Table of Contents

• Introduction
• Modeling
• How to Evaulate
  • #id
  • #access
  • #collection
  • #super
  • #schema
  • #access
  • #content
• Comparison
• Reference
  • #review
  • #formats

Introduction

Geographic Information Systems have an interesting problem domain, much like with programming GIS Hacks are involved in modeling the real world.

• Programmers model with Software (and use abstractions like Object and Class).
• Geographers model with Maps (and use abstractions like Feature and FeatureType).

There is more then one way to do it, programers used to use procedures, geographers used to use a pen and paper. Lets get on with it.

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Ack - try and stay awake. Nothing is as silly as trying to explain why creating a model is useful. Why create a model when you could solve a real problem?
Well for starters models are simpler and more flexable.

• A Geographer could model what it would be like to flood a river, this is much more pragmatic then driving around with sticks of dynamite and blowing up a few dikes (although perhaps not as much fun). Hacking models is easier then hacking reality.
• A programmer can model code (using UML diagrams and such), hacking the model is often a lot quicker then hacking the code.

One can also view "proper" Object-Oriented programming as constructing a model of a problem space in the real world - and then using it to solve actual problems. There is even a branch of practice called Model Driven Development & Domain Driven Design to try and steer us back in the direction of OO prinicples.

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What to we want (aka what is our task) - Geographic Markup Language:

• GML3 - this is the "mandate" mentioned at an IRC meeting a couple of months ago.
  • Programers would use UML2 these days (right?)

Here are some of the constructs in GML that we need to ensure we handle well:

• Feature - something that can be drawn on a map
  • Programers would call these objects (or instances if that makes you happy)
• FeatureType - kind of thing on a map.
  • Programmers would call these classes
• FeatureCollection
  • a bunch of features, can also be considered a "Feature" (Cause you can draw it on a map)
  • Programers would consider this a data structure
• id - formally FID or "FeatureID"
  • used to unqiuely identify the Feature, the real world object

These are informal working definitions, for a formal definition consult a specification.

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The exact requiremetns are outlined here:

• FeatureType Survey

In breif:

• complex type support
• subset of geometry types

There is some discussion of references and dealing multiple feature collections and feature references.

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The XPath view of a document is limited to the following:

• nodelists

To support this model we do not need the full set of validation constructs requried for GML3 (such as choice, restrictions or even multiplicity).

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Node breakdown of example:

Root:

• xml-stylesheet type="application/xml" href="sco.xsl"
  • gml:FeatureCollection
  • gml:boundedBy: Envelope
  • gml:featureMember
    • sco:wq_plus
      • sco:sitename: BALRANALD WEIR
      • sco:measurement
        • sco:determineand_description: 16/JAN/94
        • sco:result: Turbidity
      • sco:measurement
        • sco:determineand_description: 24/JAN/94
        • sco:result: Turbidity
      • sco:location: Point( 22, 134.5399658, EPSG:4283 )
      • sco:nearestSlimePit: Point( 22.1,143.53399658, EPSG:4283 )
      • sco:sitename: RWWQ004
  • gml:name: My Boreholes

Points of interest:

• Root node is not the same as the root element
• Root node contains the entire document including the xml-stylesheet (xmlns and xmlns:prefix are not covered though)

Location Paths:

Similar to unix shell:

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Reference: GML-3.1.pdf

• Page 67 for Feature
• Page 68 & 528 for FeatureCollection
• Page 530 for "Annex E UML-to-GML Application Schema Encoding Rules"

GML3 includes a more complicated Geometry model then supported by JTS. Higher order geometries (like surface) are supported in addition to curves and topologies.

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GeoTools 2.1

GeoAPI 1.0

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DeeGree

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GML2 & GML3