4

I'm trying to set up a QGIS project for creating a map of a fictional planet smaller than Earth. See here for a helpful question that doesn't quite touch on what I'm trying to accomplish (I don't mind a non-cylindrical projection, and don't have any shapefiles or rasters that the map has to "fit").

My primary goal is to be able to draw continents as if viewing their orthographic projection, while having the ability to check their ellipsoid measurements. After several hours of searching, I still haven't found if that capability exists in QGIS, so I've contented myself with a lonlat world map. The problem is, I still can't get the equatorial distance from -180 to 180 degrees to match the circumference I calculated.

Here is the proj4 string for the custom CRS:

+proj=lonlat +a=5733900 +lat_ts=0 +lon_0=0 +rf=700 +units=m +no_defs

(The fictional planet has an equatorial radius of 5733,9 km and a polar radius of 5725,7 km, and should have an equatorial circumference of 36027,1 km if I've calculated right.)

I have set Project Properties>General>Measurements to custom ellipsoid with the above mentioned equatorial and polar radii, and set the custom CRS as the project's CRS.

I have created a vector layer with the custom CRS, a polyline that should show the bounds of the map. It is a rectangle from degree coordinates -180 to +180 x and -90 to +90 y. However, when I measure the equatorial distance across this rectangle (at y=0), I get wildly varying distances. Here is an example: measurement issue from degree coordinates 0,0 to 0,+180 - half the circumference should be 18013,6 Even when I have defined the coordinate reference system as eqc, I still get an incorrect circumference at the equator.

What do I need to change in order to be able to get realistic measurements? Is there any way to work on all parts of the ellipsoid in one QGIS project (and have them have fairly orthographic dimensions when I zoom in on them), or should I be looking into some other software?

2
  • 1
    I'm not sure why it doesn't take your custom ellipsoid into account by default when you create a new crs, but I know you can change the reference ellipsoid of the measure tool by setting it manually in the Project menu > General > Measures and choose a custom ellipsoid and enter your a and b parameters there, and the measure tool should work then. – FSimardGIS Jul 19 '20 at 0:11
  • Hey @FSimardGIS, thanks for the tip! For whatever reason, changing the ellipsoid that QGIS measures with didn't solve my problem, but it's good to have your comment in case anyone else stumbles across this and it works for them. – Coel Jul 22 '20 at 18:14
1

Forget all kind of ellipsoidal calculations for non-Earth CRSes in QGIS.

But you can use the CRSes for the layers, and transform between geographic and projected non-Earth CRSes. If you export the layer to the custom eqc system, you can measure the Cartesian length of the geometries and they will be right.

1
  • Thanks Gabriel, I'll try this out when I can figure out how exactly I worded my eqc definition, haha. – Coel Jul 22 '20 at 18:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.