Save GDT20 - Polar Stereographic #122
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| Stereographic, | ||
| LambertConformal, | ||
| AlbersEqualArea) | ||
| from iris.cube import Cube |
| # Is this a north or south polar stereographic projection? | ||
| if cs.central_lat == -90.0: | ||
| # XXX: is this the correct value? We read bit 0x80 in load, but set bit | ||
| # 0x1 when saving GDT30... |
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I don't know, is it the correct value? This is a very unnerving comment.
I reckon it is though. At any rate, it seems to line up with GDT3.30, so I'm good with that.
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short version : No, this isn't the correct value, it should be 0x80.
long version : because in GRIB terminology "bit 1" is the most-significant (top) bit, i.e. 0x80, and "bit 8" is the least(bottom) i.e. 0x01.
So, this is already wrong in the existing code here, where it has
centre_flag = 0x0 if poliest_sec > 0 else 0x1
Whereas, just above that here the code uses the magic global _RESOLUTION_AND_COMPONENTS_GRID_WINDS_BIT to set a value, and checking where that value is defined, this one is correct
-- i.e. for grib 'bit 5' we use '1 << 3'.
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This is a very unnerving comment
Yup, it is. I assumed that it should be 0x80 but it was a bit odd that the GDT30 saver did not set that value...
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it was a bit odd that the GDT30 saver did not set that value
Bugs in existing implementation -- doesn't sound that unusual to me 🤣
corinnebosley
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Looks good to me. I'm happy with it.
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I'm not happy with this as-is, 'cos I think we have some outstanding problems + we want some more real(er) testing. Given the bug uncovered above, I'm thinking that at least one of NorthPolar/SouthPolar will currently fail a save-then-load roundtrip check. |
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@DPeterK Fancy re-basing? 😄 |
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@bjlittle on it! |
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@pp-mo Do you fancy taking this PR on, and getting across the finish line? |
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Add save support for Grid Definition Template 20 - Polar Stereographic.