The Role of Error

Map and attribute data errors are the data producer's responsibility, GIS user must understand error.

Accuracy scales.

and precision of map and attribute data in a GIS affect all other operations, especially when maps are compared across

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 products database = accuracy of the 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 57912 4.349

m Northing 519473 2.247

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 Record Attribute Value Attribute Value Record Record Value Value Value Value Attribute 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

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Role of DBMS

System Geographic Information System Database Management System Data Task • Data loading • Editing • Visualization • Mapping • Analysis • Storage • Indexing • Security • Query

“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

Column = attribute Row = object Vector feature

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

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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 SELECT GRANT , , , CREATE INDEX Data Manipulation Language (DML) Retrieve and manipulate data UPDATE , DELETE Control security of data CREATE USER , , INSERT Data Control Languages (DCL) 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

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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 Output Grid C

Compared with

Buffer

Recode

OR

And many more ….

See spatial analysis handout on course web site.