This VA SIGMET is intended as
an example and does not represent actual conditions.


The year and month of the issue of this VA SIGMET
are unknown; these have been substituted with YYYY and MM respectively.


This tutorial is intended as a follow-on from the simple METAR tutorial.
Please work through the METAR example first as this VA SIGMET tutorial assumes
that many of the key points will already be understood.


The full XML for this example can be found here.

TAC-encoding

This example is based on the
following VA SIGMET:

EGGX
SIGMET 4 VALID 251600/252200 EGRR –

EGGX
SHANWICK OCEANIC FIR VA ERUPTION MT HEKLA PSN N6359 W01940 VA CLD OBS AT 1600Z
SFC/FL550 N6000 W01150 - N5900 W01300 - N6000 W01600 - N6000 W01150 MOV S 60KT
NC FCST 2200Z SFC/FL550 N6000 W01200 - N5800 W01400 - N6000 W01535 - N6000
W01200=

XML Namespaces

The following namespaces are used either directly or indirectly in this
example:

Description	XML Namespace	Default namespace prefix
XML Schema	http://www.w3.org/2001/XMLSchema	xsd
XML Linking Language	http://www.w3.org/1999/xlink	xlink
OGC GML 3.2.1	http://www.opengis.net/gml/3.2	gml
ISO/TS 19139:2007 metadata XML schema implementation	http://www.isotc211.org/2005/gmd	gmd
OGC OMXML Observations and Measurements	http://www.opengis.net/om/2.0	om
OGC OMXML Sampling Features	http://www.opengis.net/sampling/2.0	sf
OGC OMXML Spatial Sampling Features	http://www.opengis.net/samplingSpatial/2.0	sams
WMO Observable Property Model(1.2)	http://def.wmo.int/opm/2013	opm
WMO METCE (1.2)	http://def.wmo.int/metce/2013	metce
ICAO Meteorological Information Exchange Model (2.1)	http://icao.int/iwxxm/2.1	iwxxm

Note that the XML Namespace is distinct from the schema locations. For
reference, these are provided here.

Overview: the VA
SIGMET report

The XML encoded VA SIGMET has a root element <iwxxm:VolcanicAshSIGMET> within which the XML Namespace
information is declared.


The VA SIGMET report is comprised of:

• report metadata (e.g. status,
  issuing centre, originating
  meteorological watch office etc.);



• one analysis
  providing one up to two observed or forecast position and motion of the
  volcanic ash cloud; and



• one forecast
  providing one up to two predicted positions of the volcanic ash cloud.



• if there are more than one
  observed and/or predicted positions of volcanic ash clouds, it is expected
  that the first predicted position corresponds to the first observed
  position and so forth.

The code fragment below illustrates the report structure:

 1 . <?xml version="1.0" encoding="UTF-8"?>

 2 .     <iwxxm:VolcanicAshSIGMET xmlns:iwxxm="http://icao.int/iwxxm/2.1"   

 3 .                              xmlns:aixm="http://www.aixm.aero/schema/5.1.1"   

 4 .                              xmlns:metce="http://def.wmo.int/metce/2013"    

 5 .                              xmlns:om="http://www.opengis.net/om/2.0"   

 6 .                              xmlns:sam="http://www.opengis.net/sampling/2.0"   

 7 .                              xmlns:sams="http://www.opengis.net/samplingSpatial/2.0"   

 8 .                              xmlns:gml="http://www.opengis.net/gml/3.2"    

 9 .                              xmlns:xlink="http://www.w3.org/1999/xlink"  

10 .                              xmlns:xsd="http://www.w3.org/2001/XMLSchema"  

11 .                              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"   

12 .                              xsi:schemaLocation="http://icao.int/iwxxm/2.1 http://schemas.wmo.int/iwxxm/2.1/iwxxm.xsd  

13 .                                                  http://def.wmo.int/metce/2013 http://schemas.wmo.int/metce/1.2/metce.xsd"  

14 .                              gml:id="va-sigmet-EGXX-YYYYMM25T160000Z"  

15 .                              permissibleUsage="OPERATIONAL"

16 .                              status="NORMAL">  

17 .         <iwxxm:issuingAirTrafficServicesUnit>...</iwxxm:issuingAirTrafficServicesUnit>  

18 .         <iwxxm:originatingMeteorologicalWatchOffice>...</iwxxm:originatingMeteorologicalWatchOffice>  

19 .         <iwxxm:sequenceNumber>4</iwxxm:sequenceNumber>  

20 .         <iwxxm:validPeriod>...</iwxxm:validPeriod>  

21 .         <iwxxm:phenomenon xlink:href="http://codes.wmo.int/49-2/SigWxPhenomena/VA" xlink:title="Volcanic Ash"/>  

22 .         <iwxxm:analysis>...</iwxxm:analysis>  

23 .         <iwxxm:forecastPositionAnalysis>...</iwxxm:forecastPositionAnalysis>  

24 .         <iwxxm:eruptingVolcano>...</iwxxm:eruptingVolcano>  

25 .     </iwxxm:VolcanicAshSIGMET> 

The VA SIGMET report comprises:

• (Line 14) a unique
  identifier for the report (e.g.
  "va-sigmet-EGXX-YYYYMM25T160000Z"); each VA SIGMET report should
  have a unique identifier of the form: va-sigmet-{issuing Air Traffic Service
  Unit}-{issue date-time}.



• (Line 15) permissible
  usage; in this case operational



• (Line 16) report status;
  in this case normal



• (Line 17) information
  about the issuing Air Traffic Services Unit (elaborated below)



• (Line 18) information
  about the originating Meteorological Watch Office (elaborated below)



• (Line 19) report
  sequence number - following the standard ICAO Annex 3 (WMO No. 49 Vol 2)
  rules for report assigning sequence numbers



• (Line 20) definition of
  the validity period for this report (elaborated below)



• (Line 21) definition of
  the significant weather phenomenon that the report describes; in this case
  Volcanic Ash. Terms are referenced using xlink:href from the WMO Codes Register http://codes.wmo.int/49-2/SigWxPhenomena.
  xlink:title is provided
  to supplement human readability. 



• (line
  22) the analysis provides one up to two observed or forecast positions and
  motions of the volcanic ash clouds. This is encoded as an instance of <om:OM_Observation>
  (elaborated below).



• (Line 23) the forecast
  provides one up to two predicted positions of the volcanic ash clouds.
  This is encoded as an instance of <om:OM_Observation> (elaborated below).



• (Line 24) provides
  information about the source of the volcanic ash; the name and position of
  the erupting volcano (elaborated below).

 

Issuing Air Traffic
Services Unit and Originating Meteorological Watch Office

  

...

    <iwxxm:issuingAirTrafficServicesUnit>

        <aixm:Unit gml:id="fic-EGGX">

            <aixm:timeSlice>

                <aixm:UnitTimeSlice gml:id="fic-EGGX-ts">

                    <gml:validTime/>

                    <aixm:interpretation>SNAPSHOT</aixm:interpretation>

                    <aixm:name>SHANWICK OCEANIC AREA CONTROL CENTRE</aixm:name>

                    <aixm:type>ATCC</aixm:type>

                    <aixm:designator>EGGX</aixm:designator>

                </aixm:UnitTimeSlice>

            </aixm:timeSlice>

        </aixm:Unit>

    </iwxxm:issuingAirTrafficServicesUnit>

    <iwxxm:originatingMeteorologicalWatchOffice>

        <aixm:Unit gml:id="wmo-EGRR">

            <aixm:timeSlice>

                <aixm:UnitTimeSlice gml:id="mwo-EGRR-ts">

                    <gml:validTime/>

                    <aixm:interpretation>SNAPSHOT</aixm:interpretation>

                    <aixm:name>UK METEOROLOGICAL OFFICE - EXETER</aixm:name>

                    <aixm:type>MWO</aixm:type>

                    <aixm:designator>EGRR</aixm:designator>

                </aixm:UnitTimeSlice>

            </aixm:timeSlice>

        </aixm:Unit>

    </iwxxm:originatingMeteorologicalWatchOffice>

...

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• Information about both Issuing
  Air Traffic Services Unit (ATSU) and Originating
  Meteorological Watch Office (MWO) is included in the SIGMET
  report. Both of these entities conform to the Unit data
  model defined in the Aeronautical Information eXchange
  Model (AIXM). This is intended to align with the broader ICAO Aeronautical
  Information Reference Model (AIRM); albeit only a subset of the
  information required for meteorological products is required.



• The gml:ids shown here are fictitious.  A core feature enabling consistency and
  alignment within the AIRM, is the use of UUIDs (RFC 4122) for identifying
  and referencing information about aeronautical features. The use of UUIDs
  is adopted from AIXM 5, conjointly developed by EuroControl
  and US FAA. A managed set of information describing an aeronautical
  feature is allocated a UUID by the data publisher. The UUID is an opaque
  identifier and may be automatically generated (e.g. using an online UUID-generator
  or other mechanism). Once assigned by a data publisher, the UUID may be
  used as a unique reference to the given set of information, thus enabling
  distributed systems to assert that they are, indeed, using consistent
  information.



• Additional information
  provided about the ATSU and MWO includes:
  • name; this must be
    provided in BLOCK CAPITALS

  
  
  • type; one of the
    following options - ARO (Aerodrome Reporting Office/ATS Reporting
    Office), ATCC (Air Traffic Control Centre), ATSU (Air Traffic Services
    Unit), FIC (Flight Information Centre) or MWO (Meteorological Watch
    Office)

  
  
  • designator; the coded designator
    associated with the Unit - this may be the ICAO 4-letter Location
    Indicator as specified in ICAO Doc 7910.

  
  

 

Valid period

 

...

    <iwxxm:validPeriod>

        <gml:TimePeriod gml:id="tp-25T16Z-25T22Z">

            <gml:beginPosition>YYYY-MM-25T16:00:00Z</gml:beginPosition>

            <gml:endPosition>YYYY-MM-25T22:00:00Z</gml:endPosition>

        </gml:TimePeriod>

    </iwxxm:validPeriod>

...

 

• <iwxxm:validPeriod>
  defines the period during which this SIGMET is valid;
  noting the use of YYYY and MM in place of year and month - normally these
  would be populated (e.g. 2013-05-08T15:58:00Z)



• The <om:OM_Observation>
  instances for the analysis and forecast both include the
  element <om:validTime> which has identical
  semantics to <iwxxm:validPeriod>.



• Even though information
  is logically repeated, <iwxxm:validPeriod>
  has been retained to make it easier to map the XML encoding onto the ICAO
  Annex 3 regulation.

 

SIGMET Analysis

The analysis provides one up to two observed or forecast positions and motions
of the volcanic ash clouds. This is encoded as an instance of <om:OM_Observation>.


The code fragment below illustrates the outline structure of the <om:OM_Observation> instance:

  1 . ...

  2 .     <iwxxm:analysis>

  3 .         <om:OM_Observation gml:id="va-position-and-motion-EGXX-YYYYMM25T16Z">

  4 .             <om:type xlink:href="http://codes.wmo.int/49-2/observation-type/IWXXM/2.1/SIGMETEvolvingConditionAnalysis"/>

  5 .             <om:phenomenonTime>

  6 .                 <gml:TimeInstant gml:id="ti-25T16Z">

  7 .                     <gml:timePosition>YYYY-MM-25T16:00:00Z</gml:timePosition>

  8 .                 </gml:TimeInstant>

  9 .             </om:phenomenonTime>

 10 .             <om:resultTime xlink:href="#ti-25T16Z"/>

 11 .             <om:validTime xlink:href="#tp-25T16Z-25T22Z"/>

 12 .             <om:procedure>...</om:procedure>

 13 .             <om:observedProperty

 14 .                 xlink:href="http://codes.wmo.int/49-2/SigWxPhenomena/VA"

 15 .                 xlink:title="Volcanic ash"/>

 16 .             <om:featureOfInterest>...</om:featureOfInterest>

 17 .             <om:result>...</om:result>

 18 .         </om:OM_Observation>

 19 .     </iwxxm:analysis>

 20 . ...

The analysis <om:OM_Observation>
follows the standard OM_Observation pattern:

• (Line 3) the <om:OM_Observation>
  instance is allocated a unique identifier within the
  scope of the XML document (e.g. "va-position-and-motion-EGXX-YYYYMM25T16Z");
  there are no special rules applied here with regards to the structure of
  the identifier. The only requirements are that it is unique within the
  document.



• (Line 4) as outlined in
  the METAR tutorial, the observation
  type is specified via reference to a definition published within
  the WMO Codes Registry. For SIGMET "analysis" this is SIGMETEvolvingConditionAnalysis. 



• (Lines 5-9) the phenomenon
  time indicates the time-instant associated with the given position
  and motion of the volcanic ash cloud described in the result
  below. In this example, the observed position and motion of the ash cloud
  is provided, thus the phenomenonTime
  is commensurate with the beginning of the SIGMET report validity period.
  The phenomenon time is expressed in ISO 8601 format;
  "YYYY-MM-25T16:00:00Z" (noting that YYYY and MM would normally
  specify year and month respectively).



• (Line 10) the result
  time indicates the time-instance at which the information about
  the position and motion of the volcanic ash cloud was made available. This
  always commensurate with the beginning of the SIGMET report validity
  period. In this case, this is the same time-instant that was used to
  describe the phenomenon time thus xlink:href is used to refer to the earlier
  definition.



• (Line 11) the valid
  time indicates the time-period for which the information about
  the position and motion of the volcanic ash cloud is valid - in this case
  according to international regulation. This is identical to the valid
  period defined in the body of the SIGMET report, thus xlink:href is used to refer
  to the earlier definition.



• (Line 12) the procedure
  by which the position and motion of the volcanic ash cloud was observed
  (elaborated below).



• (Lines 13-15) the observed
  property identifies the physical phenomenon being observed.
  Following consideration, it seems appropriate to reference "volcanic
  ash" as the observed property, rather than an aggregate of
  "position and motion" about some undefined significant weather
  phenomena! The observed property is used a key when
  searching for content within a service conforming to the Open Geospatial
  Consortium's Sensor Observation Service (SOS) specification; it seems
  likely that consumers of the data product will search based on the type of
  significant weather phenomena rather than the characteristics of that
  phenomenon. Given that observed property must be a
  reference (e.g. specified using xlink:href),
  we reference a term from the WMO Codes Registry (e.g. http://codes.wmo.int/49-2/SigWxPhenomena/VA).
  xlink:title provides a
  supplemental human-readable description.



• (Line 16) the feature
  of interest provides information about the real world feature
  that the observation pertains to - in this case a Flight Information
  Region (elaborated below).



• (Line 17) the result
  provides the information about the observed characteristics of the
  volcanic ash cloud (elaborated below).

 

SIGMET Analysis:
Procedure

 

  1 . ...

  2 .             <om:procedure>

  3 .                 <metce:Process gml:id="p-49-2-va-sigmet">

  4 .                     <gml:description>WMO No. 49 Volume 2 Meteorological Service for International Air Navigation 

  5 .                                      C.3.1.7 SIGMET AND AIRMET INFORMATION, AERODROME WARNINGS AND WIND SHEAR 

  6 .                                      WARNINGS AND ALERTS

  7 .                     </gml:description>

  8 .                 </metce:Process>

  9 .             </om:procedure>

 10 . ...

As outlined in the METAR example, the procedure
is constrained such that is must be an instance of METCE Process. As described
therein, <gml:description>
is used to cite a well-known reference - in this case the appropriate clause
from the ICAO Annex 3 (WMO No. 49 Vol 2) technical regulations.

SIGMET Analysis:
Feature of interest

 

  1 . ...

  2 .             <om:featureOfInterest>

  3 .                 <sams:SF_SpatialSamplingFeature gml:id="ss-SHANWICK_OCA">

  4 .                     <sam:type xlink:href="http://www.opengis.net/def/samplingFeatureType/OGC-OM/2.0/SF_SamplingSurface"/>

  5 .                     <sam:sampledFeature>

  6 .                         <aixm:Airspace gml:id="fir-EGGX">

  7 .                             <aixm:timeSlice>

  8 .                                 <aixm:AirspaceTimeSlice gml:id="fir-EGGX-ts">

  9 .                                     <gml:validtime/>

 10 .                                     <aixm:interpretation>SNAPSHOT</aixm:interpretation>

 11 .                                     <aixm:type>FIR</aixm:type>

 12 .                                     <aixm:designator>EGXX</aixm:designator>

 13 .                                     <aixm:name>SHANWICK OCEANIC FIR</aixm:name>

 14 .                                 </aixm:AirspaceTimeSlice>

 15 .                             </aixm:timeSlice>

 16 .                         </aixm:Airspace>

 17 .                     </sam:sampledFeature>

 18 .                     <! FIR boundary - not typically reported >

 19 .                     <sams:shape nilReason="withheld"/>

 20 .                 </sams:SF_SpatialSamplingFeature>

 21 .             </om:featureOfInterest>

 22 . ...

 

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The feature of interest for the SIGMET analysis is intended to
describe the Flight Information Region associated with the SIGMET report. As
such, the feature of interest is constrained such that it must
be an instance of SF_SamplingSurface and thus follows
the standard pattern for all SF_SpatialSamplingFeatures:

• (Line 3) the <sams:SF_SpatialSamplingFeature>
  instance is allocated a unique identifier within the scope of the XML
  document (e.g. "ss-SHANWICK_OCA");
  there are no special rules applied here with regards to the structure of
  the identifier. The only requirements are that it is unique within the
  document.



• (Line 4) the type
  of the <sams:SF_SpatialSamplingFeature>
  instance; in this case a SF_SamplingSurface.



• (Lines 5-17) the sampled
  feature describes the real world feature to which the observation
  pertains; in this case the Shanwick Oceanic
  Flight Information Region.
  • (Lines 6-16) in lieu of
    an authoritative web-accessible register of information about Flight
    Information Regions, the information about Shanwick
    Oceanic FIR must be embedded within the SIGMET report. The data model used
    to describe the FIR is Airspace from the Aeronautical
    Information eXchange Model (AIXM). This is
    intended to align with the broader ICAO Aeronautical Information
    Reference Model (AIRM); albeit only a subset of the information required
    for meteorological products is required. While the gml:id shown here is fictitious, a core feature
    enabling consistency and alignment within the AIRM, is the use of UUIDs (RFC 4122) as outlined
    above (ATSU and MWO) - see line 6. Additional information provided about
    the Airspace includes:
    • (Line 11) type;
      one of FIR (Flight Information Region), UIR (Upper flight Information
      Region) or CTA (Control Area)

    
    
    • (Line 12) designator;
      a coded designator associated with the Airspace - in this case the ICAO
      location indicator "EGXX"

    
    
    • (Line 13) name;
      the textual name allocated to the Airspace by the responsible authority
      - this must be provided in BLOCK CAPITALS

    
    
  
  


• (Line 18) The
  FIR bounding is not typically reported.

 

SIGMET Analysis:
Result

  

 

  1 . ...

  2 .             <om:result>

  3 .                 <iwxxm:SIGMETEvolvingConditionCollection gml:id="emc-va-obs-EGXX-YYYYMM25T16Z" timeIndicator="OBSERVATION">

  4 .                     <iwxxm:member>

  5 .                         <iwxxm:SIGMETEvolvingCondition gml:id="sec1" intensityChange="NO_CHANGE">

  6 .                             <iwxxm:directionOfMotion uom="deg">180</iwxxm:directionOfMotion>

  7 .                             <iwxxm:geometry>

  8 .                                 <aixm:AirspaceVolume gml:id="av-va-obs-position-EGGX-YYYYMM25T16Z">

  9 .                                     <! omitted lower-limit implies that the volcanic ash cloud extends to ground/sea surface >

 10 .                                     <aixm:upperLimit uom="FL">550</aixm:upperLimit>

 11 .                                     <aixm:upperLimitReference>STD</aixm:upperLimitReference>

 12 .                                     <aixm:horizontalProjection>

 13 .                                         <aixm:Surface gml:id="polygon-va-obs-position-EGGX-YYYYMM25T16Z" 

 14 .                                                       uomLabels="deg deg" 

 15 .                                                       axisLabels="Lat Lon" 

 16 .                                                       srsDimension="2" 

 17 .                                                       srsName="http://www.opengis.net/def/crs/EPSG/0/4326">

 18 .                                             <gml:polygonPatches>

 19 .                                                 <gml:PolygonPatch>

 20 .                                                     <gml:exterior>

 21 .                                                         <gml:LinearRing>

 22 .                                                             <! bounding polygon clipped to edge of FIR @ 60N >

 23 .                                                             <gml:posList>

 24 .                                                                 60.0 -11.5

 25 .                                                                 59.0 -13.0

 26 .                                                                 60.0 -16.0

 27 .                                                                 60.0 -11.5

 28 .                                                             </gml:posList>

 29 .                                                         </gml:LinearRing>

 30 .                                                     </gml:exterior>

 31 .                                                 </gml:PolygonPatch>

 32 .                                             </gml:polygonPatches>

 33 .                                         </aixm:Surface>

 34 .                                     </aixm:horizontalProjection>

 35 .                                 </aixm:AirspaceVolume>

 36 .                             </iwxxm:geometry>

 37 .                             <iwxxm:speedOfMotion uom="[kn_i]">60</iwxxm:speedOfMotion>

 38 .                         </iwxxm:SIGMETEvolvingCondition>

 39 .                     </iwxxm:member>

 40 .                 </iwxxm:SIGMETEvolvingConditionCollection>

 41 .             </om:result>

 42 . ...

 

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The analysis result provides one up to two observed position
and motion characteristics of the volcanic ash clouds. In
order to align with timings for Volcanic Ash Advisory (VAA) products, the
SIGMET analysis may describe the characteristics of the volcanic ash clouds at
some point in the near future (e.g. up to 2-hours hence); e.g. a forecast of
the volcanic ash clouds characteristics.


The result is constrained such that it must be an instance of SIGMETEvolvingConditionCollection. This class has been
specifically designed to meet the requirements published in ICAO Annex 3 (WMO
No. 49 Vol 2) regulation and allows one to specify a collection of
meteorological features (e.g. the volcanic ash cloud or other significant
weather phenomena) described by one or more members each of which contain an
instance of SIGMETEvolvingCondition.


The code fragment above illustrates the encoding of <iwxxm:SIGMETEvolvingConditionCollection> and <iwxxm:SIGMETEvolvingCondition>:

• (Line 3) an XML
  attribute timeIndicator in <iwxxm:SIGMETEvolvingConditionCollection> identifies
  whether this section refers to an "OBSERVATION" or
  "FORECAST"



• (Line 4) the instance of
  <iwxxm:SIGMETEvolvingConditionCollection>
  contains <iwxxm:member> each of which has
  an instance of <iwxxm:SIGMETEvolvingCondition>



• (Line 5) the instance of
  <iwxxm:SIGMETEvolvingCondition>
  is uniquely identified within the scope of the XML document. Additionally,
  we see the intensity change specified as an XML
  attribute. In this case the value is "NO_CHANGE". The permitted
  set of options also includes "WEAKEN" and "INTENSIFY"
  as per the ICAO Annex 3 (WMO No. 49 Vol 2) technical regulation.



• (Line 6) specifies the direction of motion of the
  volcanic ash cloud specified in degrees from true-north; in this case the
  value is 180 degrees (e.g. due south).



• (Lines 7-36) specifies a
  2.5-dimensional representation of the observed geometry
  of the volcanic ash cloud using an instance of <iwxxm:AirspaceVolume>. In this case, the horizontal
  projection is given (lines 12-34) along with the upper
  limit at FL550 (line 10); the omitted lower limit implies that
  the AirspaceVolume extends to the ground/sea
  surface. Note that the upper-limit height is specified as measured in the
  ICAO standard atmosphere "STD" thus allowing for assessment of
  altitude as a linear distance when reading converting from barometric
  pressure (line 11). The horizontal projection is provided as an instance
  of <aixm:Surface>
  uniquely identified as
  "polygon-va-obs-position-EGXX-YYYYMM25T16Z" (lines 13-33). Note
  that, as per the GML specification, the first and last points defining the
  polygon are coincident. Following advice from ICAO secretariat, the
  horizontal projection of the significant meteorological feature must
  always be defined as a closed polygon.



• (Line 37) specifies the speed of motion of the volcanic
  ash cloud; in this case a velocity of 60 knots.

 

 

SIGMET Forecast:
Result

 

  1 . ...

  2 .             <om:result>

  3 .                 <iwxxm:SIGMETPositionCollection gml:id="mpc-EGGX-YYYYMM25T16Z">

  4 .                     <iwxxm:member>

  5 .                         <iwxxm:SIGMETPosition gml:id="mp-va-fcst-EGGX-YYYYMM25T16Z">

  6 .                             <iwxxm:geometry>

  7 .                                 <aixm:AirspaceVolume gml:id="av-va-fcst-position-EGGX-YYYYMM25T22Z">

  8 .                                     <aixm:horizontalProjection>

  9 .                                         <aixm:Surface gml:id="polygon-va-fcst-position-EGGX-YYYYMM25T22Z"

 10 .                                                       uomLabels="deg deg" 

 11 .                                                       axisLabels="Lat Lon" 

 12 .                                                       srsDimension="2" 

 13 .                                                       srsName="http://www.opengis.net/def/crs/EPSG/0/4326">

 14 .                                             <gml:polygonPatches>

 15 .                                                 <gml:PolygonPatch>

 16 .                                                     <gml:exterior>

 17 .                                                         <gml:LinearRing>

 18 .                                                             <! bounding polygon clipped to edge of FIR @ 60N >

 19 .                                                             <gml:posList>

 20 .                                                                 60.0 -12.0

 21 .                                                                 58.0 -14.0

 22 .                                                                 60.0 -15.35

 23 .                                                                 60.0 -12.0

 24 .                                                             </gml:posList>

 25 .                                                         </gml:LinearRing>

 26 .                                                     </gml:exterior>

 27 .                                                 </gml:PolygonPatch>

 28 .                                             </gml:polygonPatches>

 29 .                                         </aixm:Surface>

 30 .                                     </aixm:horizontalProjection>

 31 .                                 </aixm:AirspaceVolume>

 32 .                             </iwxxm:geometry>

 33 .                         </iwxxm:SIGMETPosition>

 34 .                     </iwxxm:member>

 35 .                 </iwxxm:SIGMETPositionCollection>

 36 .             </om:result>

 37 . ...

The forecast result provides one up to two predicted positions
of the volcanic ash clouds.


The result is constrained such that it must be an instance of SIGMETPositionCollection (from IWXXM). This class comprises
one or more members, each of which must be of type SIGMETPosition
(from IWXXM). Once again, these classes have been specifically designed to meet
the requirements published in ICAO Annex 3 / WMO No. 49 Volume 2 regulation.


The code fragment above illustrates the encoding of <iwxxm:MeteorologicalPositionCollection>:

• (Lines 3-35) define the collection
  - on this occasion with just a single member (e.g. only one position is
  predicted for the volcanic ash cloud at T+6hrs).
  • (Lines 5-33) define the
    position of the volcanic ash cloud at 22:00Z. The SIGMET
    "forecast" only requires the geometry of the
    volcanic ash cloud to be provided - once more via an instance of <aixm:AirspaceVolume> (lines 7-31).

  
  

Erupting volcano

 

  1 . ...

  2 .     <iwxxm:eruptingVolcano>

  3 .         <metce:Volcano gml:id="v-MT-HEKLA">

  4 .             <metce:name>MT HEKLA</metce:name>

  5 .             <metce:position>

  6 .                 <gml:Point gml:id="ref-point-MT-HEKLA"

  7 .                            uomLabels="deg deg"

  8 .                            axisLabels="Lat Lon" 

  9 .                            srsDimension="2"

 10 .                            srsName="http://www.opengis.net/def/crs/EPSG/0/4326">

 11 .                     <gml:pos>63.59 -19.40</gml:pos>

 12 .                 </gml:Point>

 13 .             </metce:position>

 14 .         </metce:Volcano>

 15 .     </iwxxm:eruptingVolcano>

 16 . ...

The final element of the VA SIGMET report is provision of information about the
source of the volcanic ash; the name and position of the erupting volcano. The
code fragment above illustrates:

• (Line 3) erupting
  volcano; a uniquely identified instance of <metce:Volcano>. METCE also
  provides a EruptingVolcano
  class which the provision for stating the date of the volcanic eruption if
  known. However, there is no requirement to provide this information within
  the VA SIGMET.



• (Line 4) volcano
  name. Note that the "Global Volcanism Program" provides
  an online, searchable catalogue
  of volcanoes which may assist in identifying the authoritative name
  for a given volcano feature.



• (Lines 5-13) position
  specifies the geographic location of the volcano using an instance of <gml:Point>.