| Global
Coordinates:
- provides a comprehensive framework for the database
- allows the database to be viewed in its entirety
so that interaction between elements can be evaluated
- permits identification of potential problems
and design alternatives
- without a good database design, there may be
- irrelevant data that will not be used
- omitted data
- no update potential
- inappropriate representation of entities
- lack of integration between various parts
of the database
- unsupported applications
- major additional costs to revise the database
Issues in database design
- what storage media to use?
- how large is the database?
- how much can be stored online? what access
speed is required for what parts of the database?
- how should the database be laid out on the
various media?
- what growth should be allowed for in acquiring
storage devices?
- how will the database change over time?
- will new attributes be added?
- will the number of features stored increase?
- how should the data be partitioned - both geographically
and thematically?
- is source data partitioned?
- will products be partitioned?
- what security is needed?
- who should be able to redefine schema -
new attributes, new objects, new object classes?
- who should be able to edit and update?
- should the database be distributed or centralized?
- if distributed, how will it be partitioned
between hosts?
- how should the database be documented?
- who is responsible for maintaining standards
of definition? standards of format? accuracy?
should documentation include access to the
compiler of the data?
- how should database creation be scheduled?
- where will the data come from?
- who determines product priorities?
- who is responsible for scheduling data availability?
- the following sections address some of these
questions.
Key hardware parameters
Volume
Access speed
Should database be centralized
or distributed?
there are two answers: 1. all departments
share one common database, or 2. parts of the database
exist on different workstations in an integrated network
with modern technology (e.g. NFS
(Network File System)) user may be unaware of actual
location of data being used
distributed databases require careful attention
to responsibilities, standards, scheduling of updates.
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Integrating Quality Assurance
into the GIS Project Life Cycle
Without data there would be no need
for the computers, software and human resources that
comprise GIS technology. Not just any data, but geographic
data. And not just any geographic data, but data that
is specific and reliable and that represents as closely
as possible the spatial world we live in. The technology
requires that the data be as clean, as healthy, as
good as it can be. Neglecting that, the usefulness
of the technology is short-lived. To maximize the
quality of GIS databases there should exist a well-designed
quality assurance plan that is strategically integrated
with all facets of the GIS project.
Categories of Quality Assurance
All well-designed QA strategies have certain things
in common. They must coexist within the processes
that create and maintain the data. When they are not
integrated within the procedures of the GIS project,
they themselves can become an entry point for error.
By definition, they must also incorporate key elements
from the classic QA categories that are discussed
below.
Completeness
Completeness is the adherence of the data to the database
design. This means that all of the data conforms to
a known standard for topology, table structure, precision,
projection and other data-model specific requirements.
Validity
Validity is a measure of the attribute accuracy of
the database. Each attribute must have a defined domain
and range. The domain is the set of all legal values
for the attribute. The range is the set of values
within which the data must fall.
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Logical Consistency
Logical consistency is a measure of the interaction
between the values of two or more functionally related
attributes. As the value of one attribute changes,
to maintain consistency, so must the values of its
functionally related attributes.. An example would
be the interaction between the attribute SLOPE and
the attribute LANDUSE. If LANDUSE is "water",
then SLOPE must be 0, any other value for SLOPE would
be illogical.
Physical Consistency
Physical consistency is a measure of the topological
correctness and geographic extent of the database.
For example, the requirement that all electrical transformers
in an electrical distribution database's GIS have
annotation denoting phasing placed within fifteen
feet of the transformer object is one that describes
a physically consistent spatial requirement.
Referential Integrity
Referential integrity is a measure of the associativity
of related tables based upon their primary and foreign
key relationships. Primary and foreign keys must exist
and they must associate sets of data in the tables
given predefined rules for each table.
Positional Accuracy
Positional accuracy is a measure of how well each
spatial object's position in the database matches
reality. Positional error can be introduced via incorrect
cartographic interpretation, through insufficient
densification of vertices in line segments or through
digital storage precision inadequacies, to name a
few. These errors can be random, systematic and/or
cumulative in nature. Positional accuracy must always
be qualified, because after all, it is only just a
map of reality.
(Acknowledgements: http://www.geog.ubc.ca/) |
