[CF-metadata] MSG CPP standard name for time_offset_of_observation / pixel_delta_time

From: Jonathan Gregory <j.m.gregory>
Date: Mon, 31 Jan 2011 13:36:09 +0000

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
Received on Mon Jan 31 2011 - 06:36:09 GMT