© crown copyright met office towards improved netcdf-gis interoperability: potential utility of the...

© Crown Copyright Met Office

Towards improvednetCDF-GIS interoperability:Potential utility of the “Well-Known Model” concept

Phil Bentley, Met Office Hadley Centre


Key Topics

This presentation visits the following topics:

• Problem: The netCDF-GIS disconnect

• Partial Solution: Use of the WKM concept

• Examples of WKM usage

• Conclusions & Further Work


Problem: The netCDF-GIS disconnect


The Scenario


The view from ncdump

netcdf tasmean-1960-90 {dimensions:

time = UNLIMITED; // (1 currently)height = 1;latitude = 73;longitude = 96;bound = 2;

variables:float time(time);float bounds_time(time, bound);float height(height);float bounds_height(height, bound);float latitude(latitude);float bounds_latitude(latitude, bound);float longitude(longitude);float bounds_longitude(longitude, bound);float air_temperature1(time, height, latitude, longitude);

air_temperature1:standard_name = "surface_temperature";air_temperature1:long_name = "Temperature at 1.5M";air_temperature1:units = "K";air_temperature1:cell_methods = "time: mean within years ...";air_temperature1:_FillValue = -1.e+30f;

}


The view from CF Checker

CHECKING NetCDF FILE: tasmean-1960-90-ll.nc=====================Using Standard Name Table Version 7 (2007-11-20T02:00:00Z)------------------Checking variable: time------------------------------------Checking variable: bounds_time------------------------------------Checking variable: height------------------------------------Checking variable: bounds_height------------------------------------Checking variable: latitude------------------------------------Checking variable: bounds_latitude------------------------------------Checking variable: longitude------------------------------------Checking variable: bounds_longitude------------------------------------Checking variable: air_temperature1------------------------------------ERRORS detected: 0WARNINGS given: 0


The view from Panoply


The view from CDAT


The view from ESRI ArcGIS


The view from ESRI Tech Support

Quote paraphrased from ESRI Technical Support email:

“Shift your longitude coordinates half a grid cell to the east”


So what's going on?

• CF convention lacks an attribute for specifying explicitly and unambiguously the spatial data model used by a variable in a netCDF file.

• Is it (the data model) a regular, structured grid?

• Is it an unordered collection of point locations?

• Is it a series of vertical profiles or trajectories?

• Hence, GIS software developers & applications are forced to make inferences from esoteric combinations of CF/netCDF variables and attributes.

• Why do we make this so complicated?!


Partial Solution: Use of the "well-known model" (WKM) concept?


Potential Utility of the Well-Known Model (WKM) Concept

• Mirrors the Well-Known Text (WKT) and Well-Known Binary (WKB) concepts used in e.g. OGC specifications

• Provides a hint to client applications as to how to interpret the spatial data model(s) encoded in a netCDF file.

• WKM values would be based on de facto and de jure geospatial data standards such as…

• OGC/ISO 19107 (spatial schema - aka feature geometry)

• OGC/ISO 19123 (schema for coverage geometry)

• Gridspec (Balaji’s schema for climate model grids)


Proposed WKM Syntax

• New CF metadata attribute called, say, data_model or spatial_data_model

• Used as a global-scope attribute: defines the default spatial data model for a netCDF file, e.g.

:data_model = "continuous coverage: rectified grid"

• Used as a variable-scope attribute: defines the spatial data model for that variable, e.g.

variables:float temp(time, regions);

temp:data_model = "feature_collection: polygons";

• Attribute values would be drawn from a controlled vocabulary, as per CF standard names.


Vector Data Models

• Based on OGC/ISO 19107 standard for spatial schema (aka feature geometry).

• Example values of data_model attribute:

feature_collection: pointsfeature_collection: multipointsfeature_collection: curves/linesfeature_collection: multicurves/linesfeature_collection: polygonsfeature_collection: multipolygons


Coverage/Grid Data Models

• Based upon combination of OGC/ISO 19123 (coverage geometry) and Gridspec standards

• Example values of data_model attribute:

// from OGC/ISO 19123...discrete coverage: pointsdiscrete coverage: curvesdiscrete coverage: surfacescontinuous coverage: thiessen polygon networkcontinuous coverage: hexagonal gridcontinuous coverage: quadrilateral gridcontinuous coverage: rectified gridcontinuous coverage: georectified grid// from GFDL Gridspec...continuous coverage: logically rectangularcontinuous coverage: structured triangularcontinuous coverage: unstructured polygonal


Examples of WKM Usage


Ex 1: Time-series of station data

// default data model for this netCDF file:data_model = "feature_collection: points";

dimensions:stations = 100;time = UNLIMITED;...

variables:float lat(stations);float lon(stations);float time(time);float temp(time, stations);

temp:standard_name = "surface_temperature";temp:coordinates = "lat lon";// following attribute is optionaltemp:data_model = "feature_collection: points";...


Ex 2: Time-series of max wind speed within geographic regions

// default data model for this netCDF file:data_model = "feature_collection: points";

dimensions:regions = 10;verts = 1000;time = UNLIMITED;...

variables:char region_names(regions, 80);int nverts(regions); // num vertices making up each regionfloat lat(verts);float lon(verts);float time(time);float windspeed(time, regions);

windspeed:standard_name = "wind_speed";windspeed:coordinates = "lat lon";windspeed:cell_methods = "lat: lon: maximum";// specific data model for this variablewindspeed:data_model = "feature_collection: polygons";...


Ex 3: 4D Global Model Grid

dimensions:lat = 96;lon = 73;level = 38;time = UNLIMITED;...

variables:float lat(lat);float lon(lon);float level(level);float time(time);float humidity(time, level, lat, lon);

humidity:standard_name = "specific_humidity";// data model based on OGC/ISO 19123...humidity:data_model = "discrete coverage: grid points";// ...or based on Gridspec.humidity:data_model = "discrete coverage: logically rectangular";...


Conclusions

The Well-Known Model concept...

• …exploits existing geospatial standards

• …is intended to act as a simple hint to client applications (and humans too) regarding the spatial data model(s) encoded in a netCDF file

• …is aimed particularly at GIS software community where CF metadata conventions appear to be less familiar / understood

• …should hopefully facilitate better manipulation of netCDF data within GIS applications

• …shouldn't conflict with existing CF attributes and usage


Further Work?

• If WKM seen as a Good Thing then…

• Undertake further analysis of current geospatial data model standards

• Elicit input/feedback from GIS community

• Synthesise ideas from existing geometry models (OGC/ISO, Gridspec, netCDF, CDM)

• Identify/refine an initial list of data_model definitions. Firm up syntax.

• Compile usage examples for commonly encountered netCDF data models

• Submit proposal to CF community


That’s all folks!Q&A

© crown copyright met office towards improved netcdf-gis interoperability: potential utility of the...

Documents

default spatial data

spatial data models

spatial schema

cdatthe view

boundfloat air

panoplythe view

esri arcgisthe view

float timetimefloat