Examples for alternative format where information is attached to subsampled variables

alternative_format/NDVI_grid_mapping_example_compacted.cdl

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+netcdf NDVI_grid_mapping_example_compacted {
+dimensions:
+  // NDVI product grid
+  y = 6 ;
+  x = 5 ;
+  // Tie points (tp)
+  tp_y = 2 ;
+  tp_x = 2 ;
+
+variables:
+  float ndvi(y, x) ;
+    ndvi:standard_name = "normalized_difference_vegetation_index" ;
+    ndvi:subsampled_coordinates = "y x" ;
+    ndvi:grid_mapping = "crs" ;
+
+  int y_indices(tp_y) ;
+    y_indices:subsamples_dimension = "y" ;
+
+  int x_indices(tp_x) ;
+    x_indices:subsamples_dimension = "x" ;
+
+  char crs ;
+    crs:grid_mapping_name = "lambert_conformal_conic" ;
+    crs:standard_parallel = 25.0 ;
+    crs:longitude_of_central_meridian = 265.0 ;
+    crs:latitude_of_projection_origin = 25.0 ;
+
+  char tp_interpolation ;
+    tp_interpolation:standard_name = "bi_linear" ;
+    tp_interpolation:formula_terms = "v1: y v2: x" ;
+
+  float y(tp_y) ;
+    y:standard_name = "projection_y_coordinate" ;
+    y:units = "km" ;
+    y:restore_method = "tp_interpolation" ;
+    y:restore_indices = "tp_y: y_indices" ;
+
+  float x(tp_x) ;
+    x:standard_name = "projection_x_coordinate" ;
+    x:units = "km" ;
+    x:restore_method = "tp_interpolation" ;
+    x:restore_indices = "tp_x: x_indices" ;
+
+data:
+  ndvi =
+    0.5119157, 0.04983568, 0.5414233, 0.3076001, 0.8931185,
+    0.8581991, 0.7848567, 0.2485297, 0.9762608, 0.4546139,
+    0.1063213, 0.8751125, 0.9819403, 0.9346204, 0.2765055,
+    0.2011242, 0.7634977, 0.7657007, 0.3465044, 0.9491135,
+    0.9431587, 0.04104269, 0.5652257, 0.5340118, 0.8907427,
+    0.3514801, 0.1451995, 0.1523716, 0.1563433, 0.7384073 ;
+  y_indices = 0, 5 ;
+  x_indices = 0, 4 ;
+  y = 10, 20 ;
+  x = 10, 30 ;
+}

alternative_format/NDVI_latlon_example_compacted.cdl

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+netcdf NDVI_example_compacted {
+dimensions:
+  // NDVI product grid
+  lat = 6 ;
+  lon = 5 ;
+  // Tie points (tp)
+  tp_lat = 2 ;
+  tp_lon = 2 ;
+
+variables:
+  float ndvi(lat, lon) ;
+    ndvi : standard_name = "normalized_difference_vegetation_index" ;
+
+  int lat_indices(tp_lat) ;
+    lat_indices:subsamples_dimension = "lat" ;
+
+  int lon_indices(tp_lon) ;
+    lon_indices:subsamples_dimension = "lon" ;
+
+  char tp_interpolation ;
+    tp_interpolation:standard_name = "latlon_interpolation" ;
+    tp_interpolation:formula_terms = "lat_var: lat lon_var: lon" ;
+
+  float lat(tp_lat) ;
+    lat:standard_name = "latitude" ;
+    lat:units = "degrees_north" ;
+    lat:restore_method = "tp_interpolation" ;
+    lat:restore_indices = "tp_lat: lat_indices" ;
+
+  float lon(tp_lon) ;
+    lon:standard_name = "longitude" ;
+    lon:units = "degrees_east" ;
+    lon:restore_method = "tp_interpolation" ;
+    lon:restore_indices = "tp_lon: lon_indices" ;
+
+data:
+  ndvi =
+    0.5119157, 0.04983568, 0.5414233, 0.3076001, 0.8931185,
+    0.8581991, 0.7848567, 0.2485297, 0.9762608, 0.4546139,
+    0.1063213, 0.8751125, 0.9819403, 0.9346204, 0.2765055,
+    0.2011242, 0.7634977, 0.7657007, 0.3465044, 0.9491135,
+    0.9431587, 0.04104269, 0.5652257, 0.5340118, 0.8907427,
+    0.3514801, 0.1451995, 0.1523716, 0.1563433, 0.7384073 ;
+  lat_indices = 0, 5 ;
+  lon_indices = 0, 4 ;
+  lat = 10, 20 ;
+  lon = 10, 30 ;
+}

alternative_format/README.md

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+Rationale
+=========
+
+The subsampling/compaction mechanism should have minimal effect on the data
+variables that use subsampled/compacted variables as coordinates.
+
+All the information required to restore the full content of a
+subsampled/compacted variable should be attached to this variable, either
+directly (attributes) or by transitivity (other variables pointed by
+attributes).
+
+Using subsampled/compacted coordinates
+--------------------------------------
+As subsampled/compacted variables are not supported by the standard yet, they
+cannot be included in the "coordinates" attribute of variables (backward
+compatibility). Another attribute, "subsampled\_coordinates" shall be used for
+these subsampled/compacted coordinates variables.
+
+Mapping subsampled and full dimensions
+--------------------------------------
+The relation between subsampled and full dimensions is expressed through
+indices variables. The index values contained in these variables refer to the
+full dimension indicated in the "subsamples\_dimension" attribute.
+
+Methods for reconstructing full variables
+-----------------------------------------
+A container variable (no data, only attributes) is used to describe methods to
+reconstruct full data from subsampled/compacted variables.
+
+These container variables have one mandatory attribute: "standard\_name".
+The value of this attribute is a string that identifies the reconstruction
+method. The CF conventions must provide a detailed description of the algorithm
+attached to each available method.
+
+If the method accepts parameters, they must be provided with the
+"formula\_terms".
+
+See Annex D for similar examples already in CF conventions:
+http://cfconventions.org/Data/cf-conventions/cf-conventions-1.7/build/apd.html
+
+Reconstructing fuil variables
+-----------------------------
+Subsampled/compacted variables need two attributes to provides the means for
+reconstructing their full content:
+ * "restore\_method": this attribute must point to the container variable that
+                     describes the reconstruction method.
+ * "restore\_indices": this attribute must associate each dimension of the
+                      subsampled variable with an indices variable.
+
+In case additional parameters must be passed to the reconstruction method, they
+should be provided with the "restore\_terms" attribute.
+
+Extra case: shared subsampled coordinates at multiple resolutions
+-----------------------------------------------------------------
+Subsampled/compacted variables as they are described above can only be used as
+coordinates for data variables that share the same full dimensions.
+
+In case subsampled/compacted coordinates should be shared by data variables
+with different full coordinates, i.e. if reconstruction parameters are specific
+to the data variables, then the data variable should have an additional
+attribute, "restore\_indices\_override", to define its own mapping between
+subsampled coordinates and indices variables.
+
+See the m\_radiance variable in  viirs\_m\_and\_i\_band\_example.cdl

alternative_format/multiband_profile_swath.cdl

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+netcdf multiband_profile_swath {
+dimensions:
+	track = 5200 ;
+	scan = 3600 ;
+	band = 3 ;
+	press = 2 ;
+	tp_track = 250 ;
+	tp_scan = 600 ;
+variables:
+	float swath_data(track, scan, press, band) ;
+		swath_data:coordinates = "time" ;
+		swath_data:subsampled_coordinates = "lat lon" ;
+
+	float band(band) ;
+		band:standard_name = "sensor_band_central_radiation_wavenumber" ;
+		band:units = "cm-1" ;
+
+	float press(press) ;
+		press:standard_name = "air_pressure" ;
+		press:units = "Pa" ;
+		press:positive = "up" ;
+
+	double time(track, scan) ;
+		time:standard_name = "time" ;
+		time:units = "<units> since <datetime string>" ;
+		time:calendar = "gregorian" ;
+
+	float lat(tp_track, tp_scan) ;
+		lat:standard_name = "latitude" ;
+		lat:units = "degrees_north" ;
+		lat:restore_method = "restore" ;
+        lat:restore_indices = "tp_track: sub_track tp_scan: sub_scan" ;
+
+	float lon(tp_track, tp_scan) ;
+		lon:standard_name = "longitude" ;
+		lon:units = "degrees_east" ;
+		lon:restore_method = "restore" ;
+        lon:restore_indices = "tp_track: sub_track tp_scan: sub_scan" ;
+
+	int sub_track(tp_track) ;
+		sub_track:subsamples_dimension = "track" ;
+
+	int sub_scan(tp_scan) ;
+		sub_scan:subsamples_dimension = "scan" ;
+
+	char restore ;
+		restore:standard_name = "lonlat_interpolation" ;
+		restore:formula_terms = "v1: lon v2: lat v3: 360" ;
+}

alternative_format/viirs_m_and_i_band_example.cdl

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+netcdf VIIRS_M_and_I_Band_example_compacted {
+dimensions:
+  // VIIRS M-Band (750 m resolution imaging) 
+  m_track = 768 ;
+  m_scan = 3200 ;
+  m_channel = 16 ;
+
+  // VIIRS I-Band (375 m resolution imaging)
+  i_track = 1536 ;
+  i_scan = 6400 ;
+  i_channel = 5 ;
+
+  // Tie points and interpolation zones (shared between VIIRS M-Band and I-Band)
+  tp_track = 96 ;
+  tp_scan = 205 ;
+  track_interpolation_zone = 48 ;
+  scan_interpolation_zone = 200 ;
+
+  // Time, stored at scan-start and scan-end of each scan
+  time_scan = 2 ;
+
+variables:
+
+  // VIIRS M-Band 
+  float m_radiance(m_track, m_scan, m_channel) ;
+    m_radiance:subsampled_coordinates = "lat lon time sen_azi_ang sen_zen_ang sol_azi_ang sol_zen_ang" ;
+	m_radiance:restore_indices_override = "tp_track: m_track_indices tp_scan: m_scan_indices" ;
+
+  int m_track_indices(tp_track) ;
+	m_track_indices:subsamples_dimension = "m_track" ;
+
+  int m_scan_indices(tp_scan) ;
+	m_scan_indices:subsamples_dimension = "m_scan" ;
+
+  // VIIRS I-Band 
+  float i_radiance(i_track, i_scan, i_channel) ;
+	i_radiance:subsampled_coordinates = "lat lon time sen_azi_ang sen_zen_ang sol_azi_ang sol_zen_ang" ;
+
+  int i_track_indices(tp_track) ;
+	i_track_indices:subsamples_dimension = "i_track" ;
+
+  int i_scan_indices(tp_scan) ;
+	i_scan_indices:subsamples_dimension = "i_scan" ;
+
+
+  // Tie points
+  float lat(tp_track, tp_scan) ;
+    lat:standard_name = "latitude" ;
+    lat:units = "degrees_north" ;
+	lat:restore_method = "tp_interpolation" ;
+	lat:restore_indices = "tp_track: i_track_indices tp_scan: i_scan_indices" ;
+	lat:restore_terms = "lat_var: lat lon_var: lon" ;
+
+  float lon(tp_track, tp_scan) ;
+    lon:standard_name = "longitude" ;
+    lon:units = "degrees_east" ;
+	lon:restore_method = "tp_interpolation" ;
+	lon:restore_indices = "tp_track: i_track_indices tp_scan: i_scan_indices" ;
+	lon:restore_terms = "lat_var: lat lon_var: lon" ;
+
+  float sen_azi_ang(tp_track, tp_scan) ;
+    sen_azi_ang:standard_name = "sensor_azimuth_angle" ;
+    sen_azi_ang:units = "degrees" ;
+	sen_azi_ang:restore_method = "tp_interpolation" ;
+	sen_azi_ang:restore_indices = "tp_track: i_track_indices tp_scan: i_scan_indices" ;
+	sen_azi_ang:restore_terms = "sen_azi_var: sen_azi_ang sen_zen_var: sen_zen_ang" ;
+
+  float sen_zen_ang(tp_track, tp_scan) ;
+    sen_zen_ang:standard_name = "sensor_zenith_angle" ;
+    sen_zen_ang:units = "degrees" ;
+	sen_zen_ang:restore_method = "tp_interpolation" ;
+	sen_zen_ang:restore_indices = "tp_track: i_track_indices tp_scan: i_scan_indices" ;
+	sen_zen_ang:restore_terms = "sen_azi_var: sen_azi_ang sen_zen_var: sen_zen_ang" ;
+
+  float sol_azi_ang(tp_track, tp_scan) ;
+    sol_azi_ang:standard_name = "solar_azimuth_angle" ;
+    sol_azi_ang:units = "degrees" ;
+	sol_azi_ang:restore_method = "tp_interpolation" ;
+	sol_azi_ang:restore_indices = "tp_track: i_track_indices tp_scan: i_scan_indices" ;
+	sol_azi_ang:restore_terms = "sol_azi_var: sol_azi_ang sol_zen_var: sol_zen_ang" ;
+
+  float sol_zen_ang(tp_track, tp_scan) ;
+    sol_zen_ang:standard_name = "solar_zenith_angle" ;
+    sol_zen_ang:units = "degrees" ;
+	sol_zen_ang:restore_method = "tp_interpolation" ;
+	sol_zen_ang:restore_indices = "tp_track: i_track_indices tp_scan: i_scan_indices" ;
+	sol_zen_ang:restore_terms = "sol_azi_var: sol_azi_ang sol_zen_var: sol_zen_ang" ;
+
+  // Reusable interpolation containers for space and time, shared by VIIRS M-Band and I-Band
+  char tp_interpolation ;
+    tp_interpolation:standard_name = "bi_quadratic" ;
+    tp_interpolation:formula_terms = "ex_track: expansion_coefficient_track al_track: alignment_coefficient_track ex_scan: expansion_coefficient_scan al_scan: alignment_coefficient_scan flags: interpolation_zone_flags" ;
+
+  // Interpolation coefficients and flags
+  short expansion_coefficient_track(track_interpolation_zone, tp_scan) ;
+  short alignment_coefficient_track(track_interpolation_zone, tp_scan) ;
+  short expansion_coefficient_scan(tp_track, scan_interpolation_zone) ;
+  short alignment_coefficient_scan(tp_track, scan_interpolation_zone) ;
+  byte interpolation_zone_flags(track_interpolation_zone, scan_interpolation_zone) ;
+    interpolation_zone_flags:valid_range = "1b, 7b" ;
+    interpolation_zone_flags:flag_masks = "1b, 2b, 4b" ;
+    interpolation_zone_flags:flag_meanings = "location_use_cartesian  sensor_direction_use_cartesian  solar_direction_use_cartesian" ;
+
+  // Time
+  double t(tp_track, time_scan) ;
+    t:long_name = "time" ;
+    t:units = "days since 1990-1-1 0:0:0" ;
+	t:restore_method = "time_interpolation" ;
+	t:restore_indices = "tp_track: i_track_indices" ;
+
+  char time_interpolation ;
+    time_interpolation:standard_name = "linear" ;
+    time_interpolation:formula_terms = "range_var: t" ;
+
+}