I'l like to be able to calculate the length in meters of a LineString, in python. I came across this question: Get LineString length in meters (Python, GEODjango) but the recommended method seems to disagree with the calculation given by ST_Length (spatialite in this example) - which also agrees the actual length of the path.
I don't want to use ST_Length, I need to find the length in python. But I can't figure out why these calculations differ. To exemplify:
My Model:
class TestRoute(models.Model):
path = models.LineStringField(null=True)
Using GeoDjango's Length annotation after creating an instance with an example path returns the correct length in meters:
In [1]: TestRoute.objects.annotate(length=Length('path'))[0].length.m
Out[1]: 41345.49910715875
But using the same instance:
In [2]: path = TestRoute.objects.all()[0].path
In [3]: path.srid
Out[3]: 4326
In [4]: path.length
Out[4]: 0.416972813296872
In [5]: path.transform(3857)
In [6]: path.length
Out[6]: 51830.398931961485
The difference is about 10.5k meters.
How can I get the total length of a LineString without needing to save it to a model?
