Orthographic projection extent in QGIS

Question

I have a world-map-layer in QGIS, such as the one by typing "world" in the coordinate box.

I then set the project-crs to a custom orthographic projection. The world-layer gets reprojected on the fly correctly.

I then create a layer from the extent of the world-map, densify it to like 1000 vertices. I expected it to just fit to the outline of the globe. however it does this:

What am I doing wrong? When using the "Globe Builder" Plugin things work and the bounding box layer, on EPSG:3857 looks like this:

Where is this coming from and how could I create it?

Answer 1

Create a grid (Menu Vector > Reasearch Tools > Create Grid), based on your layer World Map. As grid extent, use the layer extent. As this layer is in EPSG:4326, use a convenient horizontal and vertical spacing, e.g. 10 degrees.

When you now set the project CRS to your custom CRS, you get the desired effect, see screenshot. For an explanation why your approach did not work, see below.

Screenshot: To get rid of the black lines, set their color to the same color as the polygon fill. To smooth the lines, densify the grid polygons:

Explanation:

So why don't you see the extent, created in EPSG:4326 in your custom projection? You have to think about the property of the extent, which does not represent a real-world phenomenon, but refers to borders of the (projected) map canvas. Extent is from -180 (on the left) to +180 (on the right) on the map canvas.

In reality, however, 180 and +180 represent the same points (or very close), all of them laying on the antimeridian. QGIS connects them with the shortest line in the current projection, so does not use the "detour" to cover the whole globe.

So connecting all these points in your custom projection results in a very thin polygon or even a line in your custom projection: the line you have in the upper port of your first screenshot.

The red circle probably stems as well from the initial CRS that you used to create the extent.

By creating a grid, we have smaller cells (patches) with vertices evenly distributed all over the globe.