MS RFC 13: Support of Sensor Observation Service in MapServer

Date:2006/02/21
Author:Yewondwossen Assefa
Contact:yassefa at dmsolutions.ca
Status:passed

Overview

This is the a first attempt to support part of the OGC Sensor Web Enablement (SWE) in MapServer. The different specifications developed around the SWE are :

  • Sensor Observation Service (SOS) : provides an API for managing deployed sensors and retrieving sensor data.
  • Observation & Measurement: Information model and encoding for observations and measurements.
  • Sensor Alert Service : A service by which a client can register for and receive sensor alert messages. The service supports both pre-defined and custom alerts and covers the process of alert publication, subscription, and notification.
  • Sensor Model Language : Information model and XML encoding for discovering, querying and controlling Web-resident sensors.
  • Sensor Planning Service : A service by which a client can determine collection feasibility for a desired set of collection requests for one or more mobile sensors/platforms, or a client may submit collection requests directly to these sensors/platforms.
  • Transducer markup Language (TML) : General characterizations of transducers (both receivers and transmitters)
  • Web Notification Service : Executes and manages message dialogue between a client and Web service(s) for long duration asynchronous processes.

The intention here is to support the SOS mandatory operations. Please refer to the OGC site (http://www.opengeospatial.org/functional/?page=swe) for more details on the SWE initiatives.

User Interface

From the user perspective there will be an SOS interface will offer the three core operations (GetCapabilities, GetObservation and DescribeSensor). A full description of what could be available is presented in Annexe A : Sensor Observation System (SOS) support in MapServer.

Changes in MapServer

  • All the development will be localized into a mapogcsos.c file.
  • There will be additions to the mapows.c/h file to integrate the dispatch of the requests.
  • In mapgml.c (function msGMLWriteWFSQuery), extract the loop that writes features ( gml:featureMember) into a separate function so the the GetObservation can also use it to output the results.
  • The SOS capability will be available when MapServer is built using the flag USE_SOS.

Mapscript implications

The are no special implication for the MapScript module

Additional libraries

There will be an attempt to use the libxml2 (http://xmlsoft.org/) library when generating the GetCapabilities document. Decision to go ahead will be based on ease to use and speed of output.

Testing

It is proposed that automatic tests with map/data/expected results be added into the msautotest project to test the GetCapbilities and GetObservation requests.

Voting History

  • +1 : Assefa, Warmerdam, Nacionales
  • +0 : Morissette
  • -0 : Gillies

Note

discussions, concerns are available in the mapserver-dev list (Feb 2006 RFC 13 : SOS support)

Annexe A : Sensor Observation System (SOS) support in MapServer

This is a first attempt to define what will be supported in MapServer to be able to deploy a Sensor Observation System (SOS)

Specifications and useful links used :

SOS provides several operations divided into core mandatory operations (GetCapabilities, DescribeSensor and GetObservation) and optional transactional and enhanced operations. The first implementation of SOS in MapServer will only address the core operations

1. GetCapabilities Request

The GetCapabilities request will use the following parameters :

  • Request : fixed at GetCapabilities
  • Service : fixed at SOS

2. GetCapabilities returned document

Attached at the end of the document examples of a GetCapabilities document. The following elements are SOS items included in the capabilities document, with an equivalent MapServer implementation

  • ServiceIdentification (all elements are extracted from metadata at the web level)

    • Title : extracted from a metadata at web level ows/sos_title. Same concept as wms/wfs
    • Abstract : metadata ows/sos abstract
    • ServiceType : Fixed to SOS
    • ServiceType version : Fixed to 0.3
    • Fees : metadata Ows/sos_fees
    • Access Constrains : metadata : Ows/sos_constrain
  • ServiceProvider (all elements are extracted from metadata at the web level, using an equivalent name as the SOS element)

    • ProviderName :
    • ProviderSite
    • IndividualName
    • PositionName
    • Voice
    • Facsimile
    • DeliveryPoint
    • City
    • AdministrativeArea
    • PostalCode
    • Country
    • ElectronicmailAddress
    • EndAddress
    • OnlineResource
    • HoursOfService
    • ContactInstructions
  • Operation Metadata : This part of the capabilities defines the operations that will be supported which are GetCapbilities and GetObservation. For more information refer to https://svn.opengeospatial.org:8443/ogc- projects/ows-3/schema4demo/ows/1.0.30/owsOperationsMetadata.xsd

    • Operation : GetCapabilities

      • Operation Name : Fixed at GetCapabilities
      • HTTP : Connect point URL extracted. Only the Get request method is supported
      • Parameter : includes name and version. We could use the parameters to propagate the name of the service SOS and the version. Other parameters may be added if needed.
    • Operation : GetObservation

      • Operation Name : Fixed GetObservation
      • DCP (HTTP) : extracted from a metadata
      • Parameter : We could use this to propagate the parameters needed when doing a GetObservation request (Offering, eventTime)
    • Operation : DescribeSensor

  • Filter Capabilities

    The Filter Capabilities that will be supported are the same ones that are currently supported in MapServer (See WFS Filter Encoding for more info) : Spatial Capabilities, Logical Operators, Comparison Operators

    There is a mention in the specifications of ogc filter temporal capabilities, but I could not locate the exact definition of it. In any case,

  • SOS Contents (Observation Offerings)

    As explained in SOS specifications (section 6.2) , “ ...An observation offering is also analogous to a layer in a Web Map Service because each offering is intended to be a non-overlapping group of related observations. Each Observation Offering is constrained by a number of parameters including sensor system that report observation, Time, phenomena, geographical region ... “

    In MapServer an Offering will be represented by a group of layers using mapserer’s group parameter. The metadata associated with a group (offering will be taken from the first layer of the group)

    The following properties would be set at a group level.

    • Standard Properties

      • id : Unique Offering Identifier. Mandatory
      • name : name used with the offering. Optional
      • description : description of the offering. Optional
    • Bounded By : used to define the geographical boundaries of the Offering. It should be extracted from a metadata. Mandatory

    • EventTime : used to define a valid time range for the offering. It should be extracted from a metadata. Mandatory

    • Procedures : series of URLS reference to one or more systems that supply observations in the offering. It should be extracted from a metadata. Mandatory

  • Observed property : Observables/Phenomena that can be requested in this offering.

    Two specializations are identified in the specifications:

    • Constrained : modifies base phenomenon by adding a single constrains (ex surface water temperature add a constrain that depth is between 0, -0.3)
    • Compound which is either a composite (a set of component phenomena that may or may not relate to each other)or a phenomenonSeries that applies one or more constrainstList to the base phenomenon)

    There is no clear cut indication which representation would be the more natural for MapServer but if we consider the group/layer/attribute combination, we can see that a group of layers could represent an offering, a layer would be an observed property (or phenomenon) and the attributes would be the composite phenomenon defining the phenomenon.

    The capabilities document will include CompositePhenomenonType element with an mandatory id element identifying the phenomenon and optional elements such as name and component.

  • Feature Of Interest

    The definition given in the specification is : This is a single feature or a collection of features that represents the object on which the sensor systems are making observations. In case of in-situ sensor this may be a station to which the sensor is attached or the atmosphere directly surrounding it. For remote sensors this may be the area or volume that is being sensor. It is represented by GML feature type and is expected to include bounding box extents.

    In our case here, this would be equivalent to the “bounded by” element defined earlier.

    Note that in the implementation of MapServer, It is assumed that geographical information used to represent the individual sensors represent the feature of interest of the sensor. This is a requirement to be able to do spatial queries.

  • Result Format

    MIME type of the result that will be returned to a GetObservation request. Fixed to text/xml;subtype=”om1.0.30”

3. GetObservation Request

The GetObservation request used to retrieve observation data will be supported using the Get method. Post method will not be supported in the first implementation.

Here are the parameters that will be supported and their definitions :

  • Offering : Equivalent to the Offering Id identified in the capabilities (Mandatory)
  • evetnTime : single time or time period. This will be used as a Time filter to do the queries using an identified time attribute. (Optional)
  • observedProperty : Identifies the layer in MapServer (Mandatory)
  • featureOfInterest : Additional geographical filter using a bbox. (Optional)
  • Result : Will be used to filter using the OGC Filter supported capabilities.

4. Get Observation Response

An observation response should contain the following information :

  • Information describing the Offering
  • Valid time (instances or period)
  • Description of the phenomenon (like the offering name)
  • location and feature of interest for the offering
  • The result associated with the offering

In the case of MapServer implementation, what is proposed is to be returned an observation collection reflecting the query results. Here is the different elements returned :

  • name : The offering unique identifier
  • description : Description of the offering
  • time : Valid time instance or period
  • featureofinterest : Geographical extents covered of the offering
  • Member : This is repeated for all the observations returned. The following are the parameters included for each member
    • observed property : the phenomenon observed
    • location : geographical coordinates
    • procedure
    • result : Result of the observation. In the first implementation It is proposed that the gml:feature member is returned. This gives the possibility to return one/more attribute values in an easily manipulated format. The will be equivalent to a gml:feature member returned in wfs.

5. Describe Sensor

The Describe sensor request uses two parameter that are SensorId (Mandatory) and an optional outputFormat.

In this phase the DescribeSensor will use a metadata of URL type set on the layer and relay the request. There won’t be any SensorML output generation done in MapServer in this implementation.

6. Examples