Technical Overview

 

Other Mid-America National Map areas:

Currently many websites allow users to display and browse geospatial data through the use of map viewers. However, most of these sites only allow geospatial data to be displayed which is stored and served within the context of the map viewer. The OpenGIS Consortium’s Web Map Service (WMS) Implementation Specification allows any map viewer implementing the WMS interface to display maps from multiple map servers which also implement this specification.

OpenGIS Consortium Web Map Services allows map viewers to display geospatial data throughout the world. The advantage of interoperability are numerous, but the biggest advantage in a regional collaboration is geospatial data can be seamlessly displayed from different state or regional authorities in a manner which allows users to feel like the geospatial data they are viewing comes from the same source. Because of interoperability, geospatial data can be warehoused at authoritative organizations which have the most precise data temporally, spectrally, and spatially. This capability allows users to view the most accurate and up to date feature data on a regional scale even though the displayed features come from different sources. For example, geospatial data warehouses for individual states can be used in collaboration to display similar feature types (e.g. roads) in a seamless manner across state boundaries. This is accomplished by requesting map images from two map servers of bounding states which only warehouse their state’s data.

OGC’s Web Map Service Implementation defines parameter name-value pairs to request map images in a common manner in order to overlay map images from multiple map server to produce a seamless map. This is possible when map images are requested that have the same bounding box, image size, and spatial reference system. The transparency of the image is also very important because it allows map images to be overlaid within a map viewer. Examples of three map requests are shown below. The first requests roads from a Missouri map server, the second requests roads from an Arkansas map server, and the third requests roads from a Kansas map server.

http://mcdc-maps.missouri.edu/wms/mapserver?request=GetMap&version=1.1.1
&WIDTH=300&HEIGHT=200&TRANSPARENT=TRUE&BBOX=-95.6,33.1,-89.6,39.1
&SRS=EPSG:4326&LAYERS=Highways&STYLES=default&BGCOLOR=0XFFFFFF
&EXCEPTIONS=INIMAGE&FORMAT=image/png

http://ogc.cast.uark.edu:8080/tnm/servlet/mapserver?request=GetMap&version=1.1.1
&WIDTH=300&HEIGHT=200&TRANSPARENT=TRUE&BBOX=-95.6,33.1,-89.6,39.1
&SRS=EPSG:4326&LAYERS=roads_all_ahtd&STYLES=default&BGCOLOR=0XFFFFFF
&EXCEPTIONS=INIMAGE&FORMAT=image/png

http://deuteron.kgs.ku.edu:8080/wms/servlet/mapserver?request=GetMap&version=1.1.1
&WIDTH=300&HEIGHT=200&TRANSPARENT=TRUE&BBOX=-95.6,33.1,-89.6,39.1
&SRS=EPSG:4326&LAYERS=Highways&STYLES=default&BGCOLOR=0XFFFFFF
&EXCEPTIONS=INIMAGE&FORMAT=image/png

The returned map images would be a Missouri Roads map image within the defined bounding box from the Missouri map server, Arkansas Roads within the same bounding box from the Arkansas map server, and a Kansas Roads from the Kansas map server. A map viewer overlays these images to create a seamless map as shown below:

As you can see the default styles from the three map servers portray roads differently. Therefore, Styled Layer Descriptor documents must be used to symbolize these three similar features the same. Styled Layer Descriptors are an XML encoding defined by the OpenGIS Consortium that allows map servers to render feature data according to user defined symbolization ( see Appendix Example I). Styled Layer Descriptor documents are stored at a web accessible location, and referenced within a map request. A SLD-enabled web map service can obtain a SLD and render feature data according to the parameters defined within the document.

The three map requests below reference SLD documents. Each document defines the same line symbolization rules , therefore the created map images will symbolize the road features the same.

http://mcdc-maps.missouri.edu/wms/mapserver?request=GetMap&version=1.1.1
&WIDTH=300&HEIGHT=200&TRANSPARENT=TRUE&BBOX=-95.6,33.1,-89.6,39.1
&SRS=EPSG:4326&BGCOLOR=0XFFFFFF&EXCEPTIONS=INIMAGE&FORMAT=image/png
&SLD=http://ogc.cast.uark.edu/tnm/sld/1.0.0/MissouriStateRoads.sld

http://ogc.cast.uark.edu:8080/tnm/servlet/mapserver?request=GetMap&version=1.1.1
&WIDTH=300&HEIGHT=200&TRANSPARENT=TRUE&BBOX=-95.6,33.1,-89.6,39.1
&SRS=EPSG:4326&BGCOLOR=0XFFFFFF&EXCEPTIONS=INIMAGE&FORMAT=image/png
&SLD=http://ogc.cast.uark.edu/tnm/sld/1.0.0/ArkansasRoads.sld

http://deuteron.kgs.ku.edu:8080/wms/servlet/mapserver?request=GetMap&version=1.1.1
&WIDTH=300&HEIGHT=200&TRANSPARENT=TRUE&BBOX=-95.6,33.1,-89.6,39.1
&SRS=EPSG:4326&BGCOLOR=0XFFFFFF&EXCEPTIONS=INIMAGE&FORMAT=image/png
&SLD=http://ogc.cast.uark.edu/tnm/sld/1.0.0/KansasRoads.sld

A major component of this regional collaboration is the definition of Style Layer Descriptors which will allow geospatial data from multiple map servers to be commonly symbolized. This would be an easy effort if the feature data from all map servers had common semantics to define feature data such USGS DLG. If this was the case the same rules and filters defined for a feature type could easily be reused, however this is not always true. For example USGS DLG Roads define features as Primary Route Class 1, Secondary Route Class 2, etc. and another road feature type may define roads as interstates, US Highway, etc. The difficulty in symbolizing similar feature types with different semantics is acknowledged now, and through collaboration SLD documents will be created so similar feature types will be symbolized the same.