For continuity, I went with `LatLon` as that was how it was defined in the docstring (see [iris.analysis L683 onward](https://github.com/SciTools/iris/blob/master/lib/iris/analysis/__init__.py#L683)) I used as the basis for the kwarg definition in the original commit for this PR. As it's moved on a bit since then I've no issue with changing it to something a little more english!