Ptolemy's Error:Truths and falsehoods in

heterogeneous spatial data

Leif IsaksenOxford Archaeology

Ptolemy's World Map

Spatial or textual Data and

representation separate – catalogue and instructions

Much less potential for error (perhaps)

But what about his data quality?

Ptolemy's Error

Uses 12ths of a degree

But distribution uneven

We know that the source data has varying precision

Problem – we don't know which.

Cautionary Tales

Misconception of point/area data even at standards level

Part of the problem is an oversimplified division – points/lines/polygons or vector vs. raster.

TQ125127

TQ1212

The Nature of Historic Mapping

Core Components Spatial Framework Entities (points/lines/polygons/cells)

Maps of the Past Descriptive Data underdetermines basis of compilation

Truth-standard can be defined as several 'facets' Strong and weak form Simple schema

The Categories

Incomplete vs. Complete

Fuzzy vs. Hard

Durée vs. Snapshot

Marker vs. Depiction

Observed vs. Derived

Incomplete vs. Complete

Incomplete F iff non-existent entity represented

Complete F iff non-existent entity represented or existent

entity represented Notes

Even vs. Uneven as special case

Fuzzy vs. Hard

Fuzzy Vagueness or Imprecision Vagueness F iff entities which can only be one

category are represented as another Imprecision F iff entity represented outwith

boundary of that imprecision Hard

F iff entity incorrectly categorised, or boundary condition doesn't hold

Durée vs. Snapshot

Durée F iff not all entities exist within a specified timeframe

Snapshot F iff not all entities are temporally coexistent at

specified timepoint Notes

Maps of the present are special case of Snapshots

Marker vs. Depiction

Marker F iff Marker location inaccurate with reference to

spatial framework Depiction

F iff location inaccurate or entity's and object's spatial features don't coincide

Observed vs. Derived

Observed F iff entity does not correspond to real object

Derived F iff output doesn't correspond to:

function(basedata) Notes

For Derived to have meaning, both function and basedata need to be referenced.

Questions

Theoretical Are these facets orthogonal? Periodic table - Are all combinations permissible? Interoperability - Are all combinations of

combinations useful? Practical

How can we make these facets transparent? What potential might they have for supporting data

management? Hard cases?