Converting from eastings and northings to LatLong on layers in QGIS

Question

I have been prototyping a simple navigation app using QGIS. If I generate tiles from OpenStreetMap using Generate XYZ Tiles, I create a basic app using Leaflet JS. I can then use the Geolocation API to monitor my movements. I am using EPSG:3857. However, I need to use a base map generated by a third party using a CAD tool, in order to have more detailed features than is available in OSM. However, their CAD tool only supports Eastings and Northings (in metres). Once I have imported their layer group, and set the CRS to EPSG:3857 I still have the position information in Eastings and Northings.

I have tried to change this but cannot find any way, but it surely ought to be possible as I can type an individual location into, for example, Grid Reference Finder, and I get back the correct lat and long?

Addendum 10th Feb:

I think I am getting to the root of the problem. I am doing something stupid but not sure how to fix it. Let me explain what I am doing and then I will be able to rephrase my question.

If I start off with an Open Street Map base, and then try to add my DXF layers, nothing appears on top of the OSM layer, although the DXF appears to have loaded without a problem and its three layers appear in the Layers window. So, what I did was to open a new project and simply add the DXF. This is what I get: Notice the coordinates at the base. These are in metres. However, the origin was presumably set in the original CAD file so that these are correct eastings and northings. This is why I thought that the CAD file had been located correctly. But I don't believe QGIS has done anything of the sort, which is why my DXF would not register on the OSM map when I tried this. If I then add the XY fields (or just right click on the map window and copy coordinate) I get something like this: 4.548068,1.431102. It is way off the true location and I believe this is because the lat long location of the imported DXF is quite arbitrary.

So, it seems my question should be along the following lines. How can I create a custom CRS given that I know the mapping of the metre coordinates onto Lat Long? For example, I have been given that the four corners of the box around my map are: Top Left Corner X = 505900.0 Y = 160200.0 Top Right Corner X = 507400.0 Y = 160200.0 Bottom Left Corner X = 505900.0 Y = 157500.0 Bottom Right Corner X = 507400.0 Y = 157500.0 These are eastings and northings so if I convert them using Grid Reference Finder I do indeed get the correct lat and longs.

Answer 1

Don't set the projection to EPSG:3857, that does not work.

Keep it as the original CRS and run the Reproject Layer tool from Vector>Data Management Tools> Reproject Layer...

And choose EPSG:3857 as the Target CRS.

Answer 2

Your Source Data Coordinates

Your DXF file must have a valid Coordinate Reference System, or it wouldn't appear in the right place on the map. It may well be different from your Project Coordinate Reference System, but if they're different, QGIS will automatically reproject when needed. QGIS will run a little more efficiently if you reproject the geometry layers, but that's the only different reprojection will make.

The above paragraph was optimistic, wrong, and written before the addendum to the question. See other answers for more discussion on this.

Numbers Showing in the QGIS Status Bar

The units of the EPSG:3857 Coordinate Reference System are meters, and by default, this is what QGIS will show in the status bar. Specifically, they are meters from the zero latitude, zero longitude. In the screenshot below I have changed this to show "Extents" rather than the mouse cursor; note that the magnitudes of the numbers are nearly equal, because the map is (nearly) centered on 0,0.

You say that these numbers appear to be valid eastings and northings. This part I can't explain. Can you post a screenshot of your QGIS window and the values you see there, with both the position numbers and the CRS value in the lower right?

Changing the Coordinate Display in the QGIS Status Bar

If you want different numbers in the status bar, you can change the format in the Project Settings > General > Coordinate and Bearing Display. This will not affect the way your data are stored and exported. See also the question Set the QGIS display to show decimal degrees?... which also has some people reporting this not working.

Leaflet setView

According to the Leaflet Documentation, the first argument to Map.setView is a Latitude, Longitude pair. Numbers I've seen generated by QGIS for different locations are centered as expected, so I'm not sure why yours are so far from the region you are looking at.

Seeing Coordinates of Your Point Geometry

You mention running the mouse over the map to coordinates. Would you like to see where QGIS thinks your geometry is? If you have a point layer, you can run the Add X/Y fields to layer Tool, and have their coordinates appear in the Attribute Table. If using this tool, set the Coordinate system to EPSG:4326 - WGS 84 for output in degrees. Setting it to EPSG:3857 - WGS 84 / Pseudo-Mercator will generate the same coordinates in meters that you are already seeing.

Like most QGIS tools, this will generate a new layer with randomly assigned symbology in a single color.

Answer 3

Thank you for your addendum, now we're pinning this down. I shouldn't have confidently said QGIS was putting your geometry in the right place. It must be showing it in the South Atlantic somewhere.

UTM and MGRS

I was also confusing UTM (Universal Transverse Mercator) with MGRS (Military Grid Reference System). They share the 60 bands of longitude, but with MGRS you also have those latitude bands which define the grid zone.

Other people with this issue haven't received much help. There used to be an MGRS Tools plugin for QGIS 2, but the only reason I know this is people reporting it doesn't work, and now you can't even download it.

Assign Projection

If you can get the right Coordinate Reference System, you can use the Assign Projection Tool to put it in the right location. This is distinct from the Reproject Layer Tool, which assumes that your geometry is already in a known place.

Creating a Custom CRS

You could try to manually calculate the fix, as this hard-working person had to. But you may be able to work out a custom CRS and still have QGIS do the rest of the work for you. If you can work out the CRS you want, the question Creating custom CRS in QGIS shows where in the user interface to do it. And yes, you really do follow the steps to create a new User Profile and suddenly you get the menu options to create custom projections!

You can get approximately right by finding a UTM projection of the correct longitude band, copying the text from that, and editing the false northing and possibly the false easting to match the grid zone you're in. But your geometry looks precise enough that you need to be sure about the datum as well. Have they come from an existing map, from examining aerial photographs, from walking around with a GPS, or some combination of the above?