Open Packaging Convention standard applied to compressed hypermaps
The OPC standard
OPC (also known as OOXML) is a packaging strategy that integrates elements of the ZIP
compression, XML documents, and the web MIME types into an open standard that
makes it easier to organize, store, and transport data. It was defined by
Microsoft and approved as ISO 29500-2 and ECMA-376. It is used by Office 2007
and newer versions of Word (.docx), Excel (.xlsx), and PowerPoint (.pptx),
along with XPS (.xps), Autodesk AutoCAD (.dwfx), etc (Figure 1). An OPC package can contain several files with a
directory structure in it. Each file is called a “part” and all OPC package
part names have to follow the URI restrictions and conventions. The format adds some extra
files to increase interoperability. It incorporates a
standardized way of recording file relations or links. It also incorporates a
file with a few elements of metadata and a thumbnail with a small image for
presentation purposes. All these extra parts allow some basic independent data
maintenance, such as the extraction of a fragment of the package, thus
guaranteeing that all the related resources will be extracted without the need
to understand the actual encoding of the parts included in the package.
OPC is conceptually very similar to the
KMZ and MMZ format, but offers some advantages. KMZ is a compressed file that
uses a ZIP file format following the solution reuse criteria, but its structure
is deeply related to the original KML. Therefore, it does not comply with the
completeness criteria, and KMZ strategy is not accurately documented (it is not
included in the KML specification) and thus, it does not comply with the
formality and metamodel identity criteria. MMZ was never submitted for
standardization or adopted by other vendors and has not reached a high
popularity outside the GIS community. OPC uses an accepted popular compression
schema and header (ZIP compression) but combines other advantages such as it
can be directly manipulated by other OPC compatible tools that recognize the
file structure and relations even if they are not able to identify or
understand the format of some compressed parts. Phillips and Allemang (2010)
prefer it over other alternatives as a file format for data archiving.
How to apply it to geospatial information
OPC format offers an opportunity for designing an standardized way to pack geospatial information in a
single file that internally used the original data files that can be later recovered as they were produced. Due to the similarities
between the old MMZ format and the OPC standard, we call this approach MMZX. Most GIS software offer the capability save a session composed by different information layers, symbolization, presentation, metadata, etc. This format does not contains the information but links to it. Recently the OGC has introduced a new standardized format that also can to this: the OWS context. It is possible to create a software that is able to collect the "context" file plus all the data files and symbolization and configuration files and create a OPC package. Apart from the geospatial data parts of the
file, OPC specifies how to explicitly relate different parts using .rels files
(relations are composed of a source file and a target file, and the purpose of
this relation is like in the RDF format). These files are XML files that have
the same name as that of its respective source part adding “.rels” and placed
in a “rels” folder. Each of these files lists the target parts related to its
source and the semantics of this relation.
MiraMon implementation
Recently, MiraMon has complemented the old MMZ file with the new MMZX file format that is fully follows to the OPC standard. This format is able to share geospatial information in a single file that can be decompressed and visualize automatically using the MiraMon Universal Map Reader (http://miramon-universal-map-reader.en.softonic.com/) or the MiraMon professional software
How to read and see the internal OPC structure
This page provides some clues on how to work ith OPC files http://www.wikihow.com/Open-XML. Mainly, there are two general use tools for doing this the Package Explore and the OOXML Chrome viewer plug-in.
More information about the OPC format
- Wikipedia page
- Good description of the format structure
- ISO/IEC 29500-2:2012, specification document
- Very good page form the library of the congress with link to more resources
- LibOPC
