# Creating a geodetic line that goes around the whole ellipsoid with pyproj

https://jswhit.github.io/pyproj/pyproj.Geod-class.html
Pyproj's Geod class has a npts method to create points on the geodesic between two points:

``````npts(self, lon1, lat1, lon2, lat2, npts, radians=False)
``````

Given a single initial point and terminus point (specified by python floats lon1,lat1 and lon2,lat2), returns a list of longitude/latitude pairs describing npts equally spaced intermediate points along the geodesic between the initial and terminus points.

How can I use this to create points on a geodesic that goes around the whole ellipsoid? The npts method uses the shortest of the two possible paths, so you can only use it for a line spans half the ellipsoid.

My attempt so far:

I thought I could use the fwd method to build the line in multiple steps but I do not know how to calculate/find the azimuth to continue on once I created the first segment with npts. If I use the same azimuth, the result is wrong (see image below). fwd returns the back azimuth, while I think I would need the forward azimuth at the terminus point.

``````fwd(self, lons, lats, az, dist, radians=False)
``````

forward transformation - Returns longitudes, latitudes and back azimuths of terminus points given longitudes (lons) and latitudes (lats) of initial points, plus forward azimuths (az) and distances (dist).

Here is my failed attempt:

``````from pyproj import Geod
from shapely.geometry import Point, LineString

geod = Geod("+ellps=WGS84")
slon, slat = 42, -23
azimuth = 230
distance = 14600000

# first half
h1lon, h1lat, _backazi = geod.fwd(slon, slat, azimuth, distance/2)
h1points = geod.npts(slon, slat, h1lon, h1lat, npts=100)

# other half
h2lon, h2lat, _backazi = geod.fwd(h1lon, h1lat, azimuth, distance/2)
h2points = geod.npts(h1lon, h1lat, h2lon, h2lat, npts=100)

points = [Point(slon, slat)] + h1points + [Point(h1lon, h1lat)] + h2points + [Point(h2lon, h2lat)]
line = LineString(points)
``````

The line resulting from this looks like this, you can see where the second half starts, it's where the direction "skips":

This is how it would look like if the second azimuth was correct (this was created as a single line):

If we could solve that azimuth issue, creating the geodesic around the ellipsoid could simply be done step by step.