Technical Information About the National Atlas Web Map Service

These instructions are for Web map services (WMS) consumers who want to connect to nationalatlas.gov™ using Open Geospatial Consortium (OGC) protocols for interoperability and access. Anyone can connect to the National Atlas of the United States^® by supplying valid OGC parameter values to our Web services uniform resource locator (URL).

An Introduction to National Atlas Web Map Services is also available.

Accessing the Capabilities File
The National Atlas WMS currently contains over 2,400 individual layers. A single capabilities file for all layers is available, but because it is quite a large file, we have also grouped the layers into broad categories that correspond to the chapters of the National Atlas. Use the online addresses in Table 1 to access the capabilities file for the categories of interest to you.

Table 1

The above URLs will default to WMS version 1.3.0. You can change the WMS version by appending an optional parameter with the desired WMS version. For example, to specify WMS version 1.1.1, append "&VERSION=1.1.1" to the URLs in the table above. Valid version numbers are 1.3.0, 1.1.1, 1.1.0, 1.0.7, and 1.0.0. Please consult the OGC Implementation Specification for Web Map Services.

Note for ESRI Users
If you use ArcGIS 9.3.x, be sure to specify WMS version 1.1.1 or lower.

Building a Map Request
Build a map request by using the basic WMS URL (http://webservices.nationalatlas.gov/wms?) followed by valid parameter/value pairs separated by an ampersand (&). Table 2 describes the available parameter/value pairs.

Table 2

As an example, the following map request will generate the map in Figure 1. Note that the resulting map displays the Seismic Hazard layer ("seihaz") and the States layer ("states").

http://webservices.nationalatlas.gov/wms?SERVICE=WMS&REQUEST=GetMap&VERSION=1.1.1&FORMAT=PNG&WIDTH=300&HEIGHT=425&SRS=EPSG:4326&BBOX=-125,31,-110,50&LAYERS=seihaz,states

When values are invalid, an error message will indicate which parameter/value pair is invalid according to the format of the Exceptions type. For example, if you are using WMS version 1.3.0 and if the parameter value EXCEPTIONS = INIMAGE is supplied, then an image that has the error message embedded in it is returned. If the parameter value EXCEPTIONS = BLANK is supplied, then a blank image is returned. If the parameter value EXCEPTIONS = XML or no Exceptions parameter is included, then the error message is returned in XML format.

nationalatlas.gov™ map layers are listed in the capabilities file between layer tags. For example:

<Layer queryable="0" opaque="0" cascaded="0">
     <Name>seihaz</Name>
     <Title>Geology - Seismic Hazard</Title>
     <EX_GeographicBoundingBox>
          <westBoundLongitude>-179.127</westBoundLongitude>
          <eastBoundLongitude>179.765</eastBoundLongitude>
          <southBoundLatitude>18.9155</southBoundLatitude>
          <northBoundLatitude>71.3575</northBoundLatitude>
     </EX_GeographicBoundingBox>
     <BoundingBox CRS="" minx="-179.127" miny="18.9245" maxx="179.765" maxy="71.3575"/>
     <Style>
          <Name>default</Name>
          <Title>default</Title>
          <LegendURL width="71" height="21">
               <Format>image/png</Format>
               <OnlineResource xlink:type="simple" xlink:href="http://webservices.nationalatlas.gov/
                    wms?version=1.3.0&service=WMS&request=GetLegendGraphic&sld_version=1.1.0&
                    layer=seihaz&format=image/png&STYLE=default"/>
          </LegendURL>
     </Style>
</Layer>

In this example, name tags within the layer tags define the name of the map layer. All layers in the National Atlas WMS have unique names, and this map layer, named "seihaz," is titled "Geology - Seismic Hazard." Notice that the URL for the legend graphic is in the OnlineResource tag, within the LegendURL tags.

Reporting Problems with the National Atlas WMS
Contact the National Atlas WMS Team by clicking on atlasmail at the bottom of most pages on nationalatlas.gov.