Specific projection won't work, while others do

Question

When I do this in JavaScript, using OpenLayers 3, it works:

    ol.proj.transform([153.47, -28.11], 'EPSG:4326', 'EPSG:3857');

I realize that this is because OpenLayers already knows both projections and is able to transform between them.

I want to be able to project from 'EPSG:28356' (GDA94 zone 56) to 'EPSG:3857'. So, I've installed and imported proj4js, got the appropriate string from http://spatialreference.org/ref/epsg/gda94-mga-zone-56/proj4js/ and it gives me:

"+proj=utm +zone=56 +south +ellps=GRS80 +towgs84=200,0,0,0,0,0,0 +units=m +no_defs"

So, in my code:

    proj4.defs("EPSG:28356", "+proj=utm +zone=56 +south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs");

    let projection = new ol.proj.Projection({
      code: this.props.item.data.code,
      units: this.props.item.data.units,
      extent: this.props.item.data.extent
    });

    ol.proj.addProjection(projection);

    let c1 = [543100.508297332, 6880073.9025688];
    let c2 = ol.proj.transform(c1, 'EPSG:28356', 'EPSG:3857');

But this doesn't work. It didn't surprise me to get the exact same coordinates (somewhat rounded) back in c2, since the string states '+towgs84=0,0,0,0,0,0,0'.

I can also get the correct result from https://epsg.io/transform#s_srs=28356&t_srs=3857&x=543100.5082973&y=6880073.9025688 which would be [17080772.24, -3274771.77].

Does spatialreference.org give me the wrong proj4 string for this projection and if so, where to get a correct one? Or am I making some other mistake in trying to get this projection to work?

Answer 1

I put together a quick JSFiddle to have a look at this.

The string you're using for Proj4 seems to work fine in my example, so I don't believe that is the problem.

I could reproduce your issue (ol.proj.transform returns same coord) by commenting out the proj4.defs(... line. Without knowing what's in the this.props.item.data object it's a little hard to confirm, but at a guess I'd say the projection you're adding to openlayers is overriding the proj4 based projection. Furthermore, I don't believe you need the openlayers definition as it should use the definition you've given to proj4.

tl;dr, the following should work.

proj4.defs("EPSG:28356", "+proj=utm +zone=56 +south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs");
let c1 = [543100.508297332, 6880073.9025688];
let c2 = ol.proj.transform(c1, 'EPSG:28356', 'EPSG:3857');