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A-CAP Open Geospatial Consortium (OGC) Services

Overview

Open Geospatial Consortium (OGC) services allow users to access A-CAP maps and source data using their software or client of choice. These services provide an interoperable framework so that users can easily compile information from a variety of sources.

Users can access source data either directly through a URL string, or using an OGC-compatible client such as Gaia 3, MapServer, ArcGIS 9 Service Pack 2, ENVI Zoom 4.3.1, Google Earth 4, NASA World Wind, Quantum GIS (QGIS), GRASS, and uDig.

This Web page describes how to use OGC services, including Web Map Service (WMS), Web Feature Service (WFS), and Web Coverage Service (WCS), to access A-CAP images and data. For more information on OGC parameters, visit the OGC Web site.

Table 1. OGC Specifications
Web Map Service (WMS) 1.1.1 Provides map and legend images of selected data layers and basemaps.
Web Feature Service (WFS) 1.1.0 Provides vector source data (points, lines, and polygons) in Geography Markup Language (GML) format.
Web Coverage Service (WCS) 1.1.1 Provides raster source data (rectangular grids of data values) in GeoTIFF format.

Table of Contents:

  1. GetCapabilities Request
  2. Web Map Service (WMS)
  3. Web Feature Service (WFS)
  4. Web Coverage Service (WCS)

1. GetCapabilities Request

An OGC GetCapabilities request provides remote access to data through an OGC-compatible client. A GetCapabilities request produces a document that lists and describes each of the available layers for its respective OGC service, in Extensible Markup Language (XML). This document provides an OGC-compatible software client with all of the information necessary to access source data, maps, or legend images. The following examples show GetCapabilities requests for A-CAP, using WMS, WFS, and WCS Services.

Table 2. OGC GetCapabilities Request Example URL's
WMS http://nsidc.org/cgi-bin/acap.pl?service=WMS&request=GetCapabilities&version=1.1.1
WFS http://nsidc.org/cgi-bin/acap.pl?service=WFS&request=GetCapabilities&version=1.1.0
WCS http://nsidc.org/cgi-bin/acap.pl?service=WCS&request=GetCapabilities&version=1.1.1

2. Web Map Service (WMS)

OGC GetMap and GetLegendGraphic requests provide remote access to map and legend images. These requests can be customized using various parameters, including those listed in the tables below. For complete documentation on creating GetMap requests, please visit the OGC WMS specification (version 1.1.1) Web page. For complete documentation on creating GetLegendGraphic requests, please visit the OGC Styled Layer Descriptor (SLD) specification Web page.

GetMap:

Table 3. Parameters of a GetMap Request
Parameter Possible Values Description
service WMS Web Map Service
version 1.1.1 Request version, currently 1.1.1.
request GetMap Request name.
srs Southern Hemisphere: Spatial reference system. This must be specified as an EPSG code. These codes were developed and maintained by the European Petroleum Survey Group (EPSG) and are a shorthand way of specifying a map projection and other coordinate parameters. The web interface for A-CAP employs EPSG:3031.
EPSG:3031 WGS 84 / Antarctic Polar Stereographic
EPSG:3409 NSIDC EASE-Grid South
EPSG:3412 Hughes / NSIDC Polar Stereographic South
EPSG:3974 WGS 84 / NSIDC EASE-Grid South
EPSG:3976 WGS 84 / NSIDC Sea Ice Polar Stereographic South
EPSG:32761 WGS 84 / UPS South
Global:
EPSG:3410 NSIDC EASE-Grid Global
EPSG:3975 WGS 84 / NSIDC EASE-Grid Global
EPSG:4326 WGS 84 / Latlong
format image/gif Output format of requested map.
image/jpeg
image/png
width 500 Width and height of requested map, in pixels. Maximum allowable value is 2000 pixels. For a polar projection (EPSG:3031), users will likely want a square image and will set the width and height to the same value (e.g. 500). For a geographic projection (EPSG:4326) users can set the width to double the value of the height (e.g. 800 and 400) for a global map.
height 500
bbox -12500000,-12500000,12500000,12500000 Bounding box corners—lower left (min x, min y) and upper right (max x, max y)—in selected SRS units: degrees for EPSG:4326 or meters for all other EPSG codes. For full hemispheric coverage, view the projection information provided in the links above to each of the available EPSG codes.
-10700000,-10700000,14700000,14700000
-180,-90,180,90
layers sea_ice_extent_01,moa,coastlines,south_pole_geographic Comma-separated list of one or more map layers as they are named in the WMS GetCapabilities request document. Note that the order in which the layers are listed matters: the second layer in the list will overlay the first layer in the list and so on.
transparent* true Optional: Background transparency of the requested map (default = false). If transparency is not set to true, a background color defined by the web map server will be displayed in the requested map.
false
bgcolor* 0x000787 Optionl: Hexadecimal red-green-blue color value for the background color (default=0xFFFFFF) of the requested map if the default is not desired.
*Optional Parameter

How to construct a GetMap request using the above parameters:

Example. A large image (1000 x 1000 pixels) of several cryospheric parameters in a polar stereographic projection focused on Antarctica:

http://nsidc.org/cgi-bin/acap.pl?
service=WMS&
version =1.1.1&
request =GetMap&
srs=EPSG:3031&
format=image/png&
width=1000&
height=1000&
bbox=-3000000,-3000000,3000000,3000000&
layers=sea_ice_concentration_02,glas_dem,coastlines,geographic_features_ice,geographic_features_sea,south_pole_geographic

GetLegendGraphic:

Table 4. Parameters of a GetLegendGraphic Request
Parameter Possible Values Description
service WMS Web Map Service
version 1.1.1 Request version, currently 1.1.1.
request GetLegendGraphic Request name.
format image/gif Output format of requested legend.
image/jpeg
image/png
layer ramp_dem_graded Layer for which to produce a map legend, as named in the WMS GetCapabilities request document.
sea_ice_concentration_01
antarctic_circle

How to construct a GetLegendGraphic request using the above parameters:

Example. A legend for a graded map of Antarctic surface elevation from the GLAS DEM:

http://nsidc.org/cgi-bin/acap.pl?
service=WMS&
version=1.1.1&
request=GetLegendGraphic&
format=image/gif&
layer=glas_dem_graded

3. Web Feature Service (WFS)

An OGC GetFeature request provides remote access to vector source data (points, lines, and polygons). You can obtain additional information about these sources via an OGC DescribeFeatureType request including the available field names and their data types. These request strings must include various parameters, as listed in Tables 5 and 6 below.

A WFS GetFeature request returns the actual feature data to the client, encoded in Geography Markup Language (GML) format. As a result, this process is more labor-intensive than the WMS GetMap reques, which t returns a map image, and can take noticeably longer to run depending on the resulting file size. Unlike the GetMap request, you cannot specify an alternative spatial reference system (EPSG code) for the output. Data are provided in EPSG:3031. For complete documentation on creating GetFeature requests, please visit the OGC WFS specification Web page (version 1.0).

GetFeature:

Table 5. Parameters of a GetFeature Request
Parameter Possible Values Description
service WFS Web Feature Service
version 1.1.0 Request version, currently 1.1.0.
request GetFeature Request name.
typename ramp_dem_contours Layer for which to request source data as named in the WFS GetCapabilities request document.
polar_front
antarctica_coastline
maxfeatures* 2 Optional: Number of features to include in the output. Useful for inspecting the output before requesting an entire file.
*Optional Parameter

How to construct a GetFeature request using the above parameters:

Example 1. Outlines of megadune regions on the Antarctic Ice Sheet (462 KB):

http://nsidc.org/cgi-bin/acap.pl?
service=WFS&
version=1.1.0&
request=GetFeature&
typename=antarctic_megadunes_outline

Example 2. Elevation contours for the Antarctic ice sheet from the RAMP DEM (5.3 MB):

http://nsidc.org/cgi-bin/acap.pl?
service=WFS&
version=1.1.0&
request=GetFeature&
typename=ramp_dem_contours

DescribeFeatureType:

Table 6. Parameters of a DescribeFeatureType Request
Parameter Possible Values Description
service WFS Web Feature Service
version 1.1.0 Request version, currently 1.1.0.
request DescribeFeatureType Request name.
typename* ramp_dem_contours Optional: Layer for which to request information as named in the WFS GetCapabilities request document. If you do not specify a layer, all available features will be described.
polar_front
antarctica_coastline
etc.
*Optional Parameter

How to construct a DescribeFeatureType request using the above parameters. As for the OGC GetCapabilities request, clicking on this request will produce an XML document that describes the selected feature:

Example. A description for the RAMP DEM contours feature:

http://nsidc.org/cgi-bin/acap.pl?
service=WFS&
version=1.1.0&
request=DescribeFeatureType&
typename=ramp_dem_contours

4. Web Coverage Service (WCS)

An OGC GetCoverage request provides remote access to raster source data (rectangular grids of data values). You can obtain additional information about these sources, such as spatial extent, spatial resolution, grid size, data format, available coordinate reference systems, and expected pixel values using an OGC DescribeCoverage request. Requests can include various parameters including those listed in the tables below. The WCS GetCoverage request returns the actual raster data to the user in GeoTIFF format. As a result, this process is more labor-intensive than a WMS GetMap request, and can take noticeably longer to run depending on the resulting file size. For complete documentation on creating GetCoverage and DescribeCoverage requests, please visit the OGC WCS specification Web page (version 1.0).

GetCoverage:

Table 7. Parameters of a GetCoverage Request
Parameter Possible Values Description
service WCS Web Coverage Service
version 1.1.1 Request version, currently 1.1.1.
request GetCoverage Request name.
crs Southern Hemisphere: Coordinate reference system. This must be specified as an EPSG code. These codes were developed and maintained by the European Petroleum Survey Group (EPSG) and are a shorthand way of specifying a map projection and other coordinate parameters. The web interface for A-CAP employs EPSG:3031.
EPSG:3031 WGS 84 / Antarctic Polar Stereographic
EPSG:3409 NSIDC EASE-Grid South
EPSG:3412 Hughes / NSIDC Polar Stereographic South
EPSG:3974 WGS 84 / NSIDC EASE-Grid South
EPSG:3976 WGS 84 / NSIDC Sea Ice Polar Stereographic South
EPSG:32761 WGS 84 / UPS South
Global:
EPSG:3410 NSIDC EASE-Grid Global
EPSG:3975 WGS 84 / NSIDC EASE-Grid Global
EPSG:4326 WGS 84 / Latlong
format GeoTIFF Output format of requested coverage (case-insensitive). There are three varieties of GeoTIFF available: one for 8-bit (byte) data values (GeoTIFF), one for 16-bit (integer) data values (GeoTIFFInt16), and one for 32-bit (floating-point) data values (GeoTIFFFloat32). The proper format to select depends on the particular coverage you are requesting and can be determined from a DescribeCoverage request for that coverage.
GeoTIFFInt16
GeoTIFFFloat32
width 500 Width and height of requested GeoTIFF in pixels. For a polar projection (EPSG:3031), you will likely want a square image and will set the width and height to the same value (e.g. 500). For a geographic projection (EPSG:4326) you will likely set the width to double the value of the height (e.g. 800 and 400) for a global map. The data will be resampled to the appropriate resolution to satisfy the requested width and height. To request a different resolution, use resx and resy instead of width and height.
height 500
resx 25000 You must either specify a width and height for the requested GeoTIFF or a resolution in the x and y dimensions as demonstrated here—you cannot specify all four. To find out what the native resolution of the source data is, see the <gml:offsetVector> flag in the DescribeCoverage request for that coverage.
resy 25000
bbox -12500000,-12500000,12500000,12500000 Bounding box corners (lower left [min x, min y] and upper right [max x, max y])in selected CRS units: degrees for EPSG:4326, meters for all other EPSG codes. For full hemispheric coverage, view the projection information provided in the links above to each of the available EPSG codes.
-10700000,-10700000,14700000,14700000
-180,-90,180,90
coverage moa Layer for which to request source data as named in the WCS GetCapabilities request document. You cannot select multiple layers in a single request.
ramp_dem
sea_ice_concentration_01
etc.

How to construct a GetCoverage request using the above parameters:

Example. A 16-bit satellite image from MOA of Siple Dome and the adjacent Ross Ice Shelf in Antarctica at 125-m resolution (21 MB):

http://nsidc.org/cgi-bin/acap.pl?
service=WCS&
version=1.1.1&
request=GetCoverage&
crs=EPSG:3031&
format=GeoTIFFInt16&
resx=125&
resy=125&
bbox=-800000,-900000,-300000,-400000&
coverage=moa

DescribeCoverage:

Table 8. Parameters of a DescribeCoverage Request
Parameter Possible Values Description
service WCS Web Coverage Service
version 1.1.1 Request version, currently 1.1.1.
request DescribeCoverage Request name.
coverage* moa Optional: Layer for which to request information, as named in the WCS GetCapabilities request document. If you do not specify a layer, all available features will be described.
ramp_dem
sea_ice_concentration_01
etc.
*Optional Parameter

How to construct a DescribeCoverage request using the above parameters:
Note: As for the OGC GetCapabilities request, clicking on this request will produce an XML document that describes the selected coverage.

Example. A description for Antarctic surface elevation from the RAMP DEM:

http://nsidc.org/cgi-bin/acap.pl?
service=WCS&
version=1.1.1&
request=DescribeCoverage&
coverage=ramp_dem


In the resulting XML document, you can find the following information about a coverage:

Table 9. Results of a DescribeCoverage Request
XML Tag Possible Values Description
<format> GeoTIFF Expected output format of requested coverage. There are three varieties of GeoTIFF available: one for 8-bit (byte) data values (GeoTIFF); one for 16-bit (integer) data values (GeoTIFFInt16), and one for 32-bit (floating-point) data values (GeoTIFFFloat32).
GeoTIFFInt16
GeoTIFFFloat32
<requestResponseCRSs> EPSG:3031 EPSG:4326 EPSG:3409 EPSG:3410 EPSG:3412 EPSG:3974 EPSG:3975 EPSG:3976 EPSG:32761 Available coordinate reference systems for requesting this coverage.
<gml:Envelope> -179.12303011456 63.0415709487752
179.982766500967 82.4151433839318
The spatial extent of the coverage in both latitude/longitude (decimal degrees) and x/y (meters). These are expressed as bounding box corners—lower left (min x, min y) and upper right (max x, max y).
-825267.555 -1151631.237
2173789.735 2041572.863
<gml:GridEnvelope> 599 columns, 638 rows The grid size of the full coverage in number of pixels.
<gml:offsetVector> 5000 m The pixel size or resolution of the coverage.
<description> > 0 = Antarctic surface elevation (m); -999 or 0 = no data (outside of Antarctica) A description of the coverage. In the case of A-CAP, this usually describes what the different pixel values represent.

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Page last updated: 11/14/12