International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004 
years’ practical application. But with applications going deeply, 
the expression contents needed are more and more subtle, the 
expression element system can not satisfy the requirements of 
describing the city in a three-dimensional means, so it is 
imperative to build 3D Geographic Element System of city. 
Referring to cartographic element system of two-dimensional 
map and field surveying in some cities, we think that there 
should be at least ten types of things in 3D Geographic Element 
System, which are Building, Water System, Traffic Feature, 
Ambit, Terrain, Physiognomy, Vegetation, Pipe, Wall and Bar, 
and Independent Object. 
3. SPATIAL GRANULARITY FOR 3D DESCRIPTION 
The realistic world is complicated and various, there are even 
many differences among the same kind of things. Taking 
buildings for example, they include cloud-kissing skyscrapers 
as well as rough shanties. It is difficult to estimate the enormous 
workload if we want to express all objects no matter they are 
large or small. The other problem needed to be resolved is how 
to determine the hiberarchies of objects, whether or not the 
object is necessary to be expressed, and what degree is suitable 
if it is necessary, thus we should establish a uniform standard to 
follow. Therefore, in the paper we put forward a concept of 
Spatial Granularity for 3D Description (3DSG), which can be 
used to decide whether we should store the information and 
express the object by distinguishing their hiberarchy and subtle 
degree. 
The Spatial Granularity for 3D Description has important and 
instructional significance for practical work when 
reconstructing a 3D city. Firstly, it provides a standard and a 
criterion for the 3D spatial data acquisition. Date acquisition is 
a key matter in 3D city reconstruction, which also needs 
maximum workload. The difficulty to acquire  three- 
dimensional data has become a bottle-neck problem obstructing 
the development of 3D GIS. The main reason is lacking of 
related standards. 3DSG will resolve the problem to a certain 
extent by supplying standards. After the determination of 3DSG, 
we don't need to consider those features under Granularity, and 
consequently unnecessary work can be reduced. Secondly, we 
can improve modelling efficiency of complex buildings with 
3DSG, by simplifying and omitting lots of trivial things. With 
the development of modern architectural technology, 
architectural style becomes more and more multiplex. And the 
traditional “matchbox” buildings are gradually replaced by 
modern architecture which has different styles and complex 
structures, accordingly buildings are much more difficult to 
reconstruct. By partitioning buildings details according to 
3DSG, unimportant characters can be omitted and thus we can 
realize the efficient reconstruction of city. 
Because of complexity and diversity of realistic world, and 
various application domains, 3DSG is not a constant value, but 
a variable which would be determined each time according to 
the size of a description scope, the subtle degree of different 
applications and actual data collection capability. For example, 
when constructing 3D scene aiming at exhibiting city style and 
features, the emphasis is to describe the whole view of the city. 
And such as squares, evident buildings, streets and crossroads 
must be described with high precision’! but some fundamental 
infrastructure of city such as power networks, we should only 
select and express some substations and circuitroies of higher 
gerade. Or else, we would confuse description hiberarchies and 
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destroy the integrated aesthetic feeling of scene if in the terms 
of the same granularity. On the contrary, if the application 
domain is electric department, the scene is primarily served for 
inquiry and analysis of power networks, the description 
granularity of power establishment must higher than other 
things. 
In the process of 3D Digital Weihai construction, we have done 
some elementary research on the determination standard of 
3DSG. The successful construction of Digital Weihai proved 
that granularity description is necessary. Summarizing the 
material practice, we think there are four principles that should 
be considered when determining 3DSG: 
I. Application integrated, because different applications have 
different emphasis. 
2. Applicable, that is, the 3DSG should satisfy the application 
demands but should not pursue high precision eyelessly. 
3. Practicable, the determination of 3DSG should be combined 
with data acquisition methods. 
4. Regional, the 3DSG of different regions is different from 
each other, the emphases is put on important regions. 
4. 3D PARTITION OF BUILDING 
With the development of modern city, there are variety of cities 
and thousands of different geographic objects, it is difficult to 
find a single object model suitable for all geographic objects. It 
also obstructs application and development of 3DGIS in many 
fields. However, there are commonness and similarity to a 
certain extent when any object has been divided. For examples, 
a tree can be divided into the crown, the trunk and the root, a 
table is consist of board and legs, the structure of a man 
includes head, upper limbs, body and legs and so on. Therefore, 
we can similarly divide complex geographic objects into simple 
sections. The process of partition complex spatial objects into 
sets of simple objects is called 3D Partition. Buildings, as the 
main elements of city, have different structure styles. lt is 
difficult to reconstruct them one by one, while they can be 
expressed as a series of normal geometries through 3D Partition 
which improves construction efficiency of city. 
3D Partition of Building is divided into logical partition and 
physical partition, the former means dividing buildings at 
cognitive hiberarchy, such as roofs, walls and foundations; 
while the latter means dividing buildings into some simple 
geometries according to certain rules from the building 
structure point of view, such as tetrahedrons. In the paper, 3D 
Partition of Building is primarily logical partition. In the light of 
the building structure and human cognitive habits, we think that 
any building can be divided into three parts, which are 
Mainbody, Character and Adjunct. Mainbody is the chief parts 
of a building and usually with a normal shape. Character is a 
part of a building, which represents characteristic details 
different from the others buildings, such as roofs, eaves, 
outdoor-floors and etc. Adjunct is not a part of a building but 
existed with the building, such as antennae, satellite receivers 
and hanging object etc. Fig.] demonstrates 3D Partition of two 
typical buildings. 
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