Presentation on theme: "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."— Presentation transcript:
1 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.
2 Accuracy closeness to TRUE values compromise on “infinite complexity” results, computations, or estimatescompromise on “infinite complexity”generalization of the real worlddifficult to identify a TRUE valuee.g., accuracy of a contourDoes not exist in real worldCompare to other sources
3 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
4 Positional Accuracy typical UTM coordinate pair might be: Easting mNorthing 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
5 Testing Positional Accuracy Use an independent source of higher accuracy:find a larger scale map (cartographically speaking)use GPSUse 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
6 Precision not the same as accuracy! repeatability vs. “truth” not closeness of results, but number of decimal places or significant digits in a measurementA GIS works at high precision, usually much higher than the accuracy of the data themselves
9 Components of Data Quality positional accuracyattribute accuracylogical consistencycompletenesslineage
10 Lecture 10 Geographic Databases Gateway to Spatial Analysis Chapter 10 up to 10.4, Longley et al.
11 DefinitionsDatabase – 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
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 DataFlat File or DBMSRelationshipsTopology TableMap DataPoint FileLine FileArea FileTopology Type
14 Flat File or DBMS Attribute Attribute Attribute Record Value Record
18 Geographic Information System Role of DBMSTaskSystemData loadingEditingVisualizationMappingAnalysisGeographic Information SystemStorageIndexingSecurityQueryDatabase ManagementSystem“Programmable API”Data
19 Relational DBMS (1)Data stored as tuples (tup-el), conceptualized as tablesTable – data about a class of objectsTwo-dimensional list (array)Rows = objectsColumns = object states (properties, attributes)
21 Relational DBMS (2) Most popular type of DBMS Commercial systems Over 95% of data in DBMS is in RDBMSCommercial systemsMicrosoft AccessMicrosoft SQL ServerOracleIBM DB2InformixSybase
22 Relational Join Fundamental query operation Occurs because Data created/maintained by different users, but integration needed for queriesTable joins use common keys (column values)Table (attribute) join concept has been extended to geographic case
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 subtablesTax assessment database(D) joined table
27 SQL Structured (Standard) Query Language – (pronounced SEQUEL) Developed by IBM in 1970sStandard for accessing relational databasesThree types of usageStand alone queriesHigh level programmingEmbedded in other applications
28 Types of SQL Statements Data Definition Language (DDL)Create, alter and delete dataCREATE TABLE, CREATE INDEXData Manipulation Language (DML)Retrieve and manipulate dataSELECT, UPDATE, DELETE, INSERTData Control Languages (DCL)Control security of dataGRANT, 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.
35 Types of overlay operations UnionIntersectIdentityMaxMinEtc.
36 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.
37 within 25 miles of a city OR within 25 miles of a major river. Union
38 Intersectcomputes 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. Intersectwithin 25 miles of a city AND within 25 miles of a major river.
40 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.
41 Identity Union Intersect Portion of the major city buffer WITHIN the major river bufferUnionwithin 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