Dear all,
This message is to revive (part of) a short-lived thread in the last days of May 2011. Jonathan was kind enough to answer some of my questions then, but it never ended in a definite solution to my problem, thus the need (for me) to revive the subject.
I have a (satellite product) data field which is a land-surface classification. That means for each cell in my grid, I have a single "area type": "open water", "first year ice", "multi year ice", "land", "unclassified", and "no data". For those interested, an example (picture of) product is accessible here (with color scale if you zoom-in)
http://osisaf.met.no/p/ice/nh/type/imgs/OSI_HL_SAF_201109211200_pal.jpg
What is the "CF" way of storing such a dataset, and what is the associated standard_name? In the latest version of the CF document, I found a quite in-depth description of how to specify statistics per area-type inside a grid cell, and I suspect I must use the "area_type" mechanism. But when I assign an area_type to my cell, it means that "the cell is mostly covered with area_type" (and I am not guessing the area fraction).
1) When it comes to the definition of my "area_type", do I have to use the CF standard ones? I understand it is best, but on the other hand, the strings entering "flag_meanings" are not standardized in any way, yet are usefull information that a human can take advantage of, and that a machine can easily use to create a colored map, and associated legend.
2) Still, I am open to defining them:
"open_water" will probably be changed to "ice_free_sea" (already in the standard table), "first_year_sea_ice" and "multi_year_sea_ice" might be a bit tricky to define (is it "sea ice that survived a summer melting" or "sea ice whose age is larger than 1 year"?). "land" is already in the standard table. What happens then to "ambiguous" (we tried to estimate the type of sea ice, but failed: we cannot decide between first_year and multi_year). This might also be interpreted as "even mixture of several (but not all) area types". Finally "unclassified" and "no data" is where we do not have sufficient data or confidence to even start the classification: we know up-front the result will be too uncertain.
3) would the following 2 datasets be accepted (omitting the dimension and grid_mapping definition)?
byte sea_ice_type(time, xc, yc):
sea_ice_type:standard_name = "area_type" ;
sea_ice_type:long_name = "sea ice type classification" ;
sea_ice_type:_FillValue = -1b ;
sea_ice_type:valid_min = 1b ;
sea_ice_type:valid_max = 4b ;
sea_ice_type:area_type_values = 1b, 2b, 3b, 4b ;
sea_ice_type:area_type_meanings = "ice_free_sea first_year_sea_ice multi_year_sea_ice ambiguous" ;
byte sea_ice_type_qflags(time, xc, yc):
sea_ice_type_qflags:standard_name = "sea_ice_type status_flag";
sea_ice_type_qflags:_FillValue = -1b ;
sea_ice_type_qflags:valid_min = 0b ;
sea_ice_type_qflags:valid_max = 102b ;
sea_ice_type_qflags:flag_values = 0b, 100b, 101b, 102b ;
sea_ice_type_qflags:flag_meanings = "nominal_quality no_data unclassified land" ;
The first dataset is inspired by CF "flags" (with <x>_values and <x>_meanings, <x> being "area_type"). The first one documents where a sea_ice_type is actually defined (that is where we have data, and where we are not over (or close to) land). The second one is a regular CF "status_flag" describing the quality of the classification in each cell (and explain why there is no valid classification in some cells).
4) if the former is not accepted, does CF have a standard way to storing these classifications? Please direct me to the appropriate section in the doc, and sorry I missed that one.
5) if my proposal is ok, then should we define the two attributes "area_type_values" and "area_type_meanings"? Maybe they could be a generalization of <x>_values / <x>_meanings, along with "flag_values" / "flags_meanings"?
6) Is "area_type" truely the standard_name I am going for my first dataset? Maybe something like "sea_ice area_type" ("area_type" as a standard_name modifier) is what I want?
Thank you for reading so far in my question! I hope you can help me define how my file should look like before it is "released" (and later more difficult to amend). Hopefully, this will also convert in a CF-standard way of handling surface classification (if it is currently missing, that is).
All the best,
Thomas
Received on Thu Sep 22 2011 - 01:52:50 BST
