<s>
A	O
3D	B-Application
city	I-Application
model	I-Application
is	O
digital	O
model	O
of	O
urban	O
areas	O
that	O
represent	O
terrain	O
surfaces	O
,	O
sites	O
,	O
buildings	O
,	O
vegetation	O
,	O
infrastructure	O
and	O
landscape	O
elements	O
in	O
three-dimensional	O
scale	O
as	O
well	O
as	O
related	O
objects	O
(	O
e.g.	O
,	O
city	O
furniture	O
)	O
belonging	O
to	O
urban	O
areas	O
.	O
</s>
<s>
3D	B-Application
city	I-Application
models	I-Application
support	O
presentation	O
,	O
exploration	O
,	O
analysis	O
,	O
and	O
management	O
tasks	O
in	O
a	O
large	O
number	O
of	O
different	O
application	O
domains	O
.	O
</s>
<s>
In	O
particular	O
,	O
3D	B-Application
city	I-Application
models	I-Application
allow	O
"	O
for	O
visually	O
integrating	O
heterogeneous	O
geoinformation	O
within	O
a	O
single	O
framework	O
and	O
,	O
therefore	O
,	O
create	O
and	O
manage	O
complex	O
urban	O
information	O
spaces.	O
"	O
</s>
<s>
To	O
store	O
3D	B-Application
city	I-Application
models	I-Application
,	O
both	O
file-based	O
and	O
database	O
approaches	O
are	O
used	O
.	O
</s>
<s>
There	O
is	O
no	O
single	O
,	O
unique	O
representation	O
schema	O
due	O
to	O
the	O
heterogeneity	O
and	O
diversity	O
of	O
3d	B-Application
city	I-Application
model	I-Application
contents	O
.	O
</s>
<s>
The	O
Components	O
of	O
3D	B-Application
city	I-Application
models	I-Application
are	O
encoded	O
by	O
common	O
file	O
and	O
exchange	O
formats	O
for	O
2D	O
raster-based	O
GIS	B-Application
data	O
(	O
e.g.	O
,	O
GeoTIFF	B-Application
)	O
,	O
2D	O
vector-based	O
GIS	B-Application
data	O
(	O
e.g.	O
,	O
AutoCAD	O
DXF	O
)	O
,	O
3D	O
models	O
(	O
e.g.	O
,	O
.3DS	O
,	O
.OBJ	B-Application
)	O
,	O
and	O
3D	O
scenes	O
(	O
e.g.	O
,	O
Collada	O
,	O
Keyhole	B-Application
Markup	I-Application
Language	I-Application
)	O
such	O
as	O
supported	O
by	O
CAD	B-Application
,	O
GIS	B-Application
,	O
and	O
computer	O
graphics	O
tools	O
and	O
systems	O
.	O
</s>
<s>
All	O
components	O
of	O
a	O
3D	B-Application
city	I-Application
model	I-Application
have	O
to	O
be	O
transformed	O
into	O
a	O
common	O
geographic	O
coordinate	O
system	O
.	O
</s>
<s>
A	O
database	O
for	O
3D	B-Application
city	I-Application
models	I-Application
stores	O
its	O
components	O
in	O
a	O
hierarchically	O
structured	O
,	O
multi-scale	O
way	O
,	O
which	O
allows	O
for	O
a	O
stable	O
and	O
reliable	O
data	O
management	O
and	O
facilitates	O
complex	O
GIS	B-Application
modeling	O
and	O
analysis	O
tasks	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
3D	O
City	O
Database	O
is	O
a	O
free	O
3D	O
geo	O
database	O
to	O
store	O
,	O
represent	O
,	O
and	O
manage	O
virtual	O
3D	B-Application
city	I-Application
models	I-Application
on	O
top	O
of	O
a	O
standard	O
spatial	O
relational	O
database	O
.	O
</s>
<s>
A	O
database	O
is	O
required	O
if	O
3D	B-Application
city	I-Application
models	I-Application
have	O
to	O
be	O
continuously	O
managed	O
.	O
</s>
<s>
3D	B-Application
city	I-Application
model	I-Application
databases	O
form	O
a	O
key	O
element	O
in	O
3D	O
spatial	O
data	O
infrastructures	O
that	O
require	O
support	O
for	O
storing	O
,	O
managing	O
,	O
maintenance	O
,	O
and	O
distribution	O
of	O
3D	B-Application
city	I-Application
model	I-Application
contents	O
.	O
</s>
<s>
As	O
common	O
application	O
,	O
geodata	O
download	O
portals	O
can	O
be	O
set	O
up	O
for	O
3D	B-Application
city	I-Application
model	I-Application
contents	O
(	O
e.g.	O
,	O
virtualcityWarehouse	O
)	O
.	O
</s>
<s>
The	O
Open	O
Geospatial	O
Consortium	O
(	O
OGC	O
)	O
defines	O
an	O
explicit	O
XML-based	O
exchange	O
format	O
for	O
3D	B-Application
city	I-Application
models	I-Application
,	O
CityGML	O
,	O
which	O
supports	O
not	O
only	O
geometric	O
descriptions	O
of	O
3D	B-Application
city	I-Application
model	I-Application
components	O
but	O
also	O
the	O
specification	O
of	O
semantics	O
and	O
topology	O
information	O
.	O
</s>
<s>
CityJSON	O
is	O
a	O
JSON-based	O
format	O
for	O
storing	O
3D	B-Application
city	I-Application
models	I-Application
.	O
</s>
<s>
Due	O
to	O
its	O
simple	O
encoding	O
and	O
the	O
use	O
of	O
JSON	B-General_Concept
,	O
it	O
is	O
also	O
suitable	O
for	O
web	O
applications	O
.	O
</s>
<s>
3D	B-Application
city	I-Application
models	I-Application
are	O
typically	O
constructed	O
at	O
various	O
levels	O
of	O
detail	O
(	O
LOD	O
)	O
to	O
provide	O
notions	O
of	O
multiple	O
resolutions	O
and	O
at	O
different	O
levels	O
of	O
abstraction	O
.	O
</s>
<s>
There	O
exist	O
also	O
approaches	O
to	O
generalize	O
a	O
given	O
detailed	O
3D	B-Application
city	I-Application
model	I-Application
by	O
means	O
of	O
automated	O
generalization	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
hierarchical	O
road	O
network	O
(	O
e.g.	O
,	O
OpenStreetMap	O
)	O
can	O
be	O
used	O
to	O
group	O
3D	B-Application
city	I-Application
model	I-Application
components	O
into	O
"	O
cells	O
"	O
;	O
each	O
cell	O
is	O
abstracted	O
by	O
aggregating	O
and	O
merging	O
contained	O
components	O
.	O
</s>
<s>
GIS	B-Application
data	O
provide	O
the	O
base	O
information	O
to	O
build	O
a	O
3D	B-Application
city	I-Application
model	I-Application
such	O
as	O
by	O
digital	B-Application
terrain	I-Application
models	I-Application
,	O
road	O
networks	O
,	O
land	O
use	O
maps	O
,	O
and	O
related	O
geo-referenced	O
data	O
.	O
</s>
<s>
GIS	B-Application
data	O
also	O
includes	O
cadastral	O
data	O
that	O
can	O
be	O
converted	O
into	O
simple	O
3D	O
models	O
as	O
,	O
for	O
example	O
,	O
in	O
the	O
case	O
of	O
extruded	O
building	O
footprints	O
.	O
</s>
<s>
Core	O
components	O
of	O
3D	B-Application
city	I-Application
models	I-Application
form	O
digital	B-Application
terrain	I-Application
models	I-Application
(	O
DTM	O
)	O
represented	O
,	O
for	O
example	O
,	O
by	O
TINs	O
or	O
grids	O
.	O
</s>
<s>
Typical	O
sources	O
of	O
data	O
for	O
3D	B-Application
city	I-Application
model	I-Application
also	O
include	O
CAD	B-Application
models	I-Application
of	O
buildings	O
,	O
sites	O
,	O
and	O
infrastructure	O
elements	O
.	O
</s>
<s>
They	O
provide	O
a	O
high	O
level	O
of	O
detail	O
,	O
possible	O
not	O
required	O
by	O
3D	B-Application
city	I-Application
model	I-Application
applications	O
,	O
but	O
can	O
be	O
incorporated	O
either	O
by	O
exporting	O
their	O
geometry	O
or	O
as	O
encapsulated	O
objects	O
.	O
</s>
<s>
Building	B-Architecture
information	I-Architecture
models	I-Architecture
represent	O
another	O
category	O
of	O
geo-spatial	O
data	O
that	O
can	O
be	O
integrated	O
into	O
a	O
3D	B-Application
city	I-Application
model	I-Application
providing	O
the	O
highest	O
level	O
of	O
detail	O
for	O
building	O
components	O
.	O
</s>
<s>
Complex	O
3D	B-Application
city	I-Application
models	I-Application
typically	O
are	O
based	O
on	O
different	O
sources	O
of	O
geodata	O
such	O
as	O
geodata	O
from	O
GIS	B-Application
,	O
building	O
and	O
site	O
models	O
from	O
CAD	B-Application
and	O
BIM	B-Architecture
.	O
</s>
<s>
It	O
is	O
one	O
of	O
their	O
core	O
properties	O
to	O
establish	O
a	O
common	O
reference	O
frame	O
for	O
heterogeneous	O
geo-spatial	O
and	O
geo-referenced	O
data	O
,	O
i.e.	O
,	O
the	O
data	O
need	O
not	O
to	O
be	O
merged	O
or	O
fused	O
based	O
on	O
one	O
common	B-Application
data	I-Application
model	I-Application
or	O
schema	O
.	O
</s>
<s>
The	O
integration	O
is	O
possible	O
by	O
sharing	O
a	O
common	O
geo-coordinate	O
system	O
at	O
the	O
visualization	B-Application
level	O
.	O
</s>
<s>
In	O
practice	O
,	O
3D	O
models	O
of	O
buildings	O
of	O
urban	O
regions	O
are	O
generated	O
based	O
on	O
capturing	O
and	O
analyzing	O
3D	O
point	B-Algorithm
clouds	I-Algorithm
(	O
e.g.	O
,	O
sampled	O
by	O
terrestrial	O
or	O
aerial	O
laser	B-Algorithm
scanning	I-Algorithm
)	O
or	O
by	O
photogrammetric	B-Application
approaches	O
.	O
</s>
<s>
To	O
achieve	O
a	O
high	O
percentage	O
of	O
geometrically	O
and	O
topologically	O
correct	O
3D	O
building	O
models	O
,	O
digital	O
terrain	O
surfaces	O
and	O
2D	O
footprint	O
polygons	O
are	O
required	O
by	O
automated	O
building	O
reconstruction	B-Algorithm
tools	O
such	O
as	O
BREC	O
.	O
</s>
<s>
Statistical	O
approaches	O
are	O
common	O
for	O
roof	O
reconstruction	B-Algorithm
based	O
on	O
airborne	O
laser	B-Algorithm
scanning	I-Algorithm
point	B-Algorithm
clouds	I-Algorithm
.	O
</s>
<s>
The	O
visualization	B-Application
of	O
3D	B-Application
city	I-Application
models	I-Application
represents	O
a	O
core	O
functionality	O
required	O
for	O
interactive	O
applications	O
and	O
systems	O
based	O
on	O
3D	B-Application
city	I-Application
models	I-Application
.	O
</s>
<s>
Providing	O
high	O
quality	O
visualization	B-Application
of	O
massive	O
3D	B-Application
city	I-Application
models	I-Application
in	O
a	O
scalable	O
,	O
fast	O
,	O
and	O
cost	O
efficient	O
manner	O
is	O
still	O
a	O
challenging	O
task	O
due	O
to	O
the	O
complexity	O
in	O
terms	O
of	O
3D	O
geometry	O
and	O
textures	O
of	O
3D	B-Application
city	I-Application
models	I-Application
.	O
</s>
<s>
Real-time	B-General_Concept
rendering	I-General_Concept
provides	O
a	O
large	O
number	O
of	O
specialized	O
3D	O
rendering	O
techniques	O
for	O
3D	B-Application
city	I-Application
models	I-Application
.	O
</s>
<s>
Real-time	O
3D	O
rendering	O
of	O
multiperspective	O
views	O
on	O
3D	B-Application
city	I-Application
models	I-Application
.	O
</s>
<s>
Real-time	B-General_Concept
rendering	I-General_Concept
algorithms	O
and	O
data	O
structures	O
are	O
listed	O
by	O
the	O
virtual	O
terrain	O
project	O
.	O
</s>
<s>
Service-oriented	O
architectures	O
(	O
SOA	O
)	O
for	O
visualizing	O
3D	B-Application
city	I-Application
models	I-Application
offer	O
a	O
separation	O
of	O
concerns	O
into	O
management	O
and	O
rendering	O
and	O
their	O
interactive	O
provision	O
by	O
client	O
applications	O
.	O
</s>
<s>
For	O
SOA-based	O
approaches	O
,	O
3D	O
portrayal	O
services	O
are	O
required	O
,	O
whose	O
main	O
functionality	O
represents	O
the	O
portrayal	O
in	O
the	O
sense	O
of	O
3D	O
rendering	O
and	O
visualization	B-Application
.	O
</s>
<s>
Web	O
view	O
service	O
(	O
WVS	O
)	O
:	O
This	O
type	O
of	O
service	O
encapsulates	O
the	O
3D	O
rendering	O
process	O
for	O
3D	B-Application
city	I-Application
models	I-Application
at	O
the	O
server	O
side	O
.	O
</s>
<s>
Client	O
applications	O
do	O
not	O
have	O
to	O
process	O
3D	O
graphics	O
data	O
,	O
but	O
to	O
provide	O
management	O
for	O
loading	O
,	O
caching	O
,	O
and	O
displaying	O
the	O
image-based	O
representations	O
of	O
3D	O
scenes	O
and	O
do	O
not	O
have	O
to	O
process	O
the	O
original	O
(	O
and	O
possibly	O
large	O
)	O
3D	B-Application
city	I-Application
model	I-Application
.	O
</s>
<s>
A	O
map-based	O
technique	O
,	O
the	O
"	O
smart	O
map	O
"	O
approach	O
,	O
aims	O
at	O
providing	O
"	O
massive	O
,	O
virtual	O
3D	B-Application
city	I-Application
models	I-Application
on	O
different	O
platforms	O
namely	O
web	O
browsers	O
,	O
smartphones	O
or	O
tablets	O
,	O
by	O
means	O
of	O
an	O
interactive	O
map	O
assembled	O
from	O
artificial	O
oblique	O
image	O
tiles.	O
"	O
</s>
<s>
The	O
map	O
tiles	O
are	O
synthesized	O
by	O
an	O
automatic	O
3D	O
rendering	O
process	O
of	O
the	O
3D	B-Application
city	I-Application
model	I-Application
;	O
the	O
map	O
tiles	O
,	O
generated	O
for	O
different	O
levels-of-detail	O
,	O
are	O
stored	O
on	O
the	O
server	O
.	O
</s>
<s>
This	O
way	O
,	O
the	O
3D	O
rendering	O
is	O
completely	O
performed	O
on	O
the	O
server	O
's	O
side	O
,	O
simplifying	O
access	O
and	O
usage	O
of	O
3D	B-Application
city	I-Application
models	I-Application
.	O
</s>
<s>
Most	O
importantly	O
,	O
the	O
map-based	O
approach	O
allows	O
for	O
distributing	O
and	O
using	O
complex	O
3D	B-Application
city	I-Application
models	I-Application
with	O
having	O
to	O
stream	O
the	O
underlying	O
data	O
to	O
client	O
devices	O
-	O
only	O
the	O
pre-generated	O
map	O
tiles	O
are	O
sent	O
.	O
</s>
<s>
This	O
way	O
,	O
"	O
(	O
a	O
)	O
The	O
complexity	O
of	O
the	O
3D	B-Application
city	I-Application
model	I-Application
data	O
is	O
decoupled	O
from	O
data	O
transfer	O
complexity	O
(	O
b	O
)	O
the	O
implementation	O
of	O
client	O
applications	O
is	O
simplified	O
significantly	O
as	O
3D	O
rendering	O
is	O
encapsulated	O
on	O
server	O
side	O
(	O
c	O
)	O
3D	B-Application
city	I-Application
models	I-Application
can	O
be	O
easily	O
deployed	O
for	O
and	O
used	O
by	O
a	O
large	O
number	O
of	O
concurrent	O
users	O
,	O
leading	O
to	O
a	O
high	O
degree	O
of	O
scalability	O
of	O
the	O
overall	O
approach.	O
"	O
</s>
<s>
3D	B-Application
city	I-Application
models	I-Application
can	O
be	O
used	O
for	O
a	O
multitude	O
of	O
purposes	O
in	O
a	O
growing	O
number	O
of	O
different	O
application	O
domains	O
.	O
</s>
<s>
Navigation	O
systems	O
:	O
3D	O
navigation	O
maps	O
have	O
become	O
omnipresent	O
both	O
in	O
automotive	O
and	O
pedestrian	O
navigation	O
systems	O
,	O
which	O
include	O
3D	B-Application
city	I-Application
models	I-Application
,	O
in	O
particular	O
,	O
terrain	O
models	O
and	O
3D	O
building	O
models	O
,	O
to	O
enhance	O
the	O
visual	O
depiction	O
and	O
to	O
simplify	O
the	O
recognition	O
of	O
locations	O
.	O
</s>
<s>
Urban	O
planning	O
and	O
architecture	O
:	O
To	O
set	O
up	O
,	O
analyze	O
,	O
and	O
disseminate	O
urban	O
planning	O
concepts	O
and	O
projects	O
,	O
3D	B-Application
city	I-Application
models	I-Application
serve	O
as	O
communication	O
and	O
participation	O
medium	O
.	O
</s>
<s>
3D	B-Application
city	I-Application
models	I-Application
provide	O
means	O
for	O
project	O
communication	O
,	O
better	O
acceptance	O
of	O
development	O
projects	O
through	O
visualization	B-Application
,	O
and	O
therefore	O
avoid	O
monetary	O
loss	O
through	O
project	O
delays	O
;	O
they	O
also	O
help	O
to	O
prevent	O
planning	O
errors	O
.	O
</s>
<s>
Spatial	O
data	O
infrastructures	O
(	O
SDIs	O
)	O
:	O
3D	B-Application
city	I-Application
models	I-Application
extend	O
spatial	O
data	O
infrastructures	O
and	O
support	O
the	O
management	O
,	O
storage	O
,	O
and	O
usage	O
of	O
3D	O
models	O
within	O
SDIs	O
;	O
they	O
require	O
not	O
only	O
tools	O
and	O
processes	O
for	O
the	O
initial	O
construction	O
and	O
storage	O
of	O
3D	B-Application
city	I-Application
models	I-Application
but	O
also	O
have	O
to	O
provide	O
efficient	O
data	O
management	O
and	O
data	O
distribution	O
to	O
support	O
workflows	O
and	O
applications	O
.	O
</s>
<s>
GIS	B-Application
:	O
GIS	B-Application
support	O
3D	O
geodata	O
and	O
provide	O
computational	O
algorithms	O
to	O
construct	O
,	O
transform	O
,	O
validate	O
,	O
and	O
analyze	O
3D	B-Application
city	I-Application
model	I-Application
components	O
.	O
</s>
<s>
Emergency	O
management	O
:	O
For	O
emergency	O
,	O
risk	O
,	O
and	O
disaster	O
management	O
systems	O
,	O
3D	B-Application
city	I-Application
models	I-Application
provide	O
the	O
computational	O
framework	O
.	O
</s>
<s>
Spatial	O
analysis	O
:	O
3D	B-Application
city	I-Application
models	I-Application
provide	O
the	O
computational	O
framework	O
for	O
3D	O
spatial	O
analysis	O
and	O
simulation	O
.	O
</s>
<s>
Gaming	O
:	O
3D	B-Application
city	I-Application
models	I-Application
can	O
be	O
used	O
to	O
obtain	O
base	O
data	O
for	O
virtual	O
3D	O
scenes	O
used	O
in	O
online	O
and	O
video	O
games	O
.	O
</s>
<s>
Cultural	O
heritage	O
:	O
3D	B-Application
city	I-Application
model	I-Application
tools	O
and	O
systems	O
are	O
applied	O
for	O
modeling	O
,	O
design	O
,	O
exploration	O
,	O
and	O
analysis	O
tasks	O
in	O
the	O
scope	O
of	O
cultural	O
heritage	O
.	O
</s>
<s>
For	O
example	O
,	O
archeological	O
data	O
can	O
be	O
embedded	O
in	O
3D	B-Application
city	I-Application
models	I-Application
.	O
</s>
<s>
City	O
information	O
systems	O
:	O
3D	B-Application
city	I-Application
models	I-Application
represent	O
the	O
framework	O
for	O
interactive	O
3D	O
city	O
information	O
systems	O
and	O
3D	O
city	O
maps	O
.	O
</s>
<s>
For	O
example	O
,	O
municipalities	O
apply	O
3D	B-Application
city	I-Application
models	I-Application
as	O
centralized	O
information	O
platform	O
for	O
location	O
marketing	O
.	O
</s>
<s>
Property	O
management	O
:	O
3D	B-Application
city	I-Application
model	I-Application
technology	O
can	O
extend	O
systems	O
and	O
applications	O
used	O
in	O
real-estate	O
and	O
property	O
management	O
.	O
</s>
<s>
Intelligent	O
transportation	O
systems	O
:	O
3D	B-Application
city	I-Application
models	I-Application
can	O
be	O
applied	O
to	O
intelligent	O
transportation	O
systems	O
.	O
</s>
<s>
Augmented	B-General_Concept
reality	I-General_Concept
:	O
3D	B-Application
city	I-Application
models	I-Application
can	O
be	O
used	O
as	O
reference	O
frame	O
for	O
augmented	B-General_Concept
reality	I-General_Concept
applications	O
.	O
</s>
