Geoscience Reference
In-Depth Information
Table 1
Triangulated data structure
Dataset
Number of buildings
Number of individual nodes
Sheffield—LoD 1
1,560
731,124
Sheffield—generalised
108
307,431
4 Implementing the First Visualisation Algorithm
This chapter proposes a hybrid approach (
Sect. 2.1
) to vector mapping of a 3D
City Model. Firstly, data is pre-structured into a format suitable for rendering
within a database server (making use of Java algorithms to implement the required
structuring). The data can then be extracted (queried) from the spatial database
using a query language such as SQL, and pre-packaged for transmission as an
XML dataset. The extraction and packaging operations can be implemented using
a web-based programming language such as PHP. Once the data is received on
the client (mobile device), the XML can then be parsed—i.e. the data elements
extracted from the XML file and placed into a the required structure for rendering.
As described below (
Sect. 4.2
), OpenGL ES only supports data in triangulated for-
mat. A number of data preparation stages are therefore required to convert the data
into the appropriate format and structure it for rendering. These are described here,
followed by a description of Open GL ES and of the implementation of the render-
ing software itself.
4.1 Dataset Structuring and Transmission
Although it is possible to triangulate the data on the fly in the App itself (see (Ellul
and Altenbuchner
2013
) for an example of this) given the quantities of data, the
first step of the hybrid approach described here is to undertake the triangulation
process on the server-side data.
Loading the Dataset into Oracle Spatial
The dataset is provided in ESRI
Shapefile Multi-Patch format (ESRI
2013
) and is then transformed into Oracle's
Spatial Database Format (the Oracle database is used to serve the data to the test
App). As part of this transformation, the dataset is fully triangulated, as required
by OpenGL ES (Whitrow
2008
). Although the Multi-Patch format does partially
triangulate the data—specifically the walls of the building are triangulated—in
the case of a standard extrusion process the roof and floor structure are stored as
“geometry rings” and not suitable for rendering in OpenGL ES (ESRI
2013
). The
“Feature Manipulation Engine” (FME) software from SafeSoft (Software
2013
)
was used to first triangulate the data by converting it to Autodesk 3D Studio for-
mat, and then to import the resulting triangulated features into Oracle Spatial
Oracle (
2013
). Once imported into the database, the data was re-structured in
preparation for visualisation—i.e. lists of triangles and corresponding nodes were
pre-created as tables.
