Map-based Mobile Services Design,Interacton and Usability Phần 3 potx

4 An Incremental Strategy for the Fast Transmission of Multi-Resolution 63

façade elements. Three different kinds of structures can be identified, for which

appropriate reduction methods are defined: extrusion or intrusion, offset, and cor￾ner (see Fig. 4.5).

Fig. 4.5. Elimination of short facade Sn: offset (a), intrusion/extrusion (b) and corner (c)

(based on Sester et al., 2004b)

These operators are interesting in a client-server context with limited resources

by reducing the amount of transferred data. Generalised dataset can be sent before

a more detailed one. Furthermore such operators guarantee the “sharing of geome￾tries” which is essential in an incremental strategy.

Consequences on incremental transitions between different levels of detail

If we observe results of these operators on different representations of regions and

polylines, we can deduce the different changes to perform on an object in order to

rebuild its more generalised or detailed representation.

Case of simplified polylines: In consequence of the simplification process,

eliminated points need to be inserted in the generalised representation of the poly￾line in a refinement transition and must be removed from the most detailed one

during a generalisation transformation. In this last case a choice must be done be￾tween conservation of shared points or removal of details. Moreover, vertices can

be moved between different LoDs, for example in order to respect the topological

relations with neighboring objects: coordinates of these points must be changed

during a refinement or generalisation transition.

Case of simplified regions: In generalisation transition, vertices can be either

kept (for the common points), removed (for the details), or moved (for preserving

parallelism and rectangularity properties of building). In refinement transition,

vertices can be either moved or introduced (for adding details).

These generalisation operators are expected to be performed on the server side

and followed by a process of increment creation. A formalism has been defined in

order to consider different object resolutions and transformations between them.

64 Jean-Michel FOLLIN, Alain BOUJU

4.3 MR data and MR data transfer models

4.3.1 Data model

A multiresolution data model adapted to limitations of mobile context has been

defined in Follin, et al. (2005b). The data organization is based on the traditional

definition of a geographic map: objects are grouped into layers and a sequence (or

overlay) of layers forms a map (Tomlinson, 1967). As representations of objects

vary according to the level of detail, we consider different LoD objects grouped

into different LoD layers. Increments allow navigation through these different

LoD objects and in this way reuse of available LoD representations on the client￾side. In order to reduce volume of data transferred from server to client, incre￾ments are sent if their size is less important than the size of LoD objects.

Layer and object

A layer, noted l, is a collection of objects associated with a description of their at￾tributes. Each layer corresponds to a specific theme (e.g. transportation network or

buildings) that can be decomposed in different LoD layers.

A map is defined as a succession of thematic layers which aims to be manipulated

and visualized at a specific scale (Follin, et al. 2005b).

An object entity is defined by the quadruplet (o, t, g, J ) where:

x o: unique identifier,

x t: last time of modification (timestamp value),

x g: location and geometrical description (modelled by one among six

two-dimensional geographical objects of spatial domain G : Point,

Polyline or Region for simple objects, and MultiPoint, MultiPolyline

and MultiRegion for collections of objects),

x J: alphanumeric values n J J , J , , J 1 2  accessed through the set

of object’s attributes a a an , , , 1 2  (for instance, the name of a

street).

LoD layer and LoD object

LoD layers of a layer l correspond to the definitions of l’s objects in the scale

ranges where they exist. A layer l can be seen as a serie of n LoD layers. LoD ob￾jects included in LoD layers can be matched (i.e. linked) between the two or more

consecutive levels where they are represented. The matching configuration corre￾sponds to the number of matched LoD representations of the same real world enti￾ties (when objects are represented at two different LoDs).

Three different matching cases are distinguished in Ai et al. (2001): 1:1, 1:n and

n:m matching case. In our works only the 1:1 and 1:0 matching cases have been

considered, i.e. the cases where 1 LoD representation of an object is mapped to 1

4 An Incremental Strategy for the Fast Transmission of Multi-Resolution 65

representation of the same object at a different and consecutive LoD or where it is

not linked to other object (because it has been deleted between the two LoDs).

Only these matching configurations are studied because use of incremental opera￾tors seems only relevant for these cases: more complex matching configurations

should involve more complex increments which are less interesting.

The linking is based on identifier o of the object’s different representations. Some

solutions to match objects with various link cardinalities have been studied in

Hampe et al. (2003).

A LoD representation or LoD object is an object o version defined for a level i

in adequacy with a scale interval. It will be noted i o .

The below described concept of increment is applied to polylines but can also

be valid for regions. Indeed a region can be defined as a closed polyline: its

boundary.

A polyline noted P is defined as a sequence of vertices ^ ` V Vn , , 1  such that

each couple   1 1 , V Vi defines a segment> @ 1 1 , V Vi .

As we deal with multiple representations of same polylines, we use the following

definition: a vertex j Vi is a vertex V at index i of a polyline Pj

.

For example, we consider two LoDs of a polyline in Fig. 4.6: a detailed and a

simplified one. We can notice that vertices of Pn

with indexes 1 and 4, i.e. n V1 and

n V4 have the same coordinates that vertices of Pn+1 with indexes 1 and 2, i.e.

1

1

n V and 1

2

n V .

We define the vertices which have the same coordinates, i.e. are matched, in the

two LoD representations Pn

and Pn+1 as shared (or matched) vertices.

The set of matched vertices is used during the creation of increment and recon￾struction of the polyline.

Increment

An increment allows performing changes on LoD object on

in order to rebuild its

representation on-1 or on+1.

An increment point corresponds to a couple  j opi Vi , where a geometrical op￾erator opi is combined with a manipulated vertex j Vi .

An increment is defined as an ordered list of increment points. The increment

allowing transition from on

to on+1 (resp. on-1) will be noted Inco n n 1 n

, o 

(resp. Inco n n 1 n

, o  ).