[Datastream #MVG-412230]: GOES 16 sweep axis
- Subject: [Datastream #MVG-412230]: GOES 16 sweep axis
- Date: Fri, 20 Oct 2017 11:06:08 -0600
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 ID: MVG-412230
Department: Support Python
NOTE: All email exchanges with Unidata User Support are recorded in the Unidata
inquiry tracking system and then made publicly available through the web. If
you do not want to have your interactions made available in this way, you must
let us know in each email you send to us.