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stereoscopy. In a similar sense, ISO 19115 implies LoD as “
a scale factor or a
ground distance
” which is only relevant for cartography and neglects the other
considerations (the examples discussed earlier).
ISO 19109 defines model as an “
abstraction of some aspects of a universe of
discourse
”. So, is 3D geospatial model an abstraction of some aspects of a 3D
universe? Although true, such perception is not enough to differentiate between
simple processed abstractions such as 3D line drawings and geometrically modeled
ones such as solids. Apel (
2006
) and Dollner and Buchholz (
2005
) indicate that
many users associate 3D geospatial modeling with 3D visual scenes. As a result,
terms like 3D and volumetric model/analysis are mentioned for describing on-
the-fly extrusion in ESRI ArcScene when polygons suddenly become 3D objects
as integrated in a 3D universe, surface extrusion in virtual globes and thickness
in AutoCAD, surface analysis in GIS, and thematic representations like 2D foot-
prints extruded based on building prices. Many GIS users know 3D city models
as thematic-geometric data structures that explicitly differentiate terrain, buildings,
and streets (Dollner et al.
2006
, Dollner and Buchholz
2005
). Meanwhile, 3D city
models among many CAD and CGI users comprise implicit geometries (e.g. sub-
objects) rendered with realistic textures. Penninga (
2008
), Bédard et al. (
2002
), and
Pilouk (
1996
) present another point of view that associates 3D geospatial modeling
with certain 3D reconstruction methods and the dimension of geometric primitives.
In this point of view, Digital Elevation Model (DEM) and extruded surfaces are
2.5D models (Gorte and Lesparre
2012
; Kessler et al.
2009
) and multiple dimen-
sions such as Multi 2.5D (Penninga
2008
), 2.75D (Moenickes et al.
2002
), and
2.8D (Groger and Plumer
2011
) exist before arriving at 3D models. Therefore,
terms like 3D and volumetric model/analysis are mentioned exclusively with 3D
geometric primitives such as solids, tetrahedrons, and voxels.
Cellary and Walczak (
2012
) and Funkhouser et al. (
2002
) indicate that mass
dissemination of 3D geospatial models requires enriching metadata and search
interfaces with specific fields and code lists. This has an impact on the terms found
in metadata with regards to 3D. When using more rigorous definitions, one will
rather find “2.5D”, “Multi 2.5D”, and “2.75D” than “3D” datasets. Inversely, one
may find metadata loosely labeled “3D” while, in fact, it is a 2.5D dataset accord-
ing to a rigorous definition. These ambiguities in the meanings of “3” and “D”
must be removed with appropriate definitions of 3D concepts in metadata.
3 Inventory on Metadata in Sharing 3D
Geospatial Resources
One way to assess the existing metadata or to identify the new requirements is
to consider the actual practitioners who publish and seek 3D models. This can be
studied among the sharing portals which publish geospatial datasets by providing
the information that outline them. Our investigation shows that although research-
ers have considered such approach when investigating 3D metadata, the number
