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I am very new to QGIS as I'm having to migrate my work here from Google Earth in the wake of NOAA discontinuing their RNC files. All I need to do on QGIS is plot series of points 1 nm apart along a path though marine traffic areas and then export the coordinates of each point.

The current coordinate system I'm working with is EPSG:3857 - WGS 84 / Pseudo-Mercator. I believe this came from the .qlr file of the ENCs that I imported as a layer, so it's what I've been using.

I was able to add a line with the same CRS and use the "point along geometry tool" to make points every 1.852 km (nm was not an option). However, when I attempt to use the field calculator in the attributes table of the points I generated to list both x & y I get outputs that I am not familiar with. Ideally, I'd like to be able to export dms lat/long but I have no idea how to get there from the x/y output I'm getting.

View of my line, points, x/y outputs, crs, and mouse coordinates

Worth noting that my coordinates displayed in the lower bar are shown in dms & match perfectly with what I'm used to in Google Earth. I'm probably missing a lot here as, like I said, I'm brand new to all this.

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The "points along geometry" tool works using the coordinate system of the layer. Your layer is in EPSG 3857 coordinates, and so your points are 1852 metres apart in EPSG 3857 coordinates. If you use the measure tool which works out true distance based on the shape of the earth, you'll see these points are not anywhere near 1nm apart (unless you are on the equator).

To get points 1nm apart, I think the best way is to use the QChainage plugin and make sure your lines are projected to lat-long coordinates.

As an example, here is a north-south line with "points along geometry" at every 100km, the source line is in EPSG 3857 coordinates, so the points are 100,000 units apart. The QGIS display projection is set to EPSG 3857 (see bottom right corner of QGIS) and so the points look evenly spread. But if they were truly 100km apart, they shouldn't look it on a Mercator projection:

enter image description here

If I project my line to EPSG:4326 (lat-long) and then use QChainage I get points like this:

enter image description here

which seem to get further apart because of the Mercator projection. But they are all 100km apart in true ellipsoidal coordinates, which you can see when I use the "measure" tool at the south and north extremes of the line (its off slightly because I'm manually clicking here):

enter image description here enter image description here

If you then added the X and Y coords of the generated points as attributes you'll get them in EPSG 4326 lat-long which seems to be what you want.

The QChainage plugin will also let you specify nautical miles!

enter image description here

but you do have to make sure your source line layer is in lat-long coords.

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  • Thank you so much! This makes complete sense & worked great when I tried it. Noticed one quirk when I compared the coordinates I got with the same points in Google Earth. Measurement between 2 of the points in QGIS reads as 1 nm, while GE reads as 0.85 nm. Copied the coordinates directly over from QGIS so I'm curious if there's something just slightly different about the coord systems between the two? Not an issue for what I need to do, just interesting. Thanks again for your help!
    – Margaret
    Commented Apr 5, 2023 at 14:26

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