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Re: Clarification required: BP5 "describing properties that change over time" [SEC=UNCLASSIFIED]

From: Bruce Bannerman <B.Bannerman@bom.gov.au>
Date: Wed, 24 Aug 2016 00:45:38 +0000
To: Jeremy Tandy <jeremy.tandy@gmail.com>, SDW WG Public List <public-sdw-wg@w3.org>
Message-ID: <1471999530173.65311@bom.gov.au>
Hi Jeremy,

wrt: '(time-series are one-dimensional coverages)'

I suspect that I'm missing the intent here, IMO this is but one type of time-series.

I believe that we can have n-dimensional time-series of traditional coverages as well, e.g.:

  *   ?a time-series of climate grids representing the distribution of maximum temperature across a continent;
  *   ??a time-series of n-dimensional climate models across a continent, or the globe;
  *   a time-series of n-dimensional satellite observations (i.e. multi-spectral images along a swathe over multiple passes over a given region); and
  *   a time-series of derived data across a region, e.g. a time-series of NDVI analyses derived from multi-spectral satellite imagery.

And of course in many instances we need to relate the observed property of the areal coverage time-series back to the type of one-dimensional time-series that you have referred to.


From: Jeremy Tandy <jeremy.tandy@gmail.com>
Sent: Wednesday, 24 August 2016 12:50 AM
To: SDW WG Public List
Subject: Clarification required: BP5 "describing properties that change over time"


BP5 [1] addresses the situation where the properties of a spatial thing are updated due to some new measurement or assertion. For example, the boundary geometry of flooded area will change as the flood levels rise and fall, or the details of a land parcel may be updated due to a new survey.

In this case, the resource is a spatial thing whose properties are captured as a series of discrete snapshots through time. Those snapshots may be created on a fixed schedule (e.g. as a result of an annual survey) or ad-hoc (e.g. some one notices that things have changed).

There is a second case where properties change over time: when the property is a time dependent variable that is being measured or estimated according to some sampling regime. In this case, the resource seems to be a "time-series" of property values. For example, the water level of a canal, the wind speed measured at routine intervals by an anemometer, or the GPS position of a fishing vessel.

(time-series are one-dimensional coverages)

So in the example case of the canal and its water-level, the canal (a spatial object) has a property "water-level" that refers to a water-level time-series coverage. As coverages are themselves features (so says OGC et al) then we can also say that our water-level time-series coverage is also a spatial thing (albeit a conceptual spatial thing that you can't trip over in the real world).

Is this second case a valid "possible approach to implementation" that should be added to BP 5? If so, please can someone in the working group suggest some criteria as when to use the "property snapshot" pattern (case 1) or the "time-series coverage" pattern (case 2).

(for me it's subjective, but seems to be related to the intent to establish a coherent sampling regime for a property value as opposed to an ad-hoc update)


BTW, I've deliberately skipped over the "value assignment" concern (e.g. how values are assigned to properties) as described in ISO 19109 7.4.10 ValueAssignment - for example, how OM_Observation can be used to relate the "observed value" to a "feature of interest" with all the metadata about the observation or measurement process. I don't think this is relevant for this BP.

[1]: http://w3c.github.io/sdw/bp/#desc-changing-properties
Received on Wednesday, 24 August 2016 00:46:15 UTC

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