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.

The Role of ErrorMap 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.

Accuracycloseness to TRUE values

results, computations, or estimates

compromise on “infinite complexity”

generalization of the real worlddifficult to identify a TRUE value

e.g., accuracy of a contourDoes not exist in real worldCompare to other sources

Accuracy (cont.)accuracy of the database = accuracy of the products computed from databasee.g., accuracy of a slope, aspect, or watershed computed from a DEM

Positional Accuracytypical UTM coordinate pair might be:Easting 579124.349 mNorthing 5194732.247 mIf 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 AccuracyUse 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 inaccuracysizes of gaps, overshoots, etc. may be a measure of positional accuracy

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

Precision

Accuracy vs. Precision

High AccuracyLow Precision

Low AccuracyHigh Precision

Many darts in reproduceable clusters, but not in the bullseye.

Darts are near the bullseye (the "true value"), but there aren't very many clusters of them (not reproduceable).

Accuracy vs. Precision

High AccuracyLow Precision

Low AccuracyHigh Precision

Many darts in reproduceable clusters, but not in the bullseye.

Darts are near the bullseye (the "true value"), but there aren't very many clusters of them (not reproduceable).

Components of Data Quality

positional accuracyattribute accuracylogical consistencycompletenesslineage

Lecture 10Geographic Databases

Gateway to Spatial Analysis

Chapter 10 up to 10.4, Longley et al.

Definitions

Database – an integrated set of attributes on a particular subjectGeographic (=geospatial) database – set of attributes on a particular subject for a particular geographic areaDatabase 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

Record Value Value Value

Attribute Attribute Attribute

Record Value Value Value

Record Value Value Value

Types of DBMS Models

HierarchicalNetworkRelational - RDBMSObject-oriented - OODBMSObject-relational - ORDBMS

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

Relational Databases rule now

2/1/98 2/4/98

Geographic Information

System

Database Management

System

• Data loading• Editing• Visualization• Mapping• Analysis

• Storage• Indexing• Security• Query

Data

System TaskRole of DBMS

“Programmable API”

Relational DBMS (1)

Data stored as tuples (tup-el), conceptualized as tablesTable – data about a class of objects

Two-dimensional list (array)Rows = objectsColumns = object states (properties, attributes)

Table

Row = objectVector feature

Column = attribute

Relational DBMS (2)

Most popular type of DBMSOver 95% of data in DBMS is in RDBMS

Commercial systemsMicrosoft AccessMicrosoft SQL ServerOracleIBM DB2InformixSybase

Relational Join

Fundamental query operationOccurs 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

Tax assessment database

(D) joined table

(C) data partially normalized into three subtables

SQL

Structured (Standard) Query Language – (pronounced SEQUEL)Developed by IBM in 1970s

• Standard for accessing relational databasesThree types of usage

Stand alone queriesHigh level programmingEmbedded in other applications

Types of SQL Statements

Data Definition Language (DDL)Create, alter and delete dataCREATE TABLE, CREATE INDEX

Data Manipulation Language (DML)Retrieve and manipulate dataSELECT, UPDATE, DELETE, INSERT

Data Control Languages (DCL)Control security of dataGRANT, 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

UnionIntersectIdentityMaxMin

Etc.

Unioncomputes 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.

Intersectcomputes 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.

Identitycomputes 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

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.

Portion of the major city buffer WITHIN the major river buffer

Union Intersect

Identity

Intersect

Raster Retrieval: Map AlgebraRaster overlayCombinations of spatial and attribute

queries can build some complex and powerful GIS operations.

Comparedwith

Input Grid A Input Grid B Output Grid C

Buffer

RecodeOR

And many more ….See spatial analysis handout on

course web site.