Applying OLAP Pre-Aggregation Techniques to ... - Jacobs University

44 3. Fundamental Geo-Raster Operations
Proximity
The proximity operation creates a new raster where each cell value contains the distance
to a specified reference point. As an example consider the following query:
Query 3.2.5. Estimate the proximity of each cell of the raster image shown in Fig. 3.4(a)
to the reference cell located in [30,5].
The computation of this query can be formulated as:
Results are shown in Fig. 3.5.
MARRAY sdom(A),(g,h) (|g − 30| + |h − 5|)
Figure 3.5. Computation of a Proximity Operation
Overlay
The overlay operation refers to the process of stacking two or more identical georeferenced
rasters on top of each other so that each position in the covered area can be
analyzed in terms of these data. The overlay operation can be solved using arithmetic
and relational operators. For example, consider the following query:
Query 3.2.6. Given two 8-bit gray raster images A and B with identical spatial domain,
perform an overlay operation. That is, make a cell-wise comparison between
the two rasters. Each cell value of the new array must take the maximum cell value
between A and B.
The computation of this query can be formulated as:
MARRAY sdom(A),g (((A[g] > B[g]) ∗ A[g]) + ((A[g] ≤ B[g]) ∗ B[g]))
The above formulation works as follows. The left part of the arithmetic expression +
tests for the cell value of array A to be greater than the cell value of B. The result of
this operation is either 0 (condition not satisfied) or 1 (condition satisfied), which in