The International At chives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
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<schema targetNamespace="http://www.ukusa.org" 
xmlns:app="http://www.ukusa.org" 
xmlns="http://www.w3.org/2001/XMLSchema" 
xmlns:gml="http://www.opengis.net/gmr 
xmlns:ex="http://www.ukusa.org"> 
<element name="City" 
type="gml:AbstractFeatureCollectionType" 
substitutionGroup="gml:_FeatureCollection"/> 
<element name="Road" type="ex:RoadType" 
substitutionGroup="gml:_Feature"/> 
<element name="Bridge" type="ex:BridgeType" 
substitutionGroup="gml:_Feature"/> 
<element name="Building" type="ex:BuildingType" 
substitutionGroup="gml:_Feature"/> 
<complexType name="RoadType"> 
<complexContent> 
<extension base="gml:AbstractFeatureType"> 
<sequence> 
<element name="roadName" 
type="string"/> 
<element name="roadCode" 
type="string"/> 
<element ref="gml:curveProperty"/> 
</sequence> 
</extension> 
</complexContent> 
</complexType> 
<complexType name="BuiIdingType"> 
<complexContent> 
<extension base="gml:AbstractFeatureType"> 
<sequence> 
<element name="BuiIdingCode" 
type="string'7> 
<element name="BuildingHeight" 
type="string"/> 
<element ref="gml:Polygon"/> 
</sequence> 
</extension> 
</complexContent> 
</complexType> 
<complexType name="BridgeType"> 
<complexContent> 
<extension base="gml:AbstractFeatureType"> 
<sequence> 
<element name="span'7> 
<element name="height" type="integer'7> 
<element ref="gml:centerLineOf7> 
<element ref="app:spans" 
minOccurs="0'7> 
</sequence> 
</extension> 
</complexContent> 
</complexType> 
<element name="spans" 
type="app:CurvePropertyType'7> 
<complexType name="CurvePropertyType"> 
<sequence> 
<element ref="app:Gorge'7> 
</sequence> 
</complexType> 
<element name="Gorge" type="app:GorgeType" 
substitutionGroup="gml:_Feature'7> 
<complexType name="GorgeType"> 
<complexContent> 
<extension base="gml:AbstractFeatureType > 
<sequence> 
<element name="width" type="integer" 
minOccurs="0'7> 
<element name="depth" type="integer" 
minOccurs="0'7> 
<element ref="gmI:centerLineOf" 
minOccurs="0'7> 
</sequence> 
</extension> 
</complexContent> 
</complexType> 
</schema> 
Table 1. City.xsd (GML application schema instance) 
When import GML schema, one of the key points is to confirm 
storage granularity of document instance according to schema. 
Granularity is the degree of data induction and aggregation, 
storage granularity determines size of fragments partitioned 
from each GML document instance in database. Fragment could 
be the whole GML document or single element. Table 1 shows 
GML application schema City.xsd that describes city, in this 
instance, city is the root element of GML document, city 
contains Road, Building, Bridge, Gorge sub-elements. For 
example, Road is a independent road, it’s a integrated 
description of road, the sub-element of Road, such as 
roadName roadCode gmkcurvePropert is specific 
description on a certain aspect of road. If take City as root 
element of partitioned fragment, the whole GML document 
would be encapsulated as an object which would be operated as 
a whole. And this object contains many integrated objects. To 
query or reuse GML document, such granularity is too large, 
which would increase cost on calling and operation. If take 
roadName > roadCode x gmkcurveProperty as root element of 
partitioned fragment, they are only describe one aspect of 
object, it lacks of integrated meaning and reusing character. 
Moreover, query result returned by user could not be generally 
single element. Thus, appropriate granularity should be 
partitioned by feature type. The feature with the same type 
should be stored in the same disk file, for example, take Road, 
Bridge, Building, Gorge as root element of partitioned GML 
document instance fragment, the element content and its sub 
element, as an integrated object , is stored in corresponding 
database. 
When read a GML document instance, system read would 
firstly check the validity of GML document instance, after 
affirm that GML document is good format GML document, 
system would establish corresponding relationship between 
GML document and storage structure according to mapping 
rules, and decompose, save document instance. The instance 
document City.xml generated according to City.xsd would not 
be given in details due to the limitation of page. As mentioned 
above, features (Bridge, Gorge, Building, and Road) have been 
confirmed as root elements; so when it comes to Bridge, Gorge, 
Building or Road element, system would store element content 
and sub-element it contained into corresponding data collection 
as a whole fragment without parsing sub-element. Thus this 
approach does not change inner structure of element. It’s still a 
legal GML document fragment. In other words, it’s still XML 
format document that stored in database, in the database, a 
XML file is used for managing these document fragments. This 
instance includes two Bridge objects, one Gorge object, one 
Road object, one Building object. The feature object with the 
same type would be merged into one file, so this document 
would be partitioned into one Bridge fragment, one Gorge