Spatial Data Warehouses - Data Warehouse Systems: Design and Implementation - page 428

State

StateName
StatePopulation
StateArea
Capital
...

Highway

HighwayNo
...

RoadCoating

Location

CoatingName
CoatingType
CoatingDurability
...

County

Section
SectionNo
...

CountyName
CountyPopulation
CountyArea
...

HighwayStructure

Maintenance

Time

Segment
SegmentNo
RoadCondition
SpeedLimit
...

Length
CommonArea
NoCars
RepairCost

Date
DayNbWeek
DayNameWeek
...

Fig. 11.6 A spatial data warehouse for analyzing the maintenance of highways

GeoNorthwind data warehouse, the Sales fact does not impose any constraint
between its spatial dimensions Supplier and Customer . As an example of a
spatial fact with an associated topological constraint, consider the schema
of Fig. 11.6 , which can be used for the analysis of highway maintenance
costs. The spatial fact Maintenance relates two spatial levels: County and
Segment . This fact includes an Overlaps topological constraint, indicating
that a segment and a county related to a fact member must overlap.
Facts, whether spatial or not, may contain measures that represent data
that are meaningful for leaf members that are aggregated when a hierarchy
is traversed. Measures can be numeric or spatial , where the latter are
represented by a geometry. Note that numeric measures can be calculated
using spatial operations such as distance and area. For example, Fig. 11.6
contains two measures. Length is a numerical measure obtained using spatial
operations, representing the length of the part of a highway segment that
belongs to a county, and CommonArea represents the geometry of the
common part.
Measures require the specification of the function used for aggregation
along the hierarchies. By default, we assume sum for numerical measures
and spatial union for spatial measures. For example, in Fig. 11.6 ,whenusers
roll up from the County to the State level, for each state the measures Length ,
NoCars ,and RepairCost of the corresponding counties will be summed, while
the CommonArea measure will be a LineSet resulting from the spatial union
of the lines representing highway segments for the corresponding counties.