differentiation 
   
  
  
f VGI data 
MECHANISM 
ion 2, aiming at settling 
ta to update professional 
tabase timely and in turn 
enhanced data to VGI 
1ism as organic cycle to 
hanism emphasizes the 
”, the users provide more 
h as GPS, drawing the 
to SDI, SDI verify the 
erified data to the user, 
improve the efficiency of 
ent methods for VGI data 
ndly according to the 
> matching algorithms for 
SDI vector data to detect 
ic SDI database linkage 
time and verified data to 
  
¢ mechanism 
ind preprocessing 
of current VGI websites, 
llowing kinds: GPS track 
eld or on-board GPS; the 
lines and polygons drew 
ng image or aerial photos; 
ser's cognition; the texts, 
tain position information 
1, it can be divided into 
phics data includes GPS 
ry objects; semantic data 
related text, images and 
gap between VGI data 
acquisition methods and professional surveying and mapping 
methods, VGI data has some characteristics: the uncertainty of 
data quality; the differentiation of data distribution; the outlines 
of geometry objects drew by public not accord with the 
standards of professional surveying and mapping department, 
the complexity of big geometric objects edited by multi-users 
etc. So, if professional surveying and mapping department 
wants to harness VGI data, the process of data filter, cleaning, 
pretreatment and conversion would be necessary to meet the 
standards of professional surveying and mapping. The next part 
in this section will discuss the proposed problems. 
At present, the mainstream VGI web sites such as 
Openstreetmap in order to get high quality data, design some 
rules to restrict the source and quality of data. They use user 
permissions, user contribution and the level of user professional 
to judging the certainty of data source, provide certain data 
format and fixed form for uploading data, adopt the multi-user 
and third-party to verity the validity of data. We think that two 
layer should be established for data filtering and cleaning, the 
first layer for getting reliable data source, the second layer for 
achieving reliable data. The verified data should be processed 
by format conversion, transformation of global coordinates, 
topological relationship building, space entity index, the 
consistent of graphics entity expression to meet the 
requirements of matching homonymous elements. 
3.2 The algorithms of homonymous elements 
The more important aspect of VGI data is the homogenous 
clements recognition and matching, which is fundament for 
detecting the changes and updating. The earliest case is a 
project of United States census bureau, in 1983 the United 
States census bureau merged its existing data map with USGS 
data map which was new, using the rubber band technology to 
adjust and iterative the node on the map constantly, for getting 
the matching relations between homogenous elements(Saalfeld, 
1988). Since then, many scholars began to discuss the 
theoretical and practical problems in homonymous elements 
matching. The current data matching algorithm, mainly chose 
geometry, semantic and topological relations as matching 
indexes to find homogenous elements. According to the 
characteristics of VGI data, such as without scale concept, we 
provide an algorithm to extract multi-level tree data structure 
globally; choose the feature points as center points to match on 
each layer; combining multiple matching indexes, match 
homogenous elements of point, line and polygon in a given 
threshold area. For the data cannot be determined, professional 
department need to design update schedule, adopting other 
method to collect the change data, such as field collection. 
wx geometry 
semnatics 
  
— Spatial relation 
Figure 3. The matching of homonymous elements 
3.3 Detect changes and define change type 
By matching the homonymous elements between VGI and SDI 
vector data, the change information could be acquired more 
easily through the automatic or artificial interactive way, then 
the change information will be extracted and submitted to the 
professional surveying and mapping departments to identify and 
release real-time data. This update method belongs to 
incremental updating. When changes once happen, they will be 
detected and verified, and updated the professional spatial 
database, at the same time the out-of-time data will be archived. 
Lastly, SDI public the new and verified data products to end- 
users, which is a loop update mechanism for each other. Here, 
according to the type of data, the change types could be 
classified into three types, attribute change, geometry change, 
geometric and attribute both changing. Then based on the 
operation of the change, the change types could be divided into 
add, deletion and modification. We could integrate the first and 
second classification, the change types will be classified into 
the following 13 kinds of combination types: attribute add; 
attribute deletion; attribute modification; geometry add; 
geometry deletion; geometric modification; attribute add, 
geometry add; attribute add, geometry modification; attribute 
deletion, geometry add; attribute deletion, geometry deletion; 
attribute — deletion, geometric modification; ^ attributes 
modification, geometry add; attributes modification, geometric 
modification. As the attribute is affiliated with geometry, so 
when the geometry is deleted, the attribute doesn't exist. 
3.4 The confirmation and publication of change for SDI 
The data from VGI has so many uncertainty factors, such as 
possible malicious data. So after detecting the data changes, the 
changes need to be extracted and submitted to SDI department 
to verify the validation of data. The validated method mainly 
can through the way of the combination of the field surveying 
and mapping and the indoor detection. Field surveying and 
mapping usage professional equipment and experts on the field 
survey to confirm the data changes, and collect professional 
data accord with standard of professional data, the surveying 
and mapping indoor usage the latest remote sensing image or 
aerial photographs through professional interpretation to 
confirm the reliability of data. The two methods also need 
certain time period, the updating period of field surveying and 
mapping depends on the professional department update 
schedule; the updating period of surveying and mapping indoor 
depends on the collection period of remote sensing image or 
aerial photos. Generally speaking, the collection period of aerial 
photos is shorter than that of remote sensing image. After 
surveying and mapping departments verify data and collect high 
precision and reliability data, including the data indoor 
compiling, the various related databases are updated, then, 
online data products are updated, by releasing to the user. 
3.5 The ring of updating 
The current update mechanism of VGI is by amateur in VGI 
platform to contribute data, in contrast, that of professional 
surveying and mapping departments is by professional expert 
and equipment to collect and update to realize, there is so great 
gap between them. But in recent literatures and researches, 
some began to pay attentions to the topic about that professional 
departments provide data for VGI platform, such as Bing Maps 
provide remote sensing image to Openstreetmap. There are also 
some researches based on VGI data to provide real-time change 
information for professional departments such as disaster 
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