Coordinate Systems CF-1.0 Unidata

1
Coordinate SystemsCF-1.0 Unidatas Common Data
Model

• John Caron, Unidata
• Oct 16, 2006

2
Overview

• Summarize netCDF coordinate system models,
  particularly CF and Unidatas Common Data Model
• Show general coordinate systems for all kinds of
  data, not just unstructured grids

3
NetCDF

• Classic NetCDF is
• An interface for array-oriented data access
• Libraries that provide an implementation of the
  interface (with efficient subsetting)
• A machine-independent file format
• Data Structures
• Multidimensional (rectangular) arrays
• Name/value attributes
• Shared, named dimensions

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No Coordinate Systems

• NetCDF data model does not have integrated
  coordinate systems
• no explicit support in the API
• Conventions are needed to georeference
• Neither do other general models like OPeNDAP, HDF
  
• Contrast GRIB, HDF-EOS, other specialized formats

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Coordinate Variables

• dimensions
• lat 64
• lon 128
• variables
• float lat(lat)
• float lon(lon)
• double temperature(lat,lon)

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Coordinate Variables

• One-dimension variable with same name as its
  dimension
• Strictly monotonic values
• No missing values
• The coordinates of a point (i,j,k) is the tuple
  cvar1(i),cvar2(j),cvar3(k)
• Used exclusively by many early Conventions, such
  as COARDS.

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Limitations of 1D Coordinate Variables

• Non lat/lon horizontal grids
• float temperature(y,x)
• float lat(y, x)
• float lon(y, x)
• Trajectory data
• float NKoreaRadioactivity(pt)
• float lat(pt)
• float lon(pt)
• float altitude(pt)
• float time(pt)

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General Coordinates in CF

• float P(y,x)
• Pcoordinates lat lon
• float lat(y, x)
• float lon(y, x)
• float Sr90(pt)
• Sr90coordinates
• lat lon altitude time

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Coordinate Systems (abstract)

• A Coordinate System for a data variable is a set
  of Coordinate Variables2 such that the
  coordinates of the (i,j,k) data point is the
  tuple cvar1(i,j,k),cvar2(i,j,k),cvar3(i,j,k),
• The dimensions of each Coordinate Variable must
  be a subset of the dimensions of the data
  variable.

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Examples
float gridData(t,z,y,x) float time(t) float
y(y) float x(x) float lat(y,x) float
lon(y,x) float height(t,z,y,x)
float radialData(radial, gate) float
distance(gate) float azimuth(radial) float
elevation(radial) float time(radial)
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Examples (2)
float stationObs(pt) float lat(pt) float
lon(pt) float z(pt) float time(pt)
float trajectory(pt) float lat(pt) float
lon(pt) float z(pt) float time(pt)
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Coordinate Systems (2)

• Specify Coordinate Variables
• Specify Coordinate Types
• (e.g. lat, lon, time, cartesian x, y, polar
  coordinates etc.)
• Specify connectivity (implicit or explicit)
  between data points
• Implicit Neighbors in index space are
  (connected) neighbors in coordinate space. Allows
  efficient searching.

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Gridded Data

• Cartesian coordinates
• All dimensions are connected

float gridData(t,z,y,x) float time(t) float
y(y) float x(x) float lat(y,x) float
lon(y,x) float height(t,z,y,x)

• Connected means
• Neighbors in index space are neighbors in
  coordinate space

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Unstructured Grid

• Node dimension not connected
• Need to specify the connectivity

float unstructGrid(node) float lat(node)
float lon(node)
float unstructGrid(t,z,node) float lat(node)
float lon(node) float time(t) float
height(z)
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Representations of Connectivity

• Assume need efficient rendering
• Do you need efficient searching?
• Given a coordinate, find surrounding points
• Persistence format may not satisfy complex needs

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CF Status

• Encodes general Coordinate Systems
• Encodes some (not all) Coordinate Types
• Connectivity is not generally clear
• No current standard for unstructured grids
• Units, climate data conventions
• Standard names controlled vocabulary for
  physical quantities
• Governance, active committees, new web site,
  actual to continue work.

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NetCDF-Java Library

• 100 Java library
• Prototype implementation of Common Data Model
• decodes Conventions, presents Coordinate System
  API
• Scientific Data Type APIs add specialized
  subsetting like find data in bounding box
• File formats
• General NetCDF, HDF5, OPeNDAP
• Grids GRIB1, GRIB2
• Radar NEXRAD, NIDS, DORADE
• Satellite DMSP, GINI
• Point BUFR

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Common Data Model
Unstructured Grid
Coordinate Systems
Data Access
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Summary

• Understanding of Coordinate Systems is still
  evolving
• Connectivity needs to be described
• CF is a good container for an unstructured grid
  convention
• governance and support
• NetCDF Java library/Common Data Model is
  Unidatas prototyping platform
• Possible transfer of functionality to cflib C
  library