on OSM
• Transform should be defined

It's look that:

And some code:

var extent = [78912.472300001, 78524.1231000004, 875939.472300001, 948244.6231];

var projection = new ol.proj.Projection({
   code: 'EPSG:2180',
   extent: extent
});
ol.proj.addProjection(projection);
ol.proj.addCoordinateTransforms('EPSG:4326', projection,
  function(coordinate) {
    return [
      WGStoCHy(coordinate[1], coordinate[0]),
      WGStoCHx(coordinate[1], coordinate[0])
    ];
  },
  function(coordinate) {
    return [
      CHtoWGSlng(coordinate[0], coordinate[1]),
      CHtoWGSlat(coordinate[0], coordinate[1])
    ];
});

var WMS=new ol.layer.Image({
  extent: extent,
  source: new ol.source.ImageWMS({
    url: 'http://mapy.geoportal.gov.pl/wss/service/img/guest/TOPO/MapServer/WMSServer',
    crossOrigin: null,
    params: {
      'LAYERS': 'Raster',
      'FORMAT': 'image/png32',
      //'TRANSPARENT':true,
    },
    serverType: /** @type {ol.source.WMSServerType} */ ('mapserver')
})
});

Views:

 var _view=new ol.View({
    center: ol.proj.fromLonLat([0, 0], projection),
    zoom: 0,
    extent: extent,
    projection: projection,
});

Answer 1

I think you need to add:

proj4.defs("EPSG:2180","+proj=tmerc +lat_0=0 +lon_0=19 +k=0.9993 +x_0=500000 +y_0=-5300000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs");

to your code so that OpenLayers "knows" how to transform to your projection.

In general you can look that string up on epsg.io