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The Role of Error

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.

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The Role of Error

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  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 • 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 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

  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 placesor 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 10Geographic DatabasesGateway to Spatial AnalysisChapter 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. Attribute Data Flat File or DBMS Relationships Topology Table Map Data Point File Line File Area File Topology Type A GIS links attribute and spatial data

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

  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 2/1/98 2/4/98

  18. 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 • 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 2/1/98 2/4/98

  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. (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.

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

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

  32. Types of overlay operations • Union • Intersect • Identity • Max • Min Etc.

  33. 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.

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

  35. 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.

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

  37. 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.

  38. 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. within 25 miles of a city AND within 25 miles of a major river. Union Intersect

  39. Intersect Identity

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

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

  42. Buffer

  43. OR Recode

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

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