4 Progressive Techniques
73
A pointer-based encoding structure for the tree is provided and used to visit the
tree during progressive transmission and for reconstruction of each level. However,
full support for flexible navigation and browsing across map levels in the form of
vertical links is not provided.
A major drawback of the system proposed in [8] is that it does not efficiently
manage massive amounts of data as it stores vector map data in files without appropriate indexing mechanisms. This causes problems also for progressive transmission
within a multi-user Internet environment. The pre-processing phase also represents a
limitation. However, using pre-computations allows for progressive real-time analysis of generalized data that might not be possible if online simplification was applied
upon request. Another limitation relates to the fact that each line is simplified, stored,
and transmitted separately. This does not guarantee preservation of map consistency.
Full experimental results on the performance of the progressive transmission system
have not been reported.
Another system based on online simplification and developed for application to
progressive transmission has been presented by Boujou et al. [22]. The approach proposed in [22] attempts to overcome some of the limitations of other systems using
multiple map representations for progressive transmission. The idea is to try and
link pre-existing map sequences (generated applying line simplification) vertically in
order to easily access different versions of the same feature and generate variable
LOD maps based on the pre-computed map representations. This system was developed for deployment in mobile environments. The emphasis is then on reducing to
the minimum the amount of data to be transferred. Therefore it is paramount to allow
reuse of already transferred objects at previous LOD. The authors identify three data
transfer models between client and server:
1. Simple Communication: All queries are executed on the server and the entire
query result is transmitted to the client.
2. Two-step Communication: All queries are executed on the server but the client
maintains data cache and can reuse already received objects.
3. Pre-computed Answer: The client can execute some queries locally without connection to the server.
Depending on the current scenario, each of these models represents advantages
over the others. Experiments conducted on three LOD generated for the transportation network of La Rochelle (France) have shown interesting and promising results
for the gain in data transfer. Evaluation with other data sets is still ongoing.
In the following paragraphs we describe our work on progressive exchange of
simplified maps [51, 52]. The system we have developed relies on the application
of Saalfeld’s modified RDP algorithm. Several map representations are generated
and stored in a (Oracle
T M 9i [37]) Spatial Database. The different map versions are
organized into a hierarchical structure that avoids redundancy and favors efficient
transmission.
73
A pointer-based encoding structure for the tree is provided and used to visit the
tree during progressive transmission and for reconstruction of each level. However,
full support for flexible navigation and browsing across map levels in the form of
vertical links is not provided.
A major drawback of the system proposed in [8] is that it does not efficiently
manage massive amounts of data as it stores vector map data in files without appropriate indexing mechanisms. This causes problems also for progressive transmission
within a multi-user Internet environment. The pre-processing phase also represents a
limitation. However, using pre-computations allows for progressive real-time analysis of generalized data that might not be possible if online simplification was applied
upon request. Another limitation relates to the fact that each line is simplified, stored,
and transmitted separately. This does not guarantee preservation of map consistency.
Full experimental results on the performance of the progressive transmission system
have not been reported.
Another system based on online simplification and developed for application to
progressive transmission has been presented by Boujou et al. [22]. The approach proposed in [22] attempts to overcome some of the limitations of other systems using
multiple map representations for progressive transmission. The idea is to try and
link pre-existing map sequences (generated applying line simplification) vertically in
order to easily access different versions of the same feature and generate variable
LOD maps based on the pre-computed map representations. This system was developed for deployment in mobile environments. The emphasis is then on reducing to
the minimum the amount of data to be transferred. Therefore it is paramount to allow
reuse of already transferred objects at previous LOD. The authors identify three data
transfer models between client and server:
1. Simple Communication: All queries are executed on the server and the entire
query result is transmitted to the client.
2. Two-step Communication: All queries are executed on the server but the client
maintains data cache and can reuse already received objects.
3. Pre-computed Answer: The client can execute some queries locally without connection to the server.
Depending on the current scenario, each of these models represents advantages
over the others. Experiments conducted on three LOD generated for the transportation network of La Rochelle (France) have shown interesting and promising results
for the gain in data transfer. Evaluation with other data sets is still ongoing.
In the following paragraphs we describe our work on progressive exchange of
simplified maps [51, 52]. The system we have developed relies on the application
of Saalfeld’s modified RDP algorithm. Several map representations are generated
and stored in a (Oracle
T M 9i [37]) Spatial Database. The different map versions are
organized into a hierarchical structure that avoids redundancy and favors efficient
transmission.