Open Geospatial Consortium Web Mapping Service
Anyone can connect to the National Atlas of the United States® by supplying valid OGC parameter values to our uniform resource locator (URL). We currently support the parameters shown in the following table.
| Parameter | Valid Values | Default Value |
|---|---|---|
| Request | Capabilities, Map | Capabilities |
| Format | GIF, JPEG | GIF |
| Width | A valid whole number expressed in pixels | 425 pixels |
| Height | A valid whole number expressed in pixels | 300 pixels |
| Layers | Individually listed in the capabilities file. This may be qualified by a dimension tag. | The background layer displaying countries in the Northern Hemisphere within the currently defined bounding box. |
| SRS | EPSG:2163 | EPSG:2163 |
| BBox | Four comma-delimited numeric values expressed in meters. |
-5720171 -2783739.25 4279073 4487617 |
| Transparent | True, False | False |
| BGColor | A hexadecimal value | 0xFFFFFF |
| WMTVER | 1.0.0 ,1.0,1.0.7 | 1.0.7 |
| Exceptions | Inimage, Blank, WMS_XML | WMS_XML |
 |
|
|
http://nationalatlas.gov/natlas/capabilities.xml
or
http://nationalatlas.gov/natlas/WMSProcess.asp.
A map request is built using this basic URL followed by a question mark and valid parameter=value pairs separated by an ampersand (&). For example, this map lengthy request will generate the map that appears just below the request:
http://nationalatlas.gov/natlas/WMSProcess.asp?Request=Map&Width=450&Height=320 &Layers=background,states,Grid&SRS=EPSG:2163&BBOX=-5720171,-2783739.25,4279073,4487617 &Format=JPEG&Transparent=True& BGColor=0xffaadd&WMTVER=1.0.7&Exceptions=WMS_XML |
If no parameter values are supplied, a default map image is drawn.
When invalid values are supplied, a detailed error message is returned indicating
which parameter value pair is invalid in XML format if WMS_XML is the value of
Exceptions. If the parameter value of Exceptions = Inimage was supplied, the
default image is drawn with the error message embedded in the image. If the parameter
value of Exceptions = Blank was given, then a blank image will be returned.
Nationalatlas.gov map layers are listed in the capabilities file between layer
tags. For example:
<Layer queryable="0" opaque="0"
noSubsets="0">
<Name>crop</Name>
<Title>Crops - 1997</Title>
<Dimension name="crop" units=""
/>
<Extent name="crop">"Soybeans for Beans", "Cotton", "Corn
for Grain or Seed",
"Wheat for Grain"</Extent>
</Layer>
In this example, name tags define the name of the map layer, within the layer
tags. The general map layer name (crop) has a specific title of Crops - 1997.
But we have prepared more than one crop map layer. So in our example, we use
dimension tags with an extent instance to qualify the type of crop map requested.
Add a dimension tag beginning with &dim_ followed by the dimension name =<Extent
instance> after the layer tag to identify the crop layer to be displayed.
To illustrate this using another example, the syntax below will make a map using
the specific crop layer named Soybeans for Beans.
&Layers=background,states,Grid,crop&dim_Crop=Soybeans for
Beans
If a layer does not have a dimension name or extent name contained in
the <Layer></Layer> tag,
then only the layer name is required.
Please consult the OGC Implementation Specification for Web Map Services for
more information.
Notes:
At this time, map-drawing order is predetermined by the nationalatlas.gov™
map software and cannot be changed.
The data are in a Lambert azimuthal equal-area projection as defined in
the capabilities file. The OGC assigned the National Atlas implementation
of this projection a Spatial Reference System tag of 2163.
The default bounding box covers the full geographic extent of the National
Atlas.
Selected National Atlas Web Mapping Services are also available from the Geography Network.
