Natural Resources Canada (NRCAN) produced 1:50000 topo maps for all of Canada according to the Natural Topographic System grid configuration. The maps are now considered a legacy product, but GeoTIFFs are still available via FTP (e.g. here).

Text on the GeoTIFF indicates that the map's datum is NAD27, however gdalinfo reports that GeoTIFF's SRS is based on NAD83. The XML file that ships with the GeoTIFF includes the following:

The source of this raster file (paper map) is geocoded according to the North American Datum of 1927 (NAD27). The resulting raster file is georeferenced to the North American Datum of 1983 (NAD 83) according to the Universal Transverse Mercator (UTM) projection

If I load the GeoTIFF into QGIS in a new project it will default to using NAD83 UTM 9N as the project SRS. If I click on a grid line intersection that the map says is 0600000,6070000, QGIS tells me its coordinate is roughly 0600000,6070200. If I change the project SRS to NAD27 UTM 9N and click at the same point I get a much more appropriate coordinate.

What is the explanation for NRCAN distributing NAD27 data with a NAD83 SRS, and even acknowledging in the XML documentation that the wrong datum is used in the GeoTIFF? Is this likely to be a mistake, or is there something I'm missing? I know that NAD27 / NAD83 can be a complicated problem, and this is a product from the federal government, so I'm guessing it has to be my mistake.

  • 1
    Maybe the quoted text means that NADCON was performed on the digital conversion from paper source, and the data is accurately reported as being NAD83. – Vince Nov 16 '20 at 4:58
  • @Vince if this was the case shouldn't I get a more accurate coordinate from QGIS? If I click on the intersection of two labelled grid lines in QGIS the coordinate in NAD83 9N is approximately 200m off - this is 1/5 of the width of a grid cell – tomfumb Nov 16 '20 at 5:03

The printed coordinates are in Nad27, the digital coordinates are in Nad83. Both are correct. They both point to the same ground location. It is a bit like displaying the coordinates in degrees or in meters, except that when only the datum is changed the difference between the 2 coordinates is rather small. When you change the project CRS, you are doing on the fly reprojection so the digital coordinates are now in Nad27 and match the printed ones

You can read a bit more on this topic here

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