1. Updating GML datasets S-100 WG TSM5 19-21 September 2017
Raphael Malyankar, Consultant, Portolan Sciences
Eivind Mong, Consultant
Work performed under NOAA sponsorship

2. Introduction
The GML specification does not specify any particular format for updates to GML data files.
Drafts of S-122 and S-123 define a ‘whole-object- replacement’ approach to updating datasets.
At NIPWG 4 in May 2017, we were requested to write up the S-122/S-123 approach for the S-100 WG.

3. Options for updating GML datasets
Attribute-level update – update delta dataset describes updates to attributes as well as instances.
Object-level update – update delta dataset describes updates only at the instance level.
Plain text update: Update with a text-based ‘patch’ utility – update is a purely text-based delta between the current and updated dataset that ignores structure and semantics.
XML-aware patching – update is an XML-based delta that identifies the XML item (attribute or instance) being updated.
Whole-dataset replacement – the entire dataset is distributed when an update is necessary.
XML-aware patching example for an addition of a restriction attribute to a RestrictedAreaNavigational feature:
<add = ‘RESARE55’]">
<restriction>entry prohibited</restriction>
</add>

4. Summarized Analysis – 1 Factor/Approach Attribute-level Object-level
Text patch
XML patch
Whole-dataset
Update detail
Attribute, instance, spatial object, spatial primitive
Instance, spatial object
Line-based
Attribute, instance, spatial object, spatial primitive
Dataset as a whole
Schema
Modified
Same as base dataset
Not needed
Special common XML schema
Robustness
Flexible
More flexible
Least flexible
Like Attribute-level.
Robust
Volume of update delta
Low
10 X attribute level?
Comparable to Attribute level
Comparable to Attribute-level
100 X attribute level?
Tools - availability
OEMs must develop.
Off-the-shelf plus custom ‘wrappers’
Uncertain. Open source tools exist, customization may be needed.
No special functionality
Implementation effort
Highest
Moderate
Application logic
Application logic like 8211 format.
Simpler parts of 8211 logic
None for basic implementation. Error recovery TBD
Simple logic for basic implementation. Error recovery TBD
Simplest
Sotware re-use
Reusable
Re-usable

5. Summarized Analysis - 2
Factor/Approach
Attribute-level
Object-level
Text patch
XML patch
Whole-dataset
Update tracking and object history on end- user system
More complex
Simplest
Most complex
Comparable to Attribute-level
Difficult unless a ‘dataset’ contains only a single feature.
Does updating spatial type need special treatment?
Whole primitive; updating single coordinate more complex
As object; update to single coordinate more complex
Depends on text structure
As object; update to single coordinate more complex
No special treatment
Previous version requirements – producer & end-user
Minimal
Previous version needed
Not needed
Transfer format
File
Compatible with web services
Compatible with web services
Not compatible with web services
Low compatibility with web services.
As for base dataset
Data product types
Vector and/or info type
Vector and/or info type
Vector and/or info type and imagery, etc.
Compatibility with GML 3.2.1
Possible
Yes
Yes - ignores GML altogether.
Yes – update format is non-GML can reference items in any GML file.
A more detailed version of this table is included in the paper.

6. Recommended methods
Object-level updating for products based on discrete features or information types, with two caveats:
Products where datasets are very small will generally need only whole-dataset methods; and
Data or coordinate collections or sequences will need special treatment (e.g., functionality extensions to locate coordinates by their positions in the sequence) in order to reduce the average data volume of updates.
Whole-dataset updating for imagery, gridded data, etc., with the caveat below:
Methods for updating parts of gridded or pointset data in other formats should be adapted to GML encodings.
Specific products may have exceptional circumstances

7. Details
Specific details for adding whole-object update formats to a product specification are described in the paper (pages 5-10). For example:
Feature ID and gml:id of updated feature must be the same as the original.
Versioning of features – add equivalent of RVER in S-100 GML profile.
Deletions – use fixedDateRange attribute.
Loading requires all previous updates to be available (or applied).
Cancelling datasets – update with edition # = 0.
Update cannot change data coverage.
Validation checks for (1) updates in isolation and (2) datasets after updating.
Metadata for update datasets.

8. Conclusion
The ‘whole-object’ approach is a reasonable compromise between the various factors for most discrete-feature vector datasets.
Specific data products might have special circumstances which dictate a different approach.
Suggested next steps:
Evaluate the most promising approaches.
Develop specifications, rules, or guidelines about update dataset format, processing, and management, for eventual incorporation into S-100.
Develop supporting guidance for authors of product specifications and implementers.