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[Datastream #MVG-412230]: GOES 16 sweep axis


I'd seen that diagram and discussion from Proj4 before, and I find it a bit 
confusing. My take on it is this:

GOES (all of them) have the outer gimbal rotating about the x-axis 
(sweep_angle='x'), with the inner gimbal then rotating around the y-axis. This 
implies that the inner gimbal is then scanning lines, with the outer gimbal 
then controlling the position of the line along the y-axis. So it's my 
impression that GOES data are working on a line-by-line basis.


> I am writing you in a matter related to GOES 16 L1b data.  We are getting
> it from CLASS and and according to PDU this data is in geostationary
> projection.  (ABI fixed grid in radians). So far so good,  the navigation
> works well, however when I tried to visualize it using Basemap I ran
> into some problems.
> More precisely, when I overlayed the coastlines I realized there is some
> spatially varying misalignment between the Image and the coastlines. The
> same was not valid if the data was in latlon projection wich led me to
> believe it is a Basemap issue. I ask for some guidance and a guy (David
> Hoesse) form u Wisc wrote it is probably because of sweep axis option
> in proj4, pyproj, basemap I am getting this issues, as the default value
> in proj4/proj is y for this argument
> I tested the reprojection of a point using the same projections except
> sweep angle axis and i can confirm the misalignment  is caused by this.
> Then i stumbled on the page http://proj4.org/projections/geos.html
> where I read that OLD GOES instruments (pre 16) use y as their fixed or
> sweep angle axis while Meteosat uses x as its fixed angle axis.
> According to my understanding, if GOES(8-15) use y axis the scanning
> should work on a column  basis (one column of image pixels at a time)
> which means the scan time varies by column not by line. However I know
> for sure from my gvar reader implementation the scan time of individual
> pixels is a function of  pixels line (1 scan swath is 8 visible lines and
> produces 10 blocks, 8 for the visible lines and 4 for the ir lines). So
> I don't understand how can GOES 8-15 instruments use y axis as their
> sweep angle axis.

Ticket Details
Ticket ID: MVG-412230
Department: Support Python
Priority: Normal
Status: Closed
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