Nawajish Noman wrote:
Hi Russ/John,
I am just wondering if you had a chance to take a look at the
following questions. I can also call if you prefer to discuss this
over the phone.
Thanks a lot!
Noman
_________________________
Hi Noman:
Sorry, your previous email got buried under a pile!
Hi Russ/John,
We are making progress here and now working with station data which
becomes point feature class in ArcGIS. But we need to understand how
grid_mapping works with type of data or trajectories.
We understand how grid_mapping works in the following example.
float Temperature(time, y, x);
Temperature:units = "K";
Temperature:long_name = "Temperature @ surface";
Temperature:missing_value = 9999.0;
Temperature:coordinates = "lat lon";
Temperature:grid_mapping = "Lambert_Conformal";
Since grid_mapping exists – we use x and y for coordinate values and
specify spatial reference using grid_mapping. In this case we___
ignore_ the coordinate attribute and don’t use the lat lon values to
draw/create raster. We honor coordinates attribute if the grid_mapping
attribute does not exist
i would probably say this differently, though the result may be the same
you have an x, y coordinate because of the existence of the coordinate
variables x(x) and y(y). The grid_mapping tells you what the projection
function is. therefore, you dont need the lat, lon coordinates
if grid_mapping doesnt exist, or it refers to a projection you dont know
about, then you need the lat, lon coordinate.
Q1: But how does grid_mapping work with a station data structure
described in the section 5.4 or trajectories described in the section
5.5 in the CF conventions?**
(___http://www.cgd.ucar.edu/cms/eaton/cf-metadata/CF-current.html#gmap_)
float humidity(time,pressure,station) ;
humidity:long_name = "specific humidity" ;
humidity:coordinates = "lat lon" ;
I am assuming grid_mapping could be an attribute of humidity and the
coordinate attribute should refer to x and y coordinate variables
instead of lat and lon. Is this a valid assumption? If yes, then how
does one attach lat and lon with this variable?
generally, "coordinates" name coordinate variables that are to be
associated with this variable. the fact that they are named here "lat"
and "lon" means they are probably the latitude and longitude. You cant
assume the names, though, you are suppose to tell longitude because it
has units "degrees_east", and latitude has "degrees_north".
what you do know is that variables "lat" and "lon" are supposed to
exist. You check the units to be sure they are latitude and longitude.
then, for each point humidity(time,pressure,station), you find
lat(time,pressure,station) and lon(time,pressure,station).
Note that its ok if lat, lon not has only a subset of the dimensions
(time,pressure,station)
Q2: Do you have any example of station data that has grid_mapping?
Well, now that we went through all that, I would say that you should not
be using CF for station data yet, because I expect CF to develop those
station data conventions more.
However, if you really want to pursue it, I will ask the CF user group
to see if anyone has sample files.
