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Michela Bertolotto
As a consequence more recently attention has turned to the progressive transmission of vector data: several approaches based on effective compression mechanisms
have been defined and successfully implemented for the particular case of triangular
meshes [1, 16, 27–29].
In contrast, the investigation and development of progressive transmission techniques for more general vector spatial data still present many challenges. Our focus
is on map data (e.g. city street maps, road networks, hydrography networks, administrative boundary maps, etc.). Compressing this type of data is not straightforward
as many factors have to be taken into account. For example, spatial relations must be
preserved to guarantee reliable query results.
The level of detail of a vector map can be decreased by applying the so-called
map generalization process. However, such a process relies on cartographic principles whose complete formalization is still lacking (see [48] for a survey). Although
few automated solutions and algorithms have been proposed for specific cases, they
involve complex and time-consuming calculations and therefore cannot be applied
for real-time Web mapping.
Recent studies [10, 11] have distinguished between two different approaches for
the development of progressive map data transmission: on-the-fly map generation
versus pre-computation of multiple map representations. The first approach aims at
generating new generalized maps upon user request. The second approach relies on
the pre-computation of a fixed sequence of map representations that are stored and
transmitted progressively in order of increasing detail. Both approaches have advantages and drawbacks. This chapter provides a critical overview of these issues and
of the few prototype systems that have been described and developed for progressive
vector map transmission.
The remainder of this chapter is organized as follows. Sect. 4.2 reviews wellestablished implementations for progressive transmissions of data over the Internet.
These include the cases of raster images and triangular meshes. The problem of transmitting geographical map data in vector format is analyzed in the subsequent sections.
Section 4.3 discusses the use of progressive transmission techniques for Web mapping.
This is followed by an overview of the prototype systems developed and discussed in the
literature. These systems rely on techniques that can be differentiated on the basis of the
type of changes applied during generalization: geometric versus topological changes.
Therefore, in Sect. 4.4 we describe research related to changes in the geometry of the
data, while in Sect. 4.5 we focus on approaches based on topological changes. Based
on our experience in the development of progressive transmission prototype systems,
Sect. 4.6 is dedicated to a critical discussion of the main research and implementation
challenges still associated with progressive vector data transmission for Web mapping.
Finally in Sect. 4.7 we present a summary and some concluding remarks.
4.2 Progressive Transmission: Established Approaches
In this section we review existing methods commonly employed for progressive
transmission of spatial data over the Internet. The first approaches were developed for
Michela Bertolotto
As a consequence more recently attention has turned to the progressive transmission of vector data: several approaches based on effective compression mechanisms
have been defined and successfully implemented for the particular case of triangular
meshes [1, 16, 27–29].
In contrast, the investigation and development of progressive transmission techniques for more general vector spatial data still present many challenges. Our focus
is on map data (e.g. city street maps, road networks, hydrography networks, administrative boundary maps, etc.). Compressing this type of data is not straightforward
as many factors have to be taken into account. For example, spatial relations must be
preserved to guarantee reliable query results.
The level of detail of a vector map can be decreased by applying the so-called
map generalization process. However, such a process relies on cartographic principles whose complete formalization is still lacking (see [48] for a survey). Although
few automated solutions and algorithms have been proposed for specific cases, they
involve complex and time-consuming calculations and therefore cannot be applied
for real-time Web mapping.
Recent studies [10, 11] have distinguished between two different approaches for
the development of progressive map data transmission: on-the-fly map generation
versus pre-computation of multiple map representations. The first approach aims at
generating new generalized maps upon user request. The second approach relies on
the pre-computation of a fixed sequence of map representations that are stored and
transmitted progressively in order of increasing detail. Both approaches have advantages and drawbacks. This chapter provides a critical overview of these issues and
of the few prototype systems that have been described and developed for progressive
vector map transmission.
The remainder of this chapter is organized as follows. Sect. 4.2 reviews wellestablished implementations for progressive transmissions of data over the Internet.
These include the cases of raster images and triangular meshes. The problem of transmitting geographical map data in vector format is analyzed in the subsequent sections.
Section 4.3 discusses the use of progressive transmission techniques for Web mapping.
This is followed by an overview of the prototype systems developed and discussed in the
literature. These systems rely on techniques that can be differentiated on the basis of the
type of changes applied during generalization: geometric versus topological changes.
Therefore, in Sect. 4.4 we describe research related to changes in the geometry of the
data, while in Sect. 4.5 we focus on approaches based on topological changes. Based
on our experience in the development of progressive transmission prototype systems,
Sect. 4.6 is dedicated to a critical discussion of the main research and implementation
challenges still associated with progressive vector data transmission for Web mapping.
Finally in Sect. 4.7 we present a summary and some concluding remarks.
4.2 Progressive Transmission: Established Approaches
In this section we review existing methods commonly employed for progressive
transmission of spatial data over the Internet. The first approaches were developed for