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I'm working with a GeoJSON file that contains sets of coordinates associated with street corners in the city of Montreal, Canada. The spec sheet of the API delivering those GeoJSON specifies that the coordinate system uses the NAD31 projection datum, which in spite of my search I doubt exist.

Because of the street names associated to the coordinates in the GeoJSON, I am able to deduce that the erroneous NAD31 coordinates 300247.509025, 5040897.253 refer to the WGS84 latitude/longitude 45.5079942,-73.5582652.

Does anybody know what might the geodetic datum NAD31 refer to, and how I can convert it to WGS84 coordinates?

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AFAIK, The National Geodetic Survey number will either be 27 (unlikely) or 83. I guess you could try using one of the conversion forms to see if it fits? No idea otherwise. –  Sleep6 Jul 29 at 22:27
    
Thanks, @Sleep6. I have indeed tried NAD27 and NAD83, but the coordinates I have to deal with are far from meeting those standards. –  Christophe Marois Jul 30 at 0:51

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up vote 2 down vote accepted

The coordinates in your example (300247, 5040897) are in some type of projected coordinate system. Coordinates in NAD27, or NAD83 (as stated in the comments there is no such thing as NAD31) are Longitude/Latitudes which are the same form as your WGS84 sample.

There is no way to determine what coordinate system has been used for your data, although you could try guessing with some of the most commonly used ones such as UTM or MTM. Your best bet would be to contact whoever created the original data and see if they can give you the correct Coordinate Reference System (CRS) that was used. Ideally they can tell you the EPSG ID number which will make it much easier to convert your data.

EDIT: Just did a quick visual check in ArcMap, your data may be in MTM Zone 8.

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Even though I didn't succeed to convert the CRS, MTM Zone 8 matches the Quebec & Ontario Mercator; furthermore the coordinate format undoubtedly matches the data that I have. Thanks a lot for the hint! –  Christophe Marois Jul 30 at 14:49

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