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Fetching data from THREDDS

This is a draft, expect sudden changes and come back later

tags: guide - thredds

This document is a brief introduction in how to extract and visualize data from THREDDS using the OPeNDAP protocol.

For more information, please see the section on “Data Access” in the Numerical Weather Prediction wiki.

THREDDS also supports the Web Map Service protocol for interactive raster images. For a quick guide, see the WMS page.

OPeNDAP

OPeNDAP provides a way for researchers to access scientific data anywhere on the Internet, from a wide variety of new and existing programs.

It is used widely in earth-science research settings but it is not limited to that. Using a flexible data model and a well-defined transmission format, an OPeNDAP client can request data from a wide variety of OPeNDAP servers, allowing researchers to enjoy flexibility similar to the flexibility of the web.

For more information, see the OPeNDAP User Guide.

Tools

FIMEX

Fimex is a the File Interpolation, Manipulation and eXtraction library for gridded geospatial data, written by MET Norway in C/C++. It converts between different, extensible dataformats (currently netcdf, NcML, grib1/2 and felt). It enables you to change the projection and interpolation of scalar and vector grids. It makes it possible to subset the gridded data and to extract only parts of the files.

For simple usage, Fimex comes also with the command line program fimex.

For information, as well as download links, see the FIMEX wiki page.

ncdump

The ncdump tool generates an ASCII representation of a netCDF file on standard output, either with or without an ASCII representation of the variable data in the file. The ASCII representation used is the CDL notation that ncgen accepts as input. Thus ncdump and ncgen can be used as inverses to transform data representation between binary and ASCII representations.

For more information, see the ncdump man page.

Example extraction of metadata:

https://thredds.met.no/thredds/catalog/metpplatest/catalog.html?dataset=metpplatest/met_forecast_1_0km_nordic_latest.nc

$ export CMEPS=https://thredds.met.no/thredds/dodsC/metpplatest/met_forecast_1_0km_nordic_latest.nc

$ ncdump -h $CMEPS
netcdf met_forecast_1_0km_nordic_latest {
dimensions:
    time = UNLIMITED ; // (57 currently)
    x = 1796 ;
    y = 2321 ;
variables:
    double forecast_reference_time ;
        forecast_reference_time:units = "seconds since 1970-01-01 00:00:00 +00:00" ;
        forecast_reference_time:standard_name = "forecast_reference_time" ;
    int projection_lcc ;
        projection_lcc:grid_mapping_name = "lambert_conformal_conic" ;
        projection_lcc:standard_parallel = 63., 63. ;
        projection_lcc:longitude_of_central_meridian = 15. ;
        projection_lcc:latitude_of_projection_origin = 63. ;
        projection_lcc:earth_radius = 6371000. ;
        projection_lcc:proj4 = "+proj=lcc +lat_0=63 +lon_0=15 +lat_1=63 +lat_2=63 +no_defs +R=6.371e+06" ;
    double time(time) ;
        time:long_name = "time" ;
        time:standard_name = "time" ;
        time:units = "seconds since 1970-01-01 00:00:00 +00:00" ;
        time:_ChunkSizes = 1 ;
    double x(x) ;
        x:standard_name = "projection_x_coordinate" ;
        x:units = "m" ;
        x:_ChunkSizes = 1796 ;
    double y(y) ;
        y:standard_name = "projection_y_coordinate" ;
        y:units = "m" ;
        y:_ChunkSizes = 2321 ;
    float altitude(y, x) ;
        altitude:units = "m" ;
        altitude:standard_name = "surface_altitude" ;
        altitude:grid_mapping = "projection_lcc" ;
        altitude:coordinates = "longitude latitude" ;
        altitude:_ChunkSizes = 31, 1796 ;
    float land_area_fraction(y, x) ;
        land_area_fraction:standard_name = "land_area_fraction" ;
        land_area_fraction:grid_mapping = "projection_lcc" ;
        land_area_fraction:coordinates = "longitude latitude" ;
        land_area_fraction:units = "1" ;
        land_area_fraction:_ChunkSizes = 31, 1796 ;
    double latitude(y, x) ;
        latitude:units = "degree_north" ;
        latitude:long_name = "latitude" ;
        latitude:standard_name = "latitude" ;
        latitude:grid_mapping = "projection_lcc" ;
        latitude:_ChunkSizes = 31, 1796 ;
    double longitude(y, x) ;
        longitude:units = "degree_east" ;
        longitude:long_name = "longitude" ;
        longitude:standard_name = "longitude" ;
        longitude:grid_mapping = "projection_lcc" ;
        longitude:_ChunkSizes = 31, 1796 ;
    float air_temperature_2m(time, y, x) ;
        air_temperature_2m:coordinates = "longitude latitude" ;
        air_temperature_2m:units = "K" ;
        air_temperature_2m:standard_name = "air_temperature" ;
        air_temperature_2m:grid_mapping = "projection_lcc" ;
        air_temperature_2m:_ChunkSizes = 1, 31, 1796 ;
    float precipitation_amount(time, y, x) ;
        precipitation_amount:coordinates = "longitude latitude" ;
        precipitation_amount:units = "kg/m^2" ;
        precipitation_amount:standard_name = "precipitation_amount" ;
        precipitation_amount:grid_mapping = "projection_lcc" ;
        precipitation_amount:_ChunkSizes = 1, 31, 1796 ;
    float wind_direction_10m(time, y, x) ;
        wind_direction_10m:coordinates = "longitude latitude" ;
        wind_direction_10m:units = "degrees" ;
        wind_direction_10m:standard_name = "wind_direction" ;
        wind_direction_10m:grid_mapping = "projection_lcc" ;
        wind_direction_10m:_ChunkSizes = 1, 31, 1796 ;
    float wind_speed_10m(time, y, x) ;
        wind_speed_10m:coordinates = "longitude latitude" ;
        wind_speed_10m:units = "m/s" ;
        wind_speed_10m:standard_name = "wind_speed" ;
        wind_speed_10m:grid_mapping = "projection_lcc" ;
        wind_speed_10m:_ChunkSizes = 1, 31, 1796 ;
    float wind_speed_of_gust(time, y, x) ;
        wind_speed_of_gust:coordinates = "longitude latitude" ;
        wind_speed_of_gust:units = "m/s" ;
        wind_speed_of_gust:standard_name = "wind_speed_of_gust" ;
        wind_speed_of_gust:grid_mapping = "projection_lcc" ;
        wind_speed_of_gust:_ChunkSizes = 1, 31, 1796 ;
    float cloud_area_fraction(time, y, x) ;
        cloud_area_fraction:coordinates = "longitude latitude" ;
        cloud_area_fraction:units = "1" ;
        cloud_area_fraction:standard_name = "cloud_area_fraction" ;
        cloud_area_fraction:grid_mapping = "projection_lcc" ;
        cloud_area_fraction:_ChunkSizes = 1, 31, 1796 ;
    float air_pressure_at_sea_level(time, y, x) ;
        air_pressure_at_sea_level:units = "Pa" ;
        air_pressure_at_sea_level:standard_name = "air_pressure_at_sea_level" ;
        air_pressure_at_sea_level:grid_mapping = "projection_lcc" ;
        air_pressure_at_sea_level:coordinates = "longitude latitude" ;
        air_pressure_at_sea_level:_ChunkSizes = 1, 31, 1796 ;
    float integral_of_surface_downwelling_shortwave_flux_in_air_wrt_time(time, y, x) ;
        integral_of_surface_downwelling_shortwave_flux_in_air_wrt_time:coordinates = "longitude latitude" ;
        integral_of_surface_downwelling_shortwave_flux_in_air_wrt_time:units = "W s/m^2" ;
        integral_of_surface_downwelling_shortwave_flux_in_air_wrt_time:standard_name = "integral_of_surface_downwelling_shortwave_flux_in_air_wrt_time" ;
        integral_of_surface_downwelling_shortwave_flux_in_air_wrt_time:grid_mapping = "projection_lcc" ;
        integral_of_surface_downwelling_shortwave_flux_in_air_wrt_time:_ChunkSizes = 1, 31, 1796 ;
    float relative_humidity_2m(time, y, x) ;
        relative_humidity_2m:coordinates = "longitude latitude" ;
        relative_humidity_2m:units = "1" ;
        relative_humidity_2m:standard_name = "relative_humidity" ;
        relative_humidity_2m:grid_mapping = "projection_lcc" ;
        relative_humidity_2m:_ChunkSizes = 1, 31, 1796 ;

// global attributes:
        :institution = "Norwegian Meteorological Institute, met.no" ;
        :creator_url = "met.no" ;
        :geospatial_lat_min = "51.0" ;
        :geospatial_lat_max = "88.0" ;
        :geospatial_lon_min = "-20.0" ;
        :geospatial_lon_max = "80.0" ;
        :references = "unknown" ;
        :license = "http://met.no/sb5-met.no/Forskning/Informasjonsteknologi/Tilgang_til_data/" ;
        :source = "meps" ;
        :summary = "Based on model data from MEPS (MetCoOp-Ensemble Prediction System) and observations from met.no, Netatmo, and Bergensvaeret." ;
        :Conventions = "CF-1.0" ;
        :NCO = "netCDF Operators version 4.7.9 (Homepage = http://nco.sf.net, Code = http://github.com/nco/nco)" ;
        :meps_forecast_reference_time = "1583744400" ;
        :DODS_EXTRA.Unlimited_Dimension = "time" ;
}

—Geir Aalberg, 19 May 2020