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OGC Observations and Measurements - Simple Feature model & encodings (OMSF)


NOTE: This document and the defined encodings are work-in-progress, and at this point, have not been endorsed by the OGC or any other standards organization. They may (and probably will) change in a backwards incompatible way during the drafting process.

There is an on-going discussion in leveraging the OMSF encodings in the INSPIRE alternative encodings action (MIWP 2017.2) for a simplified encoding of the INSPIRE O&M datasets.

The namespaces http://www.opengis.net/omsf-gml/2.0 and http://www.opengis.net/omsf-json/2.0 have not (yet) been approved by the OGC Naming Authority, and thus may also change. Consider yourself warned.


Handling complex feature structure of the O&M XML Implementation (as in OGC 10-025r1) typically creates a considerable additional implementation cost, added code complexity and lower performance for both server and client software. The purpose of this activity is to define simple encodings for the most used O&M Observation types, and thus enable interoperable O&M data exchange between existing software applications, servers and clients limited to using simple (non-complex) GML features and/or GeoJSON.

Design considerations

The following primary design goals have been followed (in priority order):

  1. The defined feature types must be suitable for encoding using GML Simple Features Profile level SF-0 or SF-1.
  2. Each defined feature type must have a relevant geometry property for spatial processing and map visualization purposes.
  3. The defined feature types must follow the O&M model structure and property naming as long as it does not conflict with higher priority design goals.
  4. The defined GML feature types should be as simple as possible, but not simpler (so called Einstein's razor).

In contrast to the OMXML (complex feature) implementation model, hard-typing is used for the different Observation types: GenericObservation, MeasureObservation, CategoryObservation, CountObservation, TruthObservation and MeasureTimeseriesObservation are each defined as separate feature types with fixed result value types. This is an intentional trade-off between simplicity and flexibility: Hard typing allows for easier, more precise definition of the Observation content model, especially the observation result, in the APIs providing OMSF data. On the other hand, the providing Observation data for novel result content type requires changes to the OMSF model.

ISO 19156 and the OMSF Implementation model

TODO: Revise the following text and images based on the ISO 19156 Edition 2 changes:

The figure below described a basic Observation feature type as defined in Observations and Measurements v2.0 (OGC Document 10-004r3, also published as ISO 19156:2011, Geographic information — Observations and Measurements). !O&M basic Observation type, ISO 19156:2011, Figure 2

Specialization of OM_Observation class by result type is described in the Figure 6: !Specializations of Observation by result type, ISO 19156:2011, Figure 6

Specialization of the OM_Observation class for Coverage valued results is described in the Figure 7: !Coverage valued Observations, ISO 19156:2011, Figure 7

The OMSF implementation model defines a simplified profile of the full O&M Observation model and the selected Specialized Observation types mentioned in the figures above:

  • OMSF does not include all the Specialized Observation types mentioned in ISO 19156:2011 (see the table below).
  • OMSF observation model does not include the following of less commonly used OM_Observation class properties: parameter, relatedObservation and resultQuality. The rationale for these exclusions is given below.
  • OMSF properties are structured in a way that it is easy to encode them as simple features defined in GML Simple Features Profile version 2.0 (OGC Document 10-100r3).
  • Geometry and display name of the feature-of-interest (FoI) are embedded in the OMSF Observations as their primary geometry and a dedicated name properties to enable access to the Observation location without resolving the FoI feature.
  • Feature of interest of the OMSF observations is represented as separate properties for the proximateFeatureOfInterest and the ultimateFeatureOfInterest.
  • to align with the W3C Semantic Sensor Network Ontology specification, the method and the implementation of the measurement procedure has been split into two separate properties: procedure and observer.

!OMSF Implementation model

The table below lists the O&M Core and Specialized Observation UML classes implemented in the OMSF implementation model profile:

O&M v2.0 class OGC name OMSF feature
OM_CategoryObservation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_CategoryObservation omsf:CategoryObservation
OM_CountObservation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_CountObservation omsf:CountObservation
OM_Measurement http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_Measurement omsf:MeasureObservation
OM_Observation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_Observation omsf:GenericObservation
OM_TimeSeriesObservation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_TimeSeriesObservation omsf:MeasureTimeseriesObservation
OM_TruthObservation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_TruthObservation omsf:TruthObservation
OM_ComplexObservation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_ComplexObservation not implemented
OM_DiscreteCoverageObservation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_DiscreteCoverageObservation not implemented
OM_GeometryObservation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_GeometryObservation not implemented
OM_PointCoverageObservation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_PointCoverageObservation not implemented
OM_TemporalObservation http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_TemporalObservation not implemented

Observations with complicated results, such as coverages, are in most cases out-of-scope of the OMSF context. The implementation model is intentionally more limited than the one used for the Observations and Measurements - XML Implementation (OGC Document 10-025r1) and OGC Observations and Measurements – JSON implementation (OGC Document 15-100r1) which are able to express the full O&M abstract model. However, expressing any type of O&M Observation is possible by using the omsf:GenericObservation feature type with a remote reference to the Observation result.

The simplified implementation models for the proximate and the ultimate feature of interest are realised by embedding their essential properties into the Observation class itself. The sampling feature is encoded by one mandatory property geometry and an optional property proximateFeatureOfInterest. In a similar fashion the ultimate feature of interest (the model of the observed real-world object) is encoded using an optional property: ultimateFeatureOfInterest. The geometry property of a OMSF Observation feature is the geometry of the sampling feature of the observation, or if no sampling feature was used, a representative geometry of the ultimate feature of interest.

Observation mapping

The implementation model of the OMSF Observation is a simplified version of the Observation class as defined in the ISO 19156 standard. The following table summarises the simplification decisions applied:

O&M attribute/association O&M type O&M Multiplicity OMSF property OMSF type
proximateFeatureOfInterest>geometry or ultimateFeatureOfInterest>geometry association with AnyFeature 1 geometry GM_Object
ultimateFeatureOfInterest association with AnyFeature 0..n *) ultimateFeatureOfInterest Reference to an external resource
proximateFeatureOfInterest association with AnyFeature 0..n *) proximateFeatureOfInterest Reference to an external resource
metadata association with Any 0..* metadata Reference to an external resource
observedProperty association with ObservableProperty 1..n observedProperty Reference to an external code list
parameter NamedValue 0..n n/a n/a
phenomenonTime TM_Object 1 phenomenonTime TM_Object
procedure association with Procedure 1 procedure association with Procedure
observer association with Observer 1 observer association with Observer
observer>platform association with Platform 0..1 platform association with Platform
relatedObservation association with self 0..n n/a n/a
result Any 1 result varied
result Any 1 timeStep TM_Instant
result Any 1 unitOfMeasure Reference to an external code list
resultQuality DQ_Element 0..n n/a n/a
resultTime TM_Instant 1 resultTime TM_Instant
validTime TM_Period 0..1 validTime TM_Period
stimulusTime TM_Instant 0..1 stimulusTime TM_Instant

*) at least one of proximateFeatureOfInterest and ultimateFeatureOfInterest shall be given.

Rationale for the not included (n/a) properties:

  • parameter: 0..n multiplicity of name-value pairs (as user defined types) would require compliance level SF-1, or a specially encoded list type. Trade-off between completeness and simplicity.
  • relatedObservation: could be included technically (as a reference), but no way to specify the relation (role) without the user-defined types. Rarely used in practice.
  • resultQuality: embedded quality info would a user-defined type and thus SF-1 level. Rarely used in practice, trade-off between completeness and simplicity.

Procedure mapping

Attribute/association O&M type O&M Multiplicity OMSF property OMSF type
name GenericName 0..1 name GenericName
description CharacterString 0..1 name CharacterString

Observer mapping

Attribute/association O&M type O&M Multiplicity OMSF property OMSF type
name GenericName 0..1 name GenericName
description CharacterString 0..1 name CharacterString
observableProperty association with ObservableProperty 0..n observableProperty Reference to an external code list
platform association with Platform 0..1 platform Reference to Platform

Platform mapping

Attribute/association O&M type O&M Multiplicity OMSF property OMSF type
name GenericName 0..1 name GenericName
description CharacterString 0..1 name CharacterString
observer association with Observer 0..n observer association with Observer

GML and GeoJSON Encodings

OMSF contains both GML and GeoJSON encodings of the common OMSF implementation model defined above. For more details see

Acknowledgements

Work on this application profile was initiated by Ilkka Rinne for the needs of Vaisala and Finnish Meteorological Institute in 2017.