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I received a shapefile (with all its sibling files) from an urban planning agency in Paris that I'm trying to overlay on map tiles from either Google Maps or OSM. The source file uses the projection NTF (Paris) / Lambert Nord France (EPSG:27561). The file contains street-level, elaborate paths, so I need high precision.

My understanding is that, unless one wants to spin up a whole new tile server, as this question/answer discusses, the paths in the shapefile will need to be converted to Web Mercator (EPSG:3857). So far, I've had no luck reconciling these two projections.

TL:DR: Are Esri files pre-projected, such that I'll need to ask the source for a different projection, or do they natively contain lat/lng coordinates and a projection, such that I can transform them myself? I tried to answer this myself by opening them in Sublime, but I don't speak binary fluently.

Background: Wrestling with Projections

As soon as I open the shapefile in QGIS, before adding any tiles, I'm offered three options for the conversion necessary just to view the vectors. The first of three suggestions is:

Inverse of unnamed + Transformation from NTF (Paris) to NTF + Inverse of WGS 84 to NTF (3)
Scope: For applications requiring an accuracy of better than 1 metre.
Remarks: These parameter values are taken from RGF93 to NTF (2) (code 15958) as RGF93 may be considered equivalent to WGS 84 within the accuracy of the transformation.
Area of use: World, France - onshore - mainland and Corsica
Identifiers: INVERSE(DERIVED_FROM(EPSG)):15960

And the command is:

+proj=pipeline +step +inv +proj=lcc +lat_1=49.5 +lat_0=49.5 +lon_0=0 +k_0=0.999877341 +x_0=600000 +y_0=200000 +ellps=clrk80ign +pm=2.10350625000001 +step +proj=hgridshift +grids=ntf_r93.gsb +step +proj=unitconvert +xy_in=rad +xy_out=deg

I was hoping (naturally) that I could just set the projection to the Pseudo-Mercator and get it to align with the tiles, but the closest I've gotten is about 30 meters off. I have also successfully relocated Paris to the center of Atlantic and the Southern Pacific...

I believe (but am not certain) that the files were exported from ArcGIS. I can certainly ask the source for a different set of files, but would ideally know what to ask for.

UPDATE Per the request in comments, the .prj file is as follows:

PROJCS["NTF_Paris_Lambert_Nord_France",GEOGCS["GCS_NTF_Paris",DATUM["D_NTF",SPHEROID["Clarke_1880_IGN",6378249.2,293.4660212936265]],PRIMEM["Paris",2.337229166666667],UNIT["Grad",0.01570796326794897]],PROJECTION["Lambert_Conformal_Conic"],PARAMETER["False_Easting",600000.0],PARAMETER["False_Northing",200000.0],PARAMETER["Central_Meridian",0.0],PARAMETER["Standard_Parallel_1",55.0],PARAMETER["Scale_Factor",0.999877341],PARAMETER["Latitude_Of_Origin",55.0],UNIT["Meter",1.0]]

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  • The coordinates are stored into shp part in units of a coordinate system and what the system is is saved into .prj file. Normally QGIS recognizes the CRS from the .prj file automatically and can then reproject data into other systems, for example EPSG:3857. Now there seems to be different interpretations about what EPSG:27561 means. Please add the contents of the .prj file into your question.
    – user30184
    Jan 14 at 19:50
  • Thank you! Added the .prj contents at end Jan 14 at 20:39
  • Could you also add one point in EPSG:27561 and reprojected coordinates in EPSG:3857 that you believe to be correct. Tell also how do you know that they are correct.
    – user30184
    Jan 14 at 21:39
  • In order to get to Web Mercator you need to do datum transformation. On-the-fly projection should handle this but might need help. The title and body elements about "pre-projection" are confusing, because most data in reversible projections (all but a few) can be reprojected. Shapefile is just a storage format, not a prison from which math cannot escape.
    – Vince
    Jan 15 at 12:22
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    Most projections are reversible, but if datum transformation is not applied in the P1 -> G1 -> G2 -> P2 cascade (at the G1 -> G2 step) then the location could be off by as much as a kilometer. ArcGIS offers available transformations in the UI and from Python; I have no idea what QGIS does, but that's your research task here.
    – Vince
    Jan 16 at 1:03

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