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Representative Points

Creators: Tom Elliott Copyright © The Contributors. Sharing and remixing permitted under terms of the Creative Commons Attribution 3.0 License (cc-by).
Last modified Apr 05, 2022 01:42 PM
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What are "representative points" in Pleiades, what are they good for and how does one get access to them?

For each Pleiades place resource, we calculate a "representative point". This point is prominently displayed on the place page and rendered in the map on the place page with a distinctive orange circle. You can easily copy the representative point coordinates by selecting the "copy-to-clipboard" icon that appears immediately after the latitude and longitude coordinate pair under the "Representative Point" heading on the place page. Representative points are included in our CSV, JSON, and KML downloads as well.

How are these representative points calculated and why do some of them seem to be "wrong"?

In Pleiades data, a representative point for a place is usually a centroid calculated from the geometries of all the published locations associated with that place. If there is only one published location for a place, and if that location is a Point, then the representative point will have the same coordinates as that location; however if the location is a LineString or a Polygon, or if there are multiple locations, the representative point may not correspond to the coordinates in any single location, nor to the exact position of the site (if it is known).

Pleiades can record multiple locations for each place. Different source maps and gazetteers may provide different coordinates as a result of survey error, map scale limitations, and the like; Pleiades may record multiple such "conflicting" locations for a given place and so the representative point will fall between them. Both "relocated modern" and "original" locations may be recorded for a given place, causing its representative point to fall midway between the two (i.e., not coincident with either the feature's original position nor its modern spot). For some sites, incomplete published information or the results of time and change may make it impossible to pinpoint an ancient city's position, and so Pleiades will sometimes record an "associated modern" location that is nearby.

Some Pleiades places only have very large (on the order of 0.5 to 1.0 degrees on a side), rectangular Polygon locations and therefore the calculated representative points for these places may be very far indeed from the actual/original sites of the corresponding features. These big "bounding boxes" generally derive from grid squares on maps in the Barrington Atlas of the Greek and Roman World. They are still found in some Pleiades place resources for which more precise coordinates have not been digitized or imported from other sources. Work to replace these bounding boxes continues, and you can help by joining the effort.

Some Pleiades places do not have locations at all, either because the place cannot be located today on the basis of archaeological or geohistorical evidence, or because the place was a region or other spatial extent whose boundaries were imprecise or which cannot be precisely reconstructed. In these cases, Pleiades seeks to use connections between places as a basis for geospatial calculations. Thus, some places have representative points that are calculated on the basis of connections.

Pleiades calculates representative points on demand. When a person views a place page through a web browser or an automated agent requests a JSON serialization for a place or when the nightly exports are generated automatically, published locations and connections are collected and used to refresh the representative point(s). Updates to locations or to the locations of connected places, therefore, may cause the representative point of a place to change.