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GRIB2
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Question re latitudes and scanning mode in Lambert grids #55

Open shahramn opened 3 years ago

shahramn commented 3 years ago

Regarding the GRIB templates Grid definition template 3.20 – polar stereographic projection Grid definition template 3.30 – Lambert conformal

We have a question regarding the key: La1 - latitude of first grid point

Is this latitude meant to only increase, or can it decrease as well? The reason I ask is the following sentence for the LoV key (in the WMO templates doc):

"LoV is the longitude value of the meridian which is parallel to the y-axis (or columns of the grid) along which latitude increases as the y-coordinate increases (the orientation longitude may or may not appear on a particular grid)."

The phrase in bold is causing us some trouble. We have some GRIBs which have a scanning order of +i -j which different decoders treat differently. In Panoply the latitude always increases (scanning order is ignored) whereas wgrib2 uses the scanning order and the latitude decreases. If it wasn't for that sentence, I would treat the latitude just like other grids and take the scanning order into account but it seems for Lambert specially there are different interpretations and many centres are producing data which conflicts in this regard. Many thanks

efucile commented 3 years ago

Thank you @shahramn This is an interesting question and I hope someone in @wmo-im/tt-tdcf will be able to provide an answer. Otherwise we will need to take the responsibility to choose one interpretation. It looks to me that the scanning order has precedence on this description that is aimed only to describe the geometry.

sebvi commented 3 years ago

We did not discuss this during the meeting but it could be useful to have an active discussion now with possibly some clear recommendation agreed during the December meeting