OGC Standards and the Geospatial Web
CTO's Message: OGC Standards and the Geospatial Web
By Carl Reed, Chief Technology Officer
Open Geospatial Consortium, Inc.
OGC News January 2007
I recently read a number of blog entries about the role of OGC standards in the evolution of the Geospatial Web. A number of bloggers suggest that many OGC standards are neither required nor relevant for use in the Geospatial Web. Other bloggers believe that OGC standards have a central role in providing the interoperability foundation of the Geospatial Web.
My view is more centrist. The Geospatial Web has been evolving since the mid 1990's when the first mapping applications, such as MapQuest and Xerox ParcMap, were deployed. During the last few years, various applications, such as emergency services, spatial data infrastructures, and consumer mapping have accelerated the growth and evolution of the Geospatial Web. During this same time period, an increasing number of applications have implemented and use a variety of geospatial standards. Some of these standards are OGC standards but others are from the Open Mobile Alliance (OMA), from ISO, from the IEEE, or from the Internet Engineering Task Force (IETF).
The reason for this is that the Geospatial Web consists of many layers. The following is a somewhat simplistic view of the number of layers that define the geospatial web. Models for defining the layers of an IT infrastructure are typically 5 to 7 layers deep.
There is the "deep" layer that deals with location as an integral component of the Internet infrastructure. An example of this is the Internet RFC (request for comment) for location enabled DHCP [http://www.ietf.org/rfc/rfc3825.txt]. (Dynamic Host Configuration Protocol Option for Coordinate-based Location Configuration Information (RFC 3825). Then there is the Link Layer Discovery Protocol-Media Endpoint Discover, or LLDP-MED [http://en.wikipedia.org/wiki/Link_Layer_Discovery_Protocol], which is an enhancement to the Link Layer Discovery Protocol (LLDP) and is designed to allow for things such as device location discovery to allow creation of location databases and, in the case of VoIP [http://en.wikipedia.org/wiki/VoIP], E911 [http://en.wikipedia.org/wiki/Enhanced_911] services. These standards tend to be lightweight and relatively simple and very well structured semantically. These characteristics are driven by the fact that bandwidth and packet size are significant constraints. The OGC collaborates with the IETF to insure consistency in how location is expressed at this level.
Then there is the middle layer of standards that provide "gateway" access between the deep layer and the application layer. This middle layer is where standards such as OMA's Mobile Location Platform API and a variety of OGC standards, such as WMS, WFS and WCS are implemented. It should be noted that MLP uses an application schema of GML to express geometry and CRS. Standards in this layer tend to be non-trivial. A high degree of geospatial semantic richness, expressiveness and robustness is required at this layer. There are many reasons for this, such as the requirement for more precisely expressing the vast range of properties related to location, such as time, coordinate reference systems, topology, complex geometry, measurements, and metadata.
Finally, there is the application layer. The application layer is interesting in that the standards that exist at this level can be quite simple, such as GeoRSS, or quite sophisticated and semantically expressive, such as the OGC Styled Layer Descriptor standard. As with the other layers, there are other geospatial standards being developed for this level that are not OGC standards. For example the presence architecture developed in the IETF Instant Messaging and Presence Protocol (IMPP) working group has defined a format for presence information called Presence Information Data Format (PIDF). PIDF is an XML format that provides presence information about a "presentity" [http://en.wikipedia.org/wiki/Presentity]. An extension to PIDF has been defined with a Presence based Geopriv Location Object Format (PIDF-LO). PIDF-LO carries either civic, geospatial location information or both. PIDF-LO is a GML application schema. As such, PIDF-LO itself is a relatively simple standard but one that is grounded in a very rich and expressive standard — GML. And GML is grounded in the ISO Feature model. So, in the application layer, a common characteristic is that relatively simple standards can be developed and deployed that are profiles or application schemas of other, more "complex" standards such as GML.
Location content is being created and utilized at many levels in the internet/web infrastructure. Much of this content is not being created by the GIS community! Consider DSRC [http://en.wikipedia.org/wiki/DSRC] : a short to medium range (1000 meters) communications service that supports both public safety and private operations in roadside-to-vehicle and vehicle-to-vehicle communication environments. DSRC is really about developing and deploying an extensive roadside sensor and communication network. This network will generate billions of location messages on a daily basis — and is being done entirely outside the traditional geospatial domain. And this application area will be an integral component of the Geospatial Web.
OGC standards play an important role in all levels. This does not mean that the OGC is developing standards for all levels and all application areas. Instead, many other standards organizations, such as the IETF, IEEE, OMA, and OASIS are building on the work of the OGC to define profiles and application schemas for their specific requirements. Many of these profiles and schemas, such as those used in GeoRSS and IEEE 1451, are simple and lightweight. Each meets a specific requirement.
The Geospatial Web is not just a bunch of mash-ups or even the