21 May 2006 - Features, Observations and Authorization

When one says that a Road feature has 3 lanes what do we mean?  Does it mean that a friend of mine has observed this to be the case?  That a government body or private company will attest that this is a fact?  What is the difference?  Perhaps the issue is made more clearly if we replace the number of lanes property by say the centerline of the road - its geometry.  Presumably you would be much less ready to accept my friends version than that which has received the blessing of the government or a private corporation responsible for data distribution?  At the same time, you know that the government or private company had to at some point get the information, perhaps not from a friend of mine, but nonetheless from some individual or group of individuals out in the field actually measuring or observing what is "true".  So there is clearly an important distinction here.

The distinction is between what one might call an authorized feature - meaning that it has the support of some organization (e.g. government department, private company) and an observation - meaning that some person or organization has observed something, but it has not yet received the blessing of "authorization".

One may ask why such a formal distinction need be made.  Why not just treat everything as an observation and get the authorization part by simply noting who made it?  The problem with such an approach is that the same organization may be responsible for both observing and authorizing.  Additionally, an organization may be the authority (or custodian) for some types of information, while being simply an observer (albeit an authorized one) for something else.  A water company would generally be accepted as the authority for the location of water mains, while it might be considered an authorized observer (one that can be trusted by the custodian) for the location of street centerlines or parcel boundaries.  Note that this does not mean that the custodian takes the observations of the authorized observer at face value - simply that they are taken into account in determining the authorized location of these objects, where the survey conducted by my "friend" would not be.  Of course this is equally true within the organization, the authorized location, extent etc. of an object results from multiple observations, all presumably obtained from authorized observers.

The passage from observation to authorized feature is not typically a completely formal process.  It involves QA of course and very often human judgement as to the fitness for purpose of the resulting data object.

These notions of authorized feature and observation are captured in an approximate way in GML.  I say approximate in that in GML there is no "authorized" construct.  One can interpret, however, every GML feature to be an authorized feature, and then use GML observations to capture information about features which have not been "authorized".  Such observations are then authorized as observations and serve as the input to generating authorized features. 

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