The Role of Error
Map and attribute data errors are the data
producer's responsibility,
GIS user must understand error.
Accuracy and precision of map and attribute
data in a GIS affect all other operations,
especially when maps are compared across
scales.
Accuracy
closeness to TRUE values
results, computations, or estimates
compromise on “infinite complexity”
generalization of the real world
difficult to identify a TRUE value
e.g., accuracy of a contour
Does not exist in real world
Compare to other sources
Accuracy (cont.)
accuracy of the database = accuracy of the
products computed from database
e.g., accuracy of a slope, aspect, or
watershed computed from a DEM
Positional Accuracy
typical UTM coordinate pair might be:
Easting 579124.349 m
Northing 5194732.247 m
If the database was digitized from a 1:24,000
map sheet, the last four digits in each
coordinate (units, tenths, hundredths,
thousandths) would be questionable
Testing Positional Accuracy
Use an independent source of higher accuracy:
find a larger scale map (cartographically speaking)
use GPS
Use internal evidence:
digitized polygons that are unclosed, lines
that overshoot or undershoot nodes, etc. are
indications of inaccuracy
sizes of gaps, overshoots, etc. may be a
measure of positional accuracy
Precision
not the same as accuracy!
repeatability vs. “truth”
not closeness of results, but number of
decimal places or significant digits in a
measurement
A GIS works at high precision, usually much
higher than the accuracy of the data
themselves
Accuracy vs. Precision
Accuracy vs. Precision
Components of Data Quality
positional accuracy
attribute accuracy
logical consistency
completeness
lineage
Lecture 10
Geographic Databases
Gateway to Spatial Analysis
Chapter 10 up to 10.4, Longley et al.
Definitions
Database – an integrated set of
attributes on a particular subject
Geographic (=geospatial) database –
set of attributes on a particular subject
for a particular geographic area
Database Management System (DBMS)
– software to create, maintain and
access databases
A GIS can answer the question:
What is where?
WHAT: Characteristics of
features (= attributes).
WHERE: In geographic space.
A GIS links attribute and spatial data
Attribute Data
• Flat File or DBMS
• Relationships
• Topology Table
Map Data
• Point File
• Line File
• Area File
• Topology Type
Flat File or DBMS
Attribute
Attribute
Attribute
Record
Value
Value
Value
Record
Value
Value
Value
Record
Value
Value
Value
Types of DBMS Models
Hierarchical
Network
Relational - RDBMS
Object-oriented - OODBMS
Object-relational - ORDBMS
Historically, databases were
structured hierarchically in flat
files...
Relational Databases rule now
2/1/98
2/4/98
Role of DBMS
System
Task
Geographic
Information
System
•
•
•
•
•
Data loading
Editing
Visualization
Mapping
Analysis
Database
Management
System
•
•
•
•
Storage
Indexing
Security
Query
Data
“Programmable API”
Relational DBMS (1)
Data stored as tuples (tup-el),
conceptualized as tables
Table – data about a class of objects
Two-dimensional list (array)
Rows = objects
Columns = object states (properties,
attributes)
Table
Row = object
Vector feature
Column = attribute
Relational DBMS (2)
Most popular type of DBMS
Over 95% of data in DBMS is in RDBMS
Commercial systems
Microsoft Access
Microsoft SQL Server
Oracle
IBM DB2
Informix
Sybase
Relational Join
Fundamental query operation
Occurs because
Data created/maintained by different users, but
integration needed for queries
Table joins use common keys (column
values)
Table (attribute) join concept has been
extended to geographic case
Relational Databases
2/1/98
2/4/98
Parts of GIS database tables for U.S states
(A) STATES table; (B) POPULATION table
Parts of GIS database tables for U.S states
(C) joined table—COMBINED STATES and POPULATION
(C) data partially
normalized into
three subtables
Tax assessment database
(D) joined table
SQL
Structured (Standard) Query Language –
(pronounced SEQUEL)
Developed by IBM in 1970s
• Standard for accessing relational databases
Three types of usage
Stand alone queries
High level programming
Embedded in other applications
Types of SQL Statements
Data Definition Language (DDL)
Create, alter and delete data
CREATE TABLE, CREATE INDEX
Data Manipulation Language (DML)
Retrieve and manipulate data
SELECT, UPDATE, DELETE, INSERT
Data Control Languages (DCL)
Control security of data
GRANT, CREATE USER, DROP USER
Spatial Query/Search & Retrieval:
Gateway to Spatial Analysis
Overlay is a spatial retrieval operation
that is equivalent to an attribute join.
Buffering is a spatial retrieval around
points, lines, or areas based on
distance.
Overlay
Image courtesy of K. Foote/M. Lynch, UT-Austin
Overlay like an attribute join
2/1/98
2/4/98
Types of overlay operations
Union
Intersect
Identity
Max
Min
Etc.
Union
computes the geometric intersection of two
polygon coverages. All polygons from both
coverages will be split at their intersections
and preserved in the output coverage.
Union
within 25 miles of
a city OR within
25 miles of a
major river.
Intersect
computes the geometric intersection of two
coverages. Only those features in the area common
to both coverages will be preserved in the output
coverage.
Intersect
within 25 miles of
a city AND within
25 miles of a
major river.
Identity
computes the geometric intersection of two coverages. All
features of the input coverage, as well as those features of the
identity coverage that overlap the input coverage, are preserved
in the output coverage.
Identity
Portion of the major city buffer WITHIN the major river buffer
within 25 miles of a city OR within 25 miles of a major river.
Union
within 25 miles of a city AND within 25 miles of a major river.
Intersect
Intersect
Identity
Raster Retrieval: Map Algebra
Raster overlay
Combinations of spatial and attribute
queries can build some complex and
powerful GIS operations.
Input Grid A
Input Grid B
Compared
with
Output Grid C
Buffer
Recode
OR
And many more ….
See spatial analysis handout on course
web site.