Dear Maarten
On Mon, Jan 24, 2011 at 02:58:04PM +0100, plieger wrote:
> I am wondering whether anyone might know a good name for per pixel time
> offset in MSG data.
>
> A complete meteosat full disk scan takes 25 minutes, because the imager
> scans the disk line by line. This means that each line of pixels is
> observed at consecutive times. The first pixels have a time offset of
> zero, while the last pixels are measured several minutes later.
>
> We would like to make this information available in a standard way. We
> would like to define a 2D (x,y) variable field in which the time of
> observation is stored for each pixel (relative to the start of the scan,
> starting at zero).
This kind of issue has come up more than once before and CF doesn't have a
solution for it explicitly at the moment. The simplest thing to do would be
what you suggest, to define a standard name for a time offset. You suggest
> - pixel_delta_time (s)
> - time_offset_of_observation (s)
and I would suggest a more general time_offset (s), which could be used for
more purposes (not scanned data, not observational). I can imagine that similar
names might some time be needed for other coordinates e.g. longitude_offset.
I think offsets are different geophysical quantities, because time and
longitude have an absolute reference, whereas time_offset and longitude_offset
do not.
In this simple scheme, it would be assumed that a coordinate or auxiliary
coordinate variable with standard name of time_offset could be added to a
coord or aux coord var with standard name of time, I suppose. Would that work
for you?
However, we could also think about defining a new convention specifically for
this, for example with an attribute that explicitly said what could be added
together e.g.
float obs(scan,pixel); // data variable
coordinates="pixeltime";
double scan(scan); // time of scan start
scan:units="seconds since 2011-1-31";
scan:standard_name="time";
double pixeltime(scan,pixel);
pixeltime:units="seconds";
pixeltime:standard_name="time_offset";
pixeltime:relative_to="time";
where the relative_to attribute says that pixeltime can be added to the var
with name time. A scheme like this could support multiple offsets without
confusion. However, we shouldn't add complexity we don't need to the standard.
What do you think?
Best wishes
Jonathan
_______________________________________________
CF-metadata mailing list
[email protected]
http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
