Could anyone advise me as to whether it is possible to aggregate NetCDF data
using both x and y dimensions using NcML?
Aggregating on a time dimension is well covered in the many examples provided
online, but I have struggled to find much about spatial aggregation.
We have a set of NetCDF files which together represent a Land Cover map for
Great Britain at 25m resolution. Each file represents 100km square surface area
as 4000x4000 "x" & "y" grid points - each point on the grid associated with a
single numeric value representing the land cover classification at that
location.
We would like to use NcML aggregation to "stitch" them together so that users
can exploit an OPeNDAP service to access just the spatial area they need. To
test this we are using a subset of 6 of the "tiles" in a 2x3 grid.
Firstly, we aggregated "vertically" (on the "y" dimension) creating two virtual
files each 1 tile wide by 3 tiles tall:
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2";>
<aggregation dimName="y" type="joinExisting">
<netcdf location="../Detail/sd.nc"/>
<netcdf location="../Detail/sj.nc"/>
<netcdf location="../Detail/so.nc"/>
</aggregation>
</netcdf>
And:
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2";>
<aggregation dimName="y" type="joinExisting">
<netcdf location="../Detail/se.nc"/>
<netcdf location="../Detail/sk.nc"/>
<netcdf location="../Detail/sp.nc"/>
</aggregation>
</netcdf>
This seems to work just fine, we get 2 new virtual files each with 4000x12000
grid points. OPeNDAP and WMS work OK, and the NCDump seems to confirm that all
is well - the "y" variable now has 12000 values, and the shape of the LandCover
variable is now 4000 by 12000. So far so good.
Then, a further aggregation attempts to stitch these together on the "x"
dimension:
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2";>
<aggregation dimName="x" type="joinExisting">
<netcdf location="VerticalSlice01.ncml"/>
<netcdf location="VerticalSlice02.ncml"/>
</aggregation>
</netcdf>
The odd thing is that in the single resultant virtual file, the grid points for
the second "half" are not accessible. WMS only shows half the data, and OPeNDAP
gives this message when you attempt to access any "x" grid points beyond the
first 4000:
Error {
code = 3;
message = "Invalid Parameter Exception: DArrayDimension.setProjection: Bad
Projection Request: stop >= size: 5010:4000";
};
Additionally, NCDump for the overall aggregate shows a discrepancy between the
length of the x and y variables (8000 & 12000 respectively, as we would expect)
and the shape of the LandCover variable which is still only 4000x12000.
netcdf
file:W:/thredds/public/LCM/LCM2007/LCM2007_25m/Aggregation/DetailWholeDataset.ncml
{
dimensions:
y = 12000;
x = 8000;
variables:
int transverse_mercator;
:long_name = "coordinate_reference_system";
:grid_mapping_name = "transverse_mercator";
:semi_major_axis = "6377563.396";
:semi_minor_axis = "6356256.910";
:inverse_flattening = "299.3249646";
:latitude_of_projection_origin = "49.0";
:longitude_of_projection_origin = "-2.0";
:false_easting = "400000.0";
:false_northing = "-100000.0";
:scale_factor_at_projection_origin = "0.9996012717";
double y(y=12000);
:long_name = "northing - OSGB36 grid reference";
:standard_name = "projection_y_coordinate";
:units = "m";
:point_spacing = "even";
float LandCover(y=12000, x=4000);
:_FillValue = 255.0f; // float
:long_name = "Land Cover Map 2007 Great Britain 25 m";
:coordinates = "x y";
:valid_min = 0.0f; // float
:valid_max = 23.0f; // float
:grid_mapping = "transverse_mercator";
:missing_value = 255.0f; // float
double x(x=8000);
:long_name = "easting - OSGB36 grid reference";
:standard_name = "projection_x_coordinate";
:units = "m";
:point_spacing = "even";
:title = "Land Cover Map 2007 Great Britain 25 m";
:institution = "Centre for Ecology & Hydrology (CEH) Lancaster";
:source = "Centre for Ecology & Hydrology (CEH) Lancaster";
:reference = "reference";
:description = "Land Cover Map 2007 Great Britain 25 m";
:grid_mapping = "crs";
:history = "history";
:summary = "Land Cover Map 2007 Great Britain 25 m";
:keywords = "Land cover, Great Britain, 25 m";
:date_created = "2013-12-02";
:date_modified = "2013-12-02";
:date_issued = "2013-12-02";
:creator_name = "Simon Wright";
:creator_url = "http://www.ceh.ac.uk/staffwebpages/simonwright.html";;
:creator_email = "smw@xxxxxxxxx<mailto:smw@xxxxxxxxx>";
:geospatial_lon_min = -2.0014688301358983; // double
:geospatial_lat_min = 53.49670173328351; // double
:geospatial_lon_max = -0.4616603484663878; // double
:geospatial_lat_max = 54.38567886302655; // double
:licence = "ECOMAPS research.";
:publisher_name = "Centre for Ecology & Hydrology";
:publisher_url = "http://www.ceh.ac.uk";;
:publisher_email = "enquiries@xxxxxxxxx<mailto:enquiries@xxxxxxxxx>";
:Conventions = "CF-1.6";
:comment = "comment";
}
Any feedback gratefully received!
Thanks.
Peter Vodden
Informatics Data Analyst
Centre for Ecology & Hydrology
Lancaster Environment Centre
Library Avenue, Bailrigg
Lancaster, LA1 4AP, UK
www.ceh.ac.uk<http://www.ceh.ac.uk>
Tel: +44 (0)1524 595 843
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