Brad,
I've done some work on the topic myself on the linked open data side
[1]. My conclusions are similar to yours: there are three different
types of information: The Thing, the Name of the Thing and the
Location of the Thing and all three are not the same Thing.
It is probably in our interest not to make too many changes to the OSM
XML specs; perhaps we can achieve everything by simply adding
start_date/end_date to features, ways, nodes and tags? I can't think
of a case where change could not be encoded using this approach.
There is the question of imprecise dates (which really aren't all that
different from having a precise date with no time). I'd suggest that
this be handled by the renderer: most people visualize a year or era,
finding features is a query based on date overlap. My only addition
would be the before or after tags that would account for "I know it
used to be call Signal Hill but I don't remember when".
You might mean probability in your example instead of confidence
factor, but my concern with both is that they mean essentially nothing
unless you know exactly what process generated the numbers; I'm not
sure what I would do with a tag that said "There is a 45% chance that
this is the grave of XYZ".
Perhaps your Signal Hill example is not the best for confidence factor
in that toponymy is primarily a cultural artifact; I can think of
several features that appear under different names depending on the
institution that produced the map. The solution to that is the
creative use of the source tag, which in the end is no different that
the language tag for this particular case.
I am working on another project where documenting a confidence factor
would be useful in that location is estimated statistically from the
reported sinking locations of ships. The wreck location is predicted
to be within a polygon with a .95 confidence interval, which is much
more useful that a single node when trying to figure out whether an
anchor is likely to foul at that location. Still for most people, or a
renderer, the statistical information is unlikely to be useful without
a reference to a description of how it was achieved and documenting
that seems complex.
Does OSM have a tag to record Feature A is part of Feature B?
best,
rhw
[1] http://rdf.muninn-project.org/ontologies/graves.html
On 21-Jan-13, at 8:33 PM, [email protected] wrote:
Message: 4
Date: Mon, 21 Jan 2013 17:32:44 -0800
From: Brad Thompson <[email protected]>
Subject: Re: [Historic] Temporal Tagging
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
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