Enabling Federal Digital Mapping Through GeoPackage Extensions
GeoPackage has been gaining quite a bit of momentum in many geospatial communities. With its ability to handle both raster and vector data, coupled with the use of extensions to expand its utility, GeoPackage is becoming a 'tour de force' geospatial format for many applications.
However, digital mapping, even in today’s age, is still a fine art. While many facets have been improved through technology, standardization, and open formats, many things about map production remain shrouded in mystery (or rather, in proprietary workflows). Although GeoPackage is simplistic, its transparency does require different thinking in our GIS systems and workflows to fully appreciate its potential.
The core GeoPackage simply provides a base set of tables to store geographic, raster, feature, and attribute data. While maps contain the same information, more is expected from the output to make it consumable. For GeoPackage, these can be enabled through extensions.
Figure 1: (Left) Map Graphic, USGS (Public Domain); (Right) OGC GPKG (Public Domain)
As vector maps moved into the digital age, domain models and schemas were developed to store, re-use, and disseminate feature information for production. Examples of these models are the National Systems for Geospatial-Intelligence (NSG) Application Schema (NAS), US Army’s Ground-Warfighter Geospatial Data Model (GGDM), and the US Geological Survey’s (USGS) Geological Map Schema (GeMS). They provide standardized attribution and relationships of feature data to unify production, simplify visualization logic, and increase overall utility from its hard-copy predecessor.
Geospatial maps are created by people all over the world. Given their applications in urban development, disaster relief, and mission planning, inaccurate and inconsistent data generation could be very detrimental to their users. While the aforementioned logical models were designed to be independent of any physical implementation, to ensure consistency, workflows are generally aligned with a specific GIS system. This proves challenging when introducing new file formats in to the physical production of these data.
For instance, the USGS National Map Program provides various public releasable map data of the US. From this service, vector topographic data are provided in an Esri File Geodatabase (fGDB) and an Esri Map Document (MXD) template to allow the users to symbolize and portray it in their GIS system. While this works well in its original environment, as OGC’s Testbed-12 GeoPackage US Topo Engineering Report indicates, the MXD template was reconstructed to encode the portrayal rules and descriptions in order to symbolize the data in the GeoPackage format for other environments. Not an easy task.
Figure 2: (Left) Map Data © Google, Vector data USGS (Public Domain); (Right) Taken from the Testbed-12 GeoPackage US Topo Engineering Report.
Additionally, given the database-style of these models, many attributes of map data are enumerated. For instance, in NGA’s Topographic Data Store (TDS), a NAS derivative product, feature attributes are coded names that can contain a myriad of either string, boolean, and enumerated values. This information coding is great for computers, but not so great for human exploiters. Some GIS systems get around this by incorporating look up tables in their proprietary formats, which again only solves the problem for a portion of users.
Figure 3: Map Data © Stamen Design, Creative Commons, Vector Data, USDA (Public Domain).
Both examples can be addressed using GeoPackage extensions. GeoPackage can be extended by anyone. Typically, OGC GeoPackage SWG labels extensions as either Registered or Community extensions. Registered extensions are often refined through Interoperability Experiments (IE) and are always thoroughly vetted in the SWG with both public and OGC review. These events help establish a baseline understanding of what the extension is trying to accomplish while ensuring the interoperability with the GeoPackage standard is maintained. Registered extensions are also formally documented in the encoding standard. Community extensions are generally created to support research or commercial applications to serve a specific purpose or audience but are not part of the encoding standard. Community extensions can become Registered if vetted through the GeoPackage SWG and have the backing of at least three implementers.
To address symbology, two community extensions were created. One extension aimed to align with OGC Web Services (OWS) Context documents used in other OGC Services and the other was created out of the Testbed-12 GeoPackage US Topo Engineering Report
For feature attributes, the registered Schema extension allows these map models to be encoded within the GeoPackage to create user friendly map data while maintaining integrity and consistency with the logical models.
While both extensions remain in their infancy, they provide a blue print for how extensions can be leveraged to enhance GeoPackage utility in the mapping industry. The next challenge exists in the willingness of commercial and open source developers to adopt, comment on, and implement these extensions. Adoption of these extensions allows producers to maintain their logical workflows, but not restrict physical implementations based on any one particular GIS system. Support for these extensions in both encoding and client-side applications are necessary to move digital map production and exploitation into the open community, increasing their usability abroad.
If you have any questions, comments, or ideas on extensions that would further advance the utility of GeoPackage, please visit the GeoPackage OGC Standards Working Group page for information. For more information on the aforementioned OGC Testbed 12, please visit the Testbed 12 page on the OGC website.
David Wilson is a Geospatial Engineer for Strategic Alliance Consulting, Inc that specializes in Geospatial interoperability with a focus on GeoPackage standards compliance, testing and use. David has over 10 years’ experience working in the Army and the National Geospatial-Intelligence Agency’s (NGA).