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Lecture 13 Error and uncertainty

Lecture 13 Error and uncertainty. Outline terminology, types and sources why is it important? handling error and uncertainty. Introduction. GIS, great tool but what about error? data quality, error and uncertainty? error propagation? confidence in GIS outputs? NCGIA Initiative I- 1

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Lecture 13 Error and uncertainty

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  1. Lecture 13Error and uncertainty • Outline • terminology, types and sources • why is it important? • handling error and uncertainty GEOG2750 – Earth Observation and GIS of the Physical Environment

  2. Introduction • GIS, great tool but what about error? • data quality, error and uncertainty? • error propagation? • confidence in GIS outputs? • NCGIA Initiative I-1 • major research initiative? • dropped because too hard? • Be careful, be aware, be upfront... GEOG2750 – Earth Observation and GIS of the Physical Environment

  3. Terminology • Various (often confused terms) in use: • error • uncertainty • accuracy • precision • data quality GEOG2750 – Earth Observation and GIS of the Physical Environment

  4. Error and uncertainty • Error • wrong or mistaken • degree of inaccuracy in a calculation • e.g. 2% error • Uncertainty • lack of knowledge about level of error • unreliable GEOG2750 – Earth Observation and GIS of the Physical Environment

  5. 4 YO! Accuracy vs. Precision Inaccurate Accurate 1 2 Imprecise 3 4 Precise GEOG2750 – Earth Observation and GIS of the Physical Environment

  6. Question… • What does accuracy and precision mean for GIS co-ordinate systems? GEOG2750 – Earth Observation and GIS of the Physical Environment

  7. Quality • Data quality • degree of excellence • general term for how good the data is • takes all other definitions into account • error • uncertainty • precision • accuracy GEOG2750 – Earth Observation and GIS of the Physical Environment

  8. Types and sources of error • Group 1 - obvious sources: • age of data and areal coverage • map scale and density of observations • Group 2 - variation and measurement: • positional error • attribute uncertainty • generalisation • Group 3 - processing errors: • numerical computing errors • faulty topological analyses • interpolation errors GEOG2750 – Earth Observation and GIS of the Physical Environment

  9. Age of data Northallerton circa 1999 Northallerton circa 1867 GEOG2750 – Earth Observation and GIS of the Physical Environment

  10. Global DEM National DEM European DEM Scale of data Local DEM GEOG2750 – Earth Observation and GIS of the Physical Environment

  11. Digitiser error • Manual digitising • significant source of positional error • Source map error • scale related generalisation • line thickness • Operator error • under/overshoot • time related boredom factor GEOG2750 – Earth Observation and GIS of the Physical Environment

  12. Regular shift original digitised GEOG2750 – Earth Observation and GIS of the Physical Environment

  13. Distortion and edge-effects original digitised GEOG2750 – Earth Observation and GIS of the Physical Environment

  14. Systematic and random errors original digitised GEOG2750 – Earth Observation and GIS of the Physical Environment

  15. Obvious and hidden errors original digitised GEOG2750 – Earth Observation and GIS of the Physical Environment

  16. Vector to raster conversion error • coding errors • cell size • majority class • central point • grid orientation • topological mismatch errors • cell size • grid orientation GEOG2750 – Earth Observation and GIS of the Physical Environment

  17. Effects of raster size Fine raster Coarse raster GEOG2750 – Earth Observation and GIS of the Physical Environment

  18. Effects of grid orientation Original Original raster Tilted Shifted GEOG2750 – Earth Observation and GIS of the Physical Environment

  19. Attribute uncertainty • Uncertainty regarding characteristics (descriptors, attributes, etc.) of geographical entities • Types: • imprecise (numeric) or vague (descriptive) • mixed up • plain wrong! • Sources: • source document • misinterpretation (human error) • database error GEOG2750 – Earth Observation and GIS of the Physical Environment

  20. Imprecise and vague 505.9 500 500-510 238.4 240 230-240 GEOG2750 – Earth Observation and GIS of the Physical Environment

  21. Mixed up 505.9 238.4 238.4 505.9 GEOG2750 – Earth Observation and GIS of the Physical Environment

  22. Just plain wrong...! 505.9 100.3 238.4 982.3 GEOG2750 – Earth Observation and GIS of the Physical Environment

  23. Generalisation • Scale-related cartographic generalisation • simplification of reality by cartographer to meet restrictions of: • map scale and physical size • effective communication and message • can result in: • reduction, alteration, omission and simplification of map elements • passed on to GIS through digitising GEOG2750 – Earth Observation and GIS of the Physical Environment

  24. Cartographic generalisation 1:3M 1:10,000 1:500,000 1:25,000 City of Sapporo, Japan GEOG2750 – Earth Observation and GIS of the Physical Environment

  25. Question… • An appreciation of error and uncertainty is important because… GEOG2750 – Earth Observation and GIS of the Physical Environment

  26. Handling error and uncertainty • Must learn to cope with error and uncertainty in GIS applications • minimise risk of erroneous results • minimise risk to life/property/environment • More research needed: • mathematical models • procedures for handling data error and propagation • empirical investigation of data error and effects • procedures for using output data uncertainty estimates • incorporation as standard GIS tools GEOG2750 – Earth Observation and GIS of the Physical Environment

  27. Question… • What error handling facilities are their in proprietary GIS packages like ArcGIS? GEOG2750 – Earth Observation and GIS of the Physical Environment

  28. Basic error handling • Awareness • knowledge of types, sources and effects • Minimisation • use of best available data • correct choices of data model/method • Communication • to end user! GEOG2750 – Earth Observation and GIS of the Physical Environment

  29. Question… • How can error be communicated to end users? GEOG2750 – Earth Observation and GIS of the Physical Environment

  30. Quantifying error • Sensitivity analyses • Jacknifing • leave-one-out analysis • repeat analysis leaving out one data layer • test for the significance of each data layer • Bootstrapping • Monte Carlo simulation • adds random noise to data layers • Simulates the effect error/uncertainty GEOG2750 – Earth Observation and GIS of the Physical Environment

  31. Conclusions • Many types and sources of error that we need to be aware of • Environmental data is particularly prone because of high spatio-temporal variability • Few GIS tools for handling error and uncertainty… and fewer still in proprietary packages • Need to communicate potential error and uncertainty to end users GEOG2750 – Earth Observation and GIS of the Physical Environment

  32. Practical • Error in off-the-shelf datasets • Task: Assess the error in land cover data • Data: The following datasets are provided for the Leeds area… • Streets and buildings (1:10,000 OS LandLine data) • 25m resolution land cover data (ITE LCM90) GEOG2750 – Earth Observation and GIS of the Physical Environment

  33. Practical • Steps: • Display OS LandLine data over ITE LCM90 data using ArcMap. You can also add the OS 1:50,000 colour raster image and set transparency = 70%. • From your knowledge of the area identify areas of erroneous classification • What might these errors be due to? GEOG2750 – Earth Observation and GIS of the Physical Environment

  34. Learning outcomes • Familiarity with error in classified satellite imagery • Familiarity with ITE land cover map 1990 (LCM90) data • Experience with new GRID functions GEOG2750 – Earth Observation and GIS of the Physical Environment

  35. Useful web links • The Geographer’s Craft – lecture on error • http://www.colorado.edu/geography/gcraft/notes/error/error_f.html • GIGO • http://www.geoplace.com/gw/2000/1000/1000gar.asp • Disaster waiting to happen • http://www.osmose.com/utilities/articles_press_releases/data_quality/ GEOG2750 – Earth Observation and GIS of the Physical Environment

  36. Next week… • Interpolating environmental datasets • creating surfaces from points • interpolation basics • interpolation methods • common problems • Practical: Interpolating surfaces from point data GEOG2750 – Earth Observation and GIS of the Physical Environment

  37. پایگاه پاورپوینت ایرانwww.txtzoom.comبانک اطلاعات هوشمند پاورپوینت GEOG2750 – Earth Observation and GIS of the Physical Environment

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