1. 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.

2. Accuracy closeness to TRUE values compromise on “infinite complexity”
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

3. 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

4. Positional Accuracy typical UTM coordinate pair might be:
Easting m
Northing 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

5. 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

6. 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

7. Accuracy vs. Precision

8. Accuracy vs. Precision

9. Components of Data Quality
positional accuracy
attribute accuracy
logical consistency
completeness
lineage

10. Lecture 10 Geographic Databases Gateway to Spatial Analysis Chapter 10 up to 10.4, Longley et al.

11. 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

12. A GIS can answer the question: What is where?
WHAT: Characteristics of features (= attributes).
WHERE: In geographic space.

13. 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

14. Flat File or DBMS Attribute Attribute Attribute Record Value Record

15. Types of DBMS Models Hierarchical Network Relational - RDBMS
Object-oriented - OODBMS
Object-relational - ORDBMS

16. Historically, databases were structured hierarchically in flat files...

17. Relational Databases rule now
2/1/ /4/98

18. Geographic Information System
Role of DBMS
Task
System
Data loading
Editing
Visualization
Mapping
Analysis
Geographic Information System
Storage
Indexing
Security
Query
Database Management
System
“Programmable API”
Data

19. 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)

20. Table
Column = attribute
Row = object
Vector feature

21. Relational DBMS (2) Most popular type of DBMS Commercial systems
Over 95% of data in DBMS is in RDBMS
Commercial systems
Microsoft Access
Microsoft SQL Server
Oracle
IBM DB2
Informix
Sybase

22. 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

23. Relational Databases
2/1/ /4/98

24. Parts of GIS database tables for U.S states
(A) STATES table; (B) POPULATION table

25. Parts of GIS database tables for U.S states
(C) joined table—COMBINED STATES and POPULATION

26. Tax assessment database
(C) data partially normalized into three subtables
Tax assessment database
(D) joined table

27. 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

28. 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

29. 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.

31. Overlay
Image courtesy of K. Foote/M. Lynch, UT-Austin

34. Overlay like an attribute join
2/1/ /4/98

35. Types of overlay operations
Union
Intersect
Identity
Max
Min
Etc.

36. 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.

37. within 25 miles of a city OR within 25 miles of a major river.
Union

38. 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.

39. within 25 miles of a city AND within 25 miles of a major river.
Intersect
within 25 miles of a city AND within 25 miles of a major river.

40. 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.

41. Identity Union Intersect
Portion of the major city buffer WITHIN the major river buffer
Union
within 25 miles of a city OR within 25 miles of a major river.
within 25 miles of a city AND within 25 miles of a major river.
Intersect

42. Intersect
Identity

43. Raster Retrieval: Map Algebra
Raster overlay
Combinations of spatial and attribute queries can build some complex and powerful GIS operations.

44. Input Grid A Input Grid B Output Grid C
Compared
with
Input Grid A
Input Grid B
Output Grid C

45. Buffer

47. Recode OR

49. And many more ….
See spatial analysis handout on course web site.