multidimensional model and SOLAP operators with
complete and incomplete field data (cf. Section 6).
In the same way, handling multi-resolutions of
spatial data into spatial multidimensional models has
been proposed in few works (Yvan et al., 2002)
(Gascueña and Guadalupe 2009) that propose
conceptual models to represent SDW with several
representations (scales, resolutions, etc.) of spatial
dimensions and measures.
However, to best of our knowledge existing
works concerning field data and multi-resolutions
lack of a complete implementation in a full-featured
SOLAP architecture, or in other terms they do not
propose a coupled relational and SOLAP server
model for a generic SOLAP architecture allowing (i)
the map algebra operators, (ii) the multi-resolution,
and (iii) a continuous view of the field.
In order to handle the spatio-multidimensional
analysis of incomplete regular grid field data at
different resolutions, we propose in this paper: (i) a
specific logical model, extending the well-know
relational star schema; (ii) and some new MDX-
based defined functions. We validate our proposal
using a real case study concerning the odor
monitoring, and we provide some experiments
showing the feasibility also in terms of storage and
computation performances.
2 MODELING AND ANALYSIS
REQUIREMENTS
In order to show our proposal, we present a case
study based on data issued from the monitoring of
urban odor. For each 15 minutes and type of odor
(e.g. NO2) a regular grid map (field) is produced by
means of some sample points and a simulation
model (ADMS5) . The simulation model estimates
odors for a whole urban area and produces 100*100
thematic grids. Examples of points grid are provided
in figure 2-a (odor values are represented by color:
green, yellow, red) for 10:00 19-2-2012 and 10:15
19-2-2012. Let us now suppose that the user wants
to aggregate data along a temporal dimension (year,
month, day, hour, minute) using the average to
obtain an aggregated odor map. This is an OLAP
operation of RollUp on the temporal dimension that
corresponds to a local map algebra operation (Figure
2-a
).Moreover, since space is represented in a
continuous way, decision-makers should be able to
ask for the result of any OLAP query in any point of
the spatial dimension (for example, s/he should be
interested in the odor value at 10:00 in the area
behind the building) (Figure 2-c). It is also possible
to apply a spatial slice operator on the spatial
dimension (i.e. using a spatial predicate to select a
subset of warehoused data) (Figure 2-d). In order to
answer to these last two queries spatial interpolation
methods are necessary, since in incomplete field
only the values provided by the simulation model
are stored. Spatial interpolation is the process of
prediction of almost exact values of attributes at
unsampled locations from measurements made at
control points within the same area (O'Sullivan and
Unwin, 2002). In our case the interpolation function
used is the bilinear interpolation, which is a local
deterministic method. It uses the 2 * 2 grid sample
points closest to the unknown point and calculates a
distance weighted average which determines in what
proportion the value of a neighbour impact on the
value of the point to be estimated (Figure 1).
Finally, as stated in the previous section, since
visualization of spatial data at different resolutions is
mandatory for the exploration/analysis process,
decision-makers should be able querying spatial
warehoused data at different resolutions. It is very
important to note that for each spatial phenomenon a
set of useful known resolutions exist, so they could
be predefined according to data and users needs.
Moreover, in order to calculate values at finer
resolutions spatial interpolation functions as
previously described can be used.
To summarize, spatio-multidimensional analysis
of field data implies: supporting (i) OLAP classical
operators as Map Algebra, (ii) continuous view of
spatial data, (iii) spatial slice operators using field
data, and (iv) visualizing and querying data at
different predefined resolutions.
3 SPATIO-MULTIDIMENSIONAL
MODEL FOR INCOMPLETE
FIELD DATA
In this section we describe our spatio-
multidimensional model for handling incomplete
fields at different resolutions. Our model extends the
classical spatio-multidimensional models to generate
the continuity of the phenomena over the studied
area, and represents pre-defined levels of resolution.
In particular, a “Cube” is composed of “Facts”
and “Dimensions”. A “Dimension” is composed of
“Hierarchies”, which are composed of “Levels”. A
“Level” can be spatial or conventional. This means
that it can contain “Spatial attributes” (e.g. points,
etc.), or contain only alphanumerical attributes
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