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I have the map below whose projection is unknown to me. It's a PNG image without metadata.

enter image description here

It comes from a webpage that shows radar information for France.

How can I figure out the projection it implements? I clarify that I don't require a perfect fit, something reasonably close would be fine, and I'm open to accept certain level of brute force. But I'd like to gather some ideas to make a sensible search.

It seems to be some sort of Lambert Conic, that's all I can tell 'by eye'. I have been using trial and error with QGIS, but the process of "define a projection -> set it -> try to fit the map as overlay -> discard it" is slow and I have little intuition to know which parameters to tweak and in which direction.

Ideas?

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    have you tried the most common standard French projections ? RGF 93 is quite common epsg.io/?q=France%20kind%3APROJCRS but in your case the central meridian seems to be greenwich. – radouxju Feb 12 at 16:53
  • This can help you i think. In Qgis, select the layer and then click the right button of mouse, you will see properties option. Click the properties and then for the "information" option. Now you see the CRS. – Xi Jin Feb 12 at 17:07
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    By eye you can mostly guess. Your screenshot looks like a webapp with a wms to me. You can read its metadata or check the used url. – MrXsquared Feb 12 at 17:47
  • Thanks for the hints. I have edited to clarify. It's a PNG image I get without metadata from a webpage I link now in the Question. I'm not sure if and how I can get te projection from the web application. – Onturenio Feb 13 at 11:45
  • @radouxju I have tried some of them and none fits so far... – Onturenio Feb 13 at 11:45
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Seems like EPSG:3106001 as stated on https://www.rainviewer.com/radars/france.html.

If you analize the website with your favourites browsers inspector, you will find a carto.js file at https://donneespubliques.meteofrance.fr/template/js/carto.js where you can see they are using Openlayers for this with the following projection:

Proj4js.defs["EPSG:3106001"]="+title=thomas +proj=stere +lat_0=90 +lat_ts=45 +lon_0=0 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs";

Thats how it looks in QGIS as user defined projection:

enter image description here

As this seems like a non standard EPSG-Code, you need to set it up as custom projection in QGIS (couldnt find details about this Code on the web as well). If you want to do so, go to "Settings --> Custom Projections" and enter the part between the quotations (+title=thomas +proj=stere +lat_0=90 +lat_ts=45 +lon_0=0 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs) as "Proj String", also add a name:

enter image description here

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I'm interpreting your question that you have this saved image as a picture (raster), with no world file or other georeferencing information. And you say approximate is OK.

In this case, especially given the grid is shown, you could use QGIS' Raster > Georeferencer feature to force-stretch it to a desired projection rather than to try to guess/determine the projection. (Editing to add: though others' suggestions how to try to sleuth it out are good ones; if you can do so easily that's great. This is if you can't.)

  1. Set your project CRS to something you can work with afterwards, with preference given to your strongest hunch of what the image's projection might be.

  2. Load up from a known data source a simple map that lets you set the canvas extent to slightly exceed the map on your image (e.g. OpenStreetMap, Google, ...).

  3. Using Vector > Research tools > create grid, create a grid of latitude and longitude to match what you see in your image. Set the Grid CRS to EPSG:4326 to be creating the grid in Lat/Long not in metres, and set the Grid extent to the map canvas you zoomed into in step #2. Turn snapping on for this layer.

  4. Start georeferences, load up your image, and in georeferencer's Settings > Transformation settings > Transformation type choose Thin plate spline. Make sure the Target SRS matches your project CRS. (There are improvements in the about-to-be-released version 3.18 of QGIS around georeferencing between different CRSes, but in the current release and LTR versions it is best to have the same CRS.)

  5. Create GCPs along the intersection points of the grid, matching them to the grid intersections on the map canvas. The grid you created there in step #3, and with snapping on, this will be quick.

  6. Hit Georefencer's Play button to create a modified copy of the image, stretched to fit the grid, and loaded up as a layer in your canvas.

A few comments:

A. The more your project CRS is different from the unknown image's actual CRS, the more imprecision there will be in between the grid points. Conversely, if you're pretty darned close (in the same family), you might be able to get away with only 4 points and using Projective rather than Thin Plate Spline as the transformation type.

B. This georeferences this one image only. However, if you have other images (e.g. from scraping a web site over time, since this seems to be some radar/weather map) which have the exact same (unknown) projection and extents, you can manually copy over the GCP points and world file you did for one image and use them for the others. Make sure that in your transformation settings, you keep Save GCP points on.

C. You can of course undisplay or even delete the utility layers of the orientation map and grid in steps 2 and 3 once you are done. You can even reproject your fitted image to a different project CRS on the fly, though image quality may suffer a bit when you do so.

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It sounds as if you have the layer in QGIS. If I assume this right, you can just go to the properties of the layer (right or double click onto the layer) and go to the tab "information". There you have meta information about the layer at the top of the page such as Geometry, CRS (Coordinate Reference System) and Extent. If the projection is not written in the row of the CRS, you can easily define which projection your layer has with the given EPSG code (CRS: EPSG ****). If it is not already included in QGIS in your case, just go to epsg.io and search for your EPSG code and you will get plenty of information. Hope this helps and answers your question!

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  • Sorry, that's not the case. It's a PNG without metadata. I have edited my question to clarify this point. – Onturenio Feb 13 at 11:56

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