[Historic] Temporal Tagging

[mick]

On Mon, 21 Jan 2013 17:32:44 -0800
Brad Thompson <brad at pastmapper.com> wrote:

> The issue of how to handle changes in names, locations, and shapes (for
> territories, building footprints, etc) has been a favorite topic of mine in
> a few discussions to date. The problems can quickly become complex, but I
> think a simple schema could encompass all possible historical scenarios.
> I'm happy to share the observations / conclusions, which have generally
> been the following:
> 
> 1) Unlimited Changes to All Attributes: The schema should allow the
> recording of an *unlimited* number of changes to *any* attribute of any
> entity. No attribute can be used as a unique identifier, like 'name' or
> 'address' or even location-based attributes, because throughout history,
> all of these things can change. Therefore, a truly unique identifier must
> be assigned to each entity. Additionally, buildings are moved, streets are
> renamed, rerouted, and renumbered. Tim, as you've pointed out, this has
> happened many times to some entities. Therefore, a schema that simply
> allows for a single 'old-name' isn't flexible enough. All changes to
> attributes as described above must each have a time associated with them.
> 2) Confidence Factors: Because historical data inherently entail
> uncertainty, there should be a method of assigning a confidence factor to
> any attribute. (This feature has no purpose in realtime mapping, because
> all data can be verified against actual conditions.) This confidence factor
> would be applicable both to attributes like names as well as times. For
> names, for example, "we think the name of this hill was Telegraph Hill, but
> there are conflicting reports that claim it was called Signal Hill, so we
> assign a 60% confidence factor to Telegraph Hill and a 40% confidence
> factor to Signal Hill". The renderer could then decide how to display the
> name(s). For times, for example, "we know this hill changed name from Loma
> Alta to Telegraph HIll sometime between 1848 and 1852, but we don't know
> for certain when, so we assign the date of the change as January 1, 1850
> and give it a confidence factor of 4 years (creating a buffer with a
> temporal diameter of 4 years around that date). This idea is critical,
> because it allows conflicting reports and developing research to be
> displayed alongside well-established facts.
> 3) Spatial and Non-Spatial Entities: Because shapes (nodes, etc.) cannot be
> used as unique identifiers the way they can for realtime mapping, there
> exists a need to create a distinction between spatial entities and
> non-spatial entities. This way, each spatial permutation (or version) of an
> entity, like a building or a road or a territorial boundary, can have a
> distinct shape that is still linked to the nonspatial entity that
> represents the concept of its agreed-upon identity. For example, 'United
> States of America' would be a nonspatial entity with a start date of 1776
> and no end date. But linked to that entity would be dozens of spatial
> entities, because the boundaries of the United States have changed dozens
> of times, therefore changing the shape, through small border edits or
> territorial acquisitions. Each of those shapes would have its own start and
> end time, and the map would display the correct shape as determined by the
> time being viewed.
> 
> Obviously, we're talking about a dramatically different way of recording
> place data, but in my view, these levels of detail are critical to making a
> viable historical mapping platform where multiple types of data can be
> shared and displayed. Looking forward to hearing everyone else's thoughts
> on this.
> 
> Brad Thompson
> Pastmapper

You seem to have covered the issues well except for the elephant sitting on the table, if I could see just a little bit clearer we could factor it into the equation. Otherwise I'm fully behind you.

mick