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N CAAD
IPS and CAAD:
commonly
ones found
nce can be
ue points in
0 in CAAD.
s modelling
nents have
1ring means
are to this
ems. Means
of higher level groupings, which are indispensable
in CAAD, are missing in point- and lineoriented
photogrammetry systems.
Another difference is that in CAAD, there is a need
for the exchange of data between different parties
involved in a project. In photogrammetry this is
usually not a necessity. As a consequence, there is
no standard format for digital photogrammetry,
whereas for CAAD, there is a de facto standard that
is used in much of the architecture and engineering-
world: DXF.
While there are many research projects going on to
improve or refine the status-quo of this data-
exchange and the limitations of DXF have been
pointed out many times, it is impossible to ignore it
in discussions about data-structures in CAAD. The
development of a data-integration between DIPS
and CAAD must acknowledge this fact. The DXF-
structure actually can serve as a model or guideline,
how a data-structure that is compatibel with CAAD
and provides semantic and object-oriented
information, must be set up.
2.2 Data-Structure in the Digital Photogrammetry
System
The Datastructure in the DIPS developed for this
project can schematically be described as follows:
There is one 3D representation, consisting of objects
that reference points in a list of unique 3D points
(unique meaning that there are no two points with
identical coordinates). Additionally there are 2D
representations of the same objects allocated for
every image used for the evaluation. The objects and
the 2D points in every image are stored separately
but they always contain a reference to which 3D
point or object they correspond. This split in 2D and
3D representations is essential for the feature-
extraction procedure described above.
For the data-integration with CAAD this basic
structure must be treated as a given.
2.3 Data-Structures in CAAD: DXF, the De-Facto
Standard
DXF is one of the most wide-spread data exchange
formats in the computer world. Originally dating
back to 1982, Autodesk, makers of the CAD program
AutoCAD, designed it to provide an exchange-format
between different AutoCAD packages on different
operating systems. The format has been extended
and changed practically with every new version of
AutoCAD. Because of the wide spread of this
software, the support of a DXF-interface became
almost a must for any other CAD package in the
market. So, almost by accident, DXF became the de -
facto standard for drawing data it represents today.
239
We will not discuss the value or the problematic
aspects of this standard here and rather treat it as a
given that serves well to demonstrate certain aspects
of CAAD datastructures that are in some way or
another part of any CAD program.
As stated above, the main difference to
photogrammetric systems is, that in DXF,
uniqueness of points is not an issue. Rather it is a
normal situation that many elements have points in
common but each stores them individually (figure 2).
In DXF there are different basic geometric entities
that have each their own syntax. Examples are
point, line, polyline, trace, circle, arc,etc. There are
so-called 2.5D and 3D objects among them, but all
can be placed in any orientation in 3D. All entities
can be placed on different layers or have additional
information (extended entity data) attached. The
number of basic entities that can be used in a
drawing can be extended at will by creating new
complex entities, so-called blocks, out of the existing
basic ones.
2.4 The Concept of Blocks in CAAD
Figure 2: Blocks in DXF: the entity section of a DXF-file may contain
(multiple) references to blocks, which are defined in the block section
or in a separate file.
Blocks are maybe the most important feature of the
DXF format. Blocks are so to speak indipendent
mini-drawings that are defined in the block-section
of a DXF-file or in a separate file. In the entity-
section, those blocks can be referenced like normal
entities. This is useful for many things.
It enables hierarchical structures,
themselves can be part of blocks again.
It enables the formation of libraries of complex
objects that can be used many times.
It enables a compact file-storage, as reoccurring
elements must be defined in one place only.
It enables object-oriented concepts such as different
levels of detail for one object, different modes of
representation of an object, depending on the
context it is in.
The way blocks can be referenced is very powerful,
the way they can be manipulated, however, isn't. A
block can be moved, rotated and scaled in resp.
as blocks
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
