NSIDC Projections

A repo containing projection and grid definitions for projections and grids used by NSIDC. Projections and grid definitions are in a Grid class. The Grid class has pyproj.CRS and affine.Affine objects as attributes that contain projection and grid definitions. The Grid class has methods to generate cartopy.crs objects, projected coordinates for each grid, and 2D latitude and longitude arrays.

Methods for gdal crs and rasterio srs will be added soon.

Gridded data hosted by NSIDC are in a number of projections and grids. These projections and grids are used widely by the Cryospheric community. However, projections are not always the standard projections available in software packages. Moreover, projection definitions are sometimes hard to find. Defining projections in cartopy or rasterio is not always straight forward.

This repo is an attempt to solve these problems.

NSIDC Projections

Gridded data at NSIDC are most commonly archived on two families of projections: Polar Stereographic projections and Equal Area Scaleable Earth (EASE) grids. EASE-Grids are defined on north and south Lambert's equal-area, azimuthal projections for polar regions, and cylindrical equal-area projections for temperate regions and global data. Both Polar Stereographic and EASE-Grids are available at variety of grid resolutions.

The original EASE-Grids are defined on a spherical horizontal datum. An EASE-Grid version 2 was defined using the WGS84 ellipsoid as the horizontal datum to accomodate use of data in GIS packages. GIS software such as ArcGIS assumes all projections use WGS84 as a horizonal datum.

NSIDC's Polar Stereographic projection differs from standard polar stereographic projections used by cartopy etc in that the latitudes of true scale are 70 N and 70 S, not 90 N and 90 S used in standard definitions. This is done to reduce distortion of the grid in the marginal ice zones.

More information about EASE-Grid https://nsidc.org/data/ease More information about Polar Stereographic data https://nsidc.org/data/polar-stereo

More information on projections and grids can be found at https://nsidc.org/support/how/Points-Pixels-Grids-and-Cells-A-Mapping-and-Gridding-Primer

Projection	Horizontal Datum	EPSG Code	Projection	proj4 string
EASE-Grid North	1924 Authalic Sphere	3408	Lambert Azimuthal	+proj=laea +lat_0=90 +lon_0=0 +x_0=0 +y_0=0 +a=6371228 +b=6371228 +units=m +no_defs
EASE-Grid South	1924 Authalic Sphere	3409	Lambert Azimuthal	+proj=laea +lat_0=-90 +lon_0=0 +x_0=0 +y_0=0 +a=6371228 +b=6371228 +units=m +no_defs
EASE-Grid Global	1924 Authalic Sphere	3410	Cylindrical Equal-Area	+proj=cea +lon_0=0 +lat_ts=30 +x_0=0 +y_0=0 +a=6371228 +b=6371228 +units=m +no_defs
EASE-Grid 2.0 North	WGS84	6931	Lambert Azimuthal	+proj=laea +lat_0=90 +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
EASE-Grid 2.0 South	WGS84	6932	Lambert Azimuthal	+proj=laea +lat_0=-90 +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
EASE-Grid 2.0 Global	WGS84	6933	Cylindrical Equal-Area	+proj=cea +lon_0=0 +lat_ts=30 +x_0=0 +y_0=0 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
Polar Stereographic North	Hughs 1980	3411	Stereographic	+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m +no_defs
Polar Stereographic South	Hughs 1980	3412	Stereographic	+proj=stere +lat_0=-90 +lat_ts=-70 +lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m +no_defs
WGS84 Polar Stereographic North	WGS84	3413	Stereographic	+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs
WGS84 Polar Stereographic South	WGS84	3976	Stereographic	+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs

Getting Started

To import a grid definition

from nsidc_projections.grid import EASEGrid2North25km

A grid definition can be printed

print(EASEGrid2North25km)

Grid object
name: EASE-Grid 2.0 North 25 km
EPSG: 6931
rows: 720
cols: 720
cell width: 25000.0
cell height: -25000.0
upper-left x: -9000000.0
upper-left y: 9000000.0

The Coordinate Reference System can be seen with

EASEGrid2North25km.crs

<Projected CRS: EPSG:6931>
Name: WGS 84 / NSIDC EASE-Grid 2.0 North
Axis Info [cartesian]:
- X[south]: Easting (metre)
- Y[south]: Northing (metre)
Area of Use:
- name: Northern hemisphere.
- bounds: (-180.0, 0.0, 180.0, 90.0)
Coordinate Operation:
- name: US NSIDC EASE-Grid 2.0 North
- method: Lambert Azimuthal Equal Area
Datum: World Geodetic System 1984 ensemble
- Ellipsoid: WGS 84
- Prime Meridian: Greenwich

This is a proj CRS class, so has all the methods including to_wkt, to_proj4, to_dict.

A CF-compliant SRS definition can also be generated with to_cf.

A cartopy CRS can also be generated

EASEGrid2North25km.to_cartopy()

<Projected CRS: +proj=laea +datum=WGS84 +lon_0=0 +lat_0=90 +x_0=0  ...>
Name: unknown
Axis Info [cartesian]:
- E[south]: Easting (metre)
- N[south]: Northing (metre)
Area of Use:
- undefined
Coordinate Operation:
- name: unknown
- method: Lambert Azimuthal Equal Area
Datum: World Geodetic System 1984
- Ellipsoid: WGS 84
- Prime Meridian: Greenwich

In a Jupyter notebook, this produces a thumbnail of the map projection. cartopy.epsg(epsg_code) is also available but the map extent for azimuthal projections is not handled correctly. The to_cartopy method in nsidc_projections gives the correct map extent for NSIDC azimuthal projections.

Grid Methods

An important feature of the nsidc_projections package is that it does not only provides CRS definitions for NSIDC CRS but also encapsulates grid definitions for the commonly used grids.

The external boundaries of a grid are returned

EASEGrid2North25km.grid_bounds()

[(-9000000.0, 9000000.0),
 (9000000.0, 9000000.0),
 (9000000.0, -9000000.0),
 (-9000000.0, -9000000.0)]

x and y grid coordinates are returned

EASEGrid2North25km.get_coordinates()

array([-9.02430900e+06, -8.99924147e+06, -8.97417395e+06, -8.94910642e+06,
       -8.92403890e+06, -8.89897138e+06, -8.87390385e+06, -8.84883632e+06,
       -8.82376880e+06, -8.79870127e+06, -8.77363375e+06, -8.74856622e+06,
       <snip>
         8562500.,  8587500.,  8612500.,  8637500.,  8662500.,  8687500.,
         8712500.,  8737500.,  8762500.,  8787500.,  8812500.,  8837500.,
         8862500.,  8887500.,  8912500.,  8937500.,  8962500.,  8987500.]),
 array([ 8987500.,  8962500.,  8937500.,  8912500.,  8887500.,  8862500.,
         8837500.,  8812500.,  8787500.,  8762500.,  8737500.,  8712500.,
         8687500.,  8662500.,  8637500.,  8612500.,  8587500.,  8562500.,
         8537500.,  8512500.,  8487500.,  8462500.,  8437500.,  8412500.,
       <snip>
       -8712500., -8737500., -8762500., -8787500., -8812500., -8837500.,
       -8862500., -8887500., -8912500., -8937500., -8962500., -8987500.]))

andypbarrett/nsidc_projections

NSIDC Projections

NSIDC Projections

Getting Started

Grid Methods