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user defined primitives are organised by the operator.
(Figure 1.)
The object with its true geometry is the result of the
adaptation of the measures carried out on this object to
its topology.
A solution for the modelling of an undefined or not
common surface, for example a curved road on a bridge,
is given with a 3-D triangular irregular network as a
surface model, which can be created with the DTM
algorithms (Pfeiffer, Pottmann, 1996).
44. Hierarchical splitting up of the primitives
To increase the geometric reliability the different
primitives are split up into basic elements. This splitting
up make it possible to perform the selection of all
constitutive basic elements of a complex object.
The special case of the intersections between the
primitives must also be taken into account. These special
intersection are other constitutive and basic elements
that will be manipulated, transformed and requested.
After acquisition of supplementary data, the model can
be worked out again, and these new data are
incorporated into the basic and decomposed structure
through a coordinate transformation. A photogrammetric
plotted frontage, full of subtle details can in this way be
incorporated in a primitive face of an object. This face
results of the splitting up of a volumic object -like a box-
in sample surfaces surrounded by borderlines. The
different characteristics of the elements like the corners,
the nodes, the right vertical or horizontal edges are used
for the adaptation and transformation of coordinate. The
Complex Object
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finally the homogeneity of the representation in the
information system.
4.5. Local coordinate systems
Each 3-D element contains its own local coordinate
system. This coordinate system is at first a rough one for
the basic primitives. The local coordinate System is
allocated to each element -in its dimensions-
automatically or by the operator during the acquisition :
e For a point or an H-element, the local coordinate
system is defined with an origin and three
orthonormal axis.
e For a linear element, the local coordinate system is
defined with an origin on the element and a principal
direction along the element.
e For a surface element, the local coordinate system is
also defined with an origin on the surface and three
orthometric axis. The third axis allows to describe the
depth in the surface element and can be used for the
integration of a face into a surface.
e Complex 3D-objects can also have got local
coordinate systems that are defined with
characteristic points. These characteristic points allow
to place or move the complex 3D-objects in the 3D-
space.
The coordinate system can be oriented according to the
nature, the way of acquisition and the utilization of the
different elements.
The coordinate system becomes more accurate as the
geometrical precision of the element becomes higher.
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Figure 3. The hierarchical structure
storage of the two modes of initial coordinate systems
allows to save the primary data -with geometrical value-,
the adaptation from a coordinate system into another and
International Archives of Photogrammetry and
Transformations of coordinate allow the integration of
elements into higher level primitives. The different
coordinate systems are closely related to the measures
carried out on the ground objects.
463
Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
