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I recently went and collected some waypoints with a GPS that is in NAD27 UTM16N. After uploading the GPX file into QGIS, I saved it as an ESRI shapefile, and changed the CRS from WGS84 to NAD27 UTM16N. After adding it to a new file, the coordinates appeared to be in the correct place, but after comparing them to the points in the actual GPS, it appears that everything has been slightly shifted. Using one point as an example, in the GPS it is X = 327398 Y = 1836319, but in QGIS it is X = 327401 Y = 1836410.

Finding this strange, I tried using the 'Project' tool in ArcMap to convert the GPX from WGS84 to NAD27 UTM16N. Using this method, my points have the same coordinates as in the GPS.

Does anyone know how to solve this issue in QGIS, or have any insight as to why the coordinates for points are not matching up?

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    How did you "change the CRS" in QGIS?
    – MappaGnosis
    Commented Dec 7, 2018 at 16:55
  • As @MappaGnosis implies, it looks like the points underwent a reprojection in QGIS. The offsets look right for NAD27-WGS84(or NAD83).
    – mkennedy
    Commented Dec 7, 2018 at 17:34
  • @MappaGnosis From my GPX file I right clicked > Save as.. > Format 'ESRI Shapefile > Coordinate Reference System Selector 'NAD27 / UTM zone 16N'
    – Hank
    Commented Dec 7, 2018 at 17:46
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    First you said the waypoints were collected in NAD27 UTM16N. Then you said you changed the CRS from WGS84 to NAD27 UTM16N. How can those two statements both be correct? If you collected the points in NAD27 UTM16N, then they weren't in WGS84 to begin with.
    – csk
    Commented Dec 7, 2018 at 17:54
  • It's like you're saying you wrote a sentence in French, then translated it from English to French.
    – csk
    Commented Dec 7, 2018 at 17:57

3 Answers 3

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QGIS uses the NADCON grid shift files, Your GPS unit most probabaly does not have them stored inside, and uses a less accurate transformation. The GPS signals from the satellites are always WGS84, so the error is inside your GPS unit.

See https://www.usgs.gov/faqs/what-do-i-need-know-get-my-gps-correct-operation and https://www.nps.gov/gis/gps/gps4gis/how2s_bycatagories/6-gpspostprocessing/datum_advise_nps-akso.doc

Maybe related: Reprojecting between NAD27 and WGS 84 part III and Project from Datum Nad27 to WGS84 . Especially if you are outside USA and Canada.

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Make sure that the CRS of the QGIS project is the same NAD27 UTM16N.
Probably the layer is well defined and the features have their coordinates assigned correctly, but when showing them in QGIS they are reprojecting to another coordinate system.

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I've always understood that a GPX file stores data (latitude/longitude) in WGS84 coordinates. Your GPS may be displaying data to you based on a different coordinate reference system, or even recording them in a different coordinate reference system, but it seems to me that the GPX file itself probably has them in WGS84. If that is true then telling QGIS that the coordinates are in some other CRS (which they aren't) will introduce an error.

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