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Wrap longitude in the solver #23

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Apr 10, 2014
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Wrap longitude in the solver #23

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1 change: 1 addition & 0 deletions tawhiri/solver.py
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Expand Up @@ -43,6 +43,7 @@ def euler(t, lat, lng, alt, model, terminator, dt=1.0):
df = model(t, lat, lng, alt)
lat += df[0] * dt
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Do we also want to wrap latitude?

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Maybe. I'm trying to work out whether bad things happen at the poles or not.

So models.py#76 is going to go crazy (starts to become a problem when lat ~= 90- 1e-5, I reckon, since then dlng is going to be approaching 180 degrees).

However, this may not be an issue since the longitude circle is so small at the pole that the longitude value doesn’t really… matter (at 90 - 1e-5, on the order of tens of metres radius (am I right?)).

If we are going to handle latitudes at the poles we need to special case lat=90 and avoid dividing by zero on that line; then we need to have some logic to handle flipping the longitude by 180 degrees when it goes over a pole an odd number of times.

This will be quite a few more lines in solver.py, but hopefully since it will only run rarely the cost to our loop is just evaluating one if statement?

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ooooor we could not support going directly over the pole?

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Perhaps we could disregard that for now and deal with it later. A separate issue, perhaps?

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Good plan.

lng += df[1] * dt
lng %= 360.0
alt += df[2] * dt
result.append((t, lat, lng, alt))
return result