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I'm trying to figure out how to create a range ring from some location (e.g. Broome Airport "YBRM") in QGIS that looks like something in Great Circle Mapper. The issue is about how the range ring behaves around the poles.

I want to be able to show a base map in the QGIS drawing. I suspect it would be something from the OpenLayers plugin.

For example, if I want to draw a 4500 nautical mile range ring around YBRM, in Great Circle Mapper, it looks like this:

Great Circle Mapper

But when I try to get this kind of image in QGIS, even after playing around with the different projections, reprojecting the layers etc, I can't seem to get something that looks correct.

I would think that, as a test, if I create something in QGIS (e.g. 4500 nautical mile range ring around Broome (YBRM)), I should be able to import it into Google Earth and have it look like a perfect circle of 4500 nm radius around Broome.

Would someone please help me understand how I can get a Great-Circle-Mapper-style range ring nearby a pole (north pole or south pole) in QGIS?


Update:

I think the issue is about forcing the lines to continue east of the 180 degree line. At the moment, when the points get too far east (past 180 degrees), the points will jump all the way to the left / west.. looks like this:

enter image description here

So how do I make the dots continue right / east as it should do.


Another update:

I seem to get the correct shape when I create a circle in Google Earth using that measuring tool, saving it as a kml, importing that to QGIS, convert to points, add coordinate columns as an attribute, save as CSV, add 180*2 to all 'x' coordinates < 0, then import the CSV into QGIS as points. I can then join the dots up to make a line using the SAGA tool "Convert Points to Line(s)".

This option is not idea as I can't control how many points are generated when making the line. I need a way to create it purely using QGIS where I can control the number of points used, like the normal buffer tool.

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The best workaround is to clip your circle with a polygon covering nearly the world. Put

Nr;WKT
1;POLYGON ((179.9 89.9, 179.9 -89.9, -179.9 -89.9, -179.9 89.9, 179.9 89.9))

in a text file, and import it into QGIS as delimited text to WGS84. Then densify the geometry with 179 points.

For the circle, create a custom aeqd projection based on a sphere with Broome airport as center and nautical miles as units:

+proj=aeqd +lat_0=-17.949722 +lon_0=122.227778 +x_0=0 +y_0=0 +a=6371000 +b=6371000 +units=kmi +no_defs

and add another text file with center and point on the radius (in nautical miles):

name;X;Y
Broome;0;0
radius;4500;0

In the custom CRS, you can now create a circle with the CADDigitize plugin. Reproject the clippolygon to the same CRS, convert the circle polygon to a line layer, and use the difference tool to subtract the part between 179.9 East and 179.9 West. This is how it should look back in WGS84:

enter image description here

If it looks too edgy for you, increase the circle parameter in Vector -> CADDigitize -> Settings, circle tab.

  • I can't seem to get the custom CRS working. The message I get is "The proj4 definition of' *Generated CRS (+proj=longlat +ellps=IGNF:ELG037 +no_defs)' is not valid". – Joe Aug 22 '16 at 7:23
  • These are not the parameters I gave above. If you have another custom CRS with that text, better remove it from the list. – AndreJ Aug 22 '16 at 7:25
  • Deleting the other ones fixed it :-) – Joe Aug 22 '16 at 8:15

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