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

3.2 Geo-Raster Operations 45
turn is multiplied by the cell value of array A. Thus, the left part of the expression is
either 0 or the cell value of array A. Similarly, the right-hand side of the arithmetic
addition expression verifies if the cell value of array A is less than or equal to the cell
value of B. The result is either 0 or 1 depending on whether or not the condition is
satisfied. This value is multiplied for the cell value of array B. Note that only one of
the parts of the addition expression will be greater than zero, and that this value corresponds
to the highest value between arrays A and B. Results are shown in Fig. 3.6.
(a) 8-bit gray raster A (b) 8-bit gray raster B (c) Output raster
Figure 3.6. Computation of an Overlay Operation
An overlay operation can also be done considering a different condition to be tested
while determining the cell values of the output array. For example:
Query 3.2.7. Compute an overlay operation between rasters A and B. That is, compare
cell-wise the two rasters: if the cell value of B is non-zero, then set this value
as the cell value of the corresponding cell in array A. Otherwise, the cell value of A
remains unchanged.
The query can be answered as follows:
MARRAY sdom(A),g (((B[g] > 0) ∗ B[g]) + ((B[g] ≤ 0) ∗ A[g]))
Results are shown in Fig. 3.7.
3.2.2 Aggregation Operations
We now present the modeling of operations consisting of one or more aggregate
functions. An aggregate function takes a collection of cells and returns a single value
that summarizes the information contained in the set of cells. The SQL standard provides
a variety of aggregate functions. SQL-92 includes count, sum, average, min,