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.

The above URLs will default to WMS version 1.3.0. You can change the WMS version by appending an optional version parameter with the desired WMS version. For example, to specify WMS version 1.1.1, append "&VERSION=1.1.1" to the URLs in Table 1. Details of the GetCapabilities Request parameters are included in Table 2.

Table 2: GetCapabilities Request Parameters

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 3 describes the available parameter/value pairs.

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

All layers in the National Atlas WMS have unique names. You can find map layer names by accessing the capabilities files in Table 1. The name of the map layer is defined between the Name tag, which is within the Layer tag (Figure 2).

When GetMap parameter 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.

Getting the Legend Graphic
The URL for the legend graphic is available in the GetCapabilities file between the OnlineResource tag, which is within the LegendURL tag (Figure 2). We suggest you use the legend graphic request supplied in the GetCapabilities file, but you can also build it using the basic WMS URL (http://webservices.nationalatlas.gov/wms?) followed by valid parameter/value pairs separated by an ampersand (&). Table 4 describes the available parameter/value pairs.

Table 4: GetLegendGraphic Request Parameters

Using the Seismic Hazard layer from Figure 1 as an example, the following GetLegendGraphic request will generate the image in Figure 3.

http://webservices.nationalatlas.gov/wms?SERVICE=WMS&VERSION=1.1.1&REQUEST=GetLegendGraphic&
LAYER=seihaz&FORMAT=image/png&STYLE=default

Getting Feature Information
You can get information about features displayed in National Atlas WMS map layers by using the GetFeatureInfo request. A GetFeatureInfo request is much like the GetMap request, but with several additional parameters to specify the point on the map to be queried, which layers to query, and how the output should be displayed. To build a feature information request, start by using the original map request URL, replace the REQUEST parameter with "REQUEST=GetFeatureInfo" and then add the particular GetFeatureInfo paramer/value pairs (separated by "&") as indicated in Table 5. Before building the GetFeatureInfo request, you can check that the layer that interests you can be queried by going to the capabilities file and verifying that queryable=1 in the layer definition.

Using the Seismic Hazard map request from Figure 1 as an example, start with the original GetMap request:

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

To get information about the seismic hazard of a point on the San Francisco peninsula:

• replace "REQUEST=GetMap" with "REQUEST=GetFeatureInfo"
• add the parameter to query just the seismic hazard layer ("QUERY_LAYERS=seihaz")
• add the X and Y coordinate of a point on the San Francisco peninsula ("X=51" and "Y=274")
• and specify the parameter to return the results as plain text ("INFO_FORMAT=text/plain")

The resulting GetFeatureInfo request would be:

http://webservices.nationalatlas.gov/wms?SERVICE=WMS&REQUEST=GetFeatureInfo&VERSION=1.1.1&
FORMAT=PNG&WIDTH=300&HEIGHT=425&SRS=EPSG:4326&BBOX=-125,31,-110,50&LAYERS=seihaz,states&
QUERY_LAYERS=seihaz&X=51&Y=274&INFO_FORMAT=text/plain

and the result, in plain text (Figure 4), indicates a value of 40-60% g for this part of the San Francisco peninsula:

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.