Re: Station data/Trajectory and grid_mapping

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.