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.