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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 l.jpg

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


Introduction l.jpg
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


Terminology l.jpg
Terminology

  • Various (often confused terms) in use:

    • error

    • uncertainty

    • accuracy

    • precision

    • data quality

GEOG2750 – Earth Observation and GIS of the Physical Environment


Error and uncertainty l.jpg
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


Accuracy vs precision l.jpg

4

YO!

Accuracy vs. Precision

Inaccurate

Accurate

1

2

Imprecise

3

4

Precise

GEOG2750 – Earth Observation and GIS of the Physical Environment


Question l.jpg
Question…

  • What does accuracy and precision mean for GIS co-ordinate systems?

GEOG2750 – Earth Observation and GIS of the Physical Environment


Quality l.jpg
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


Types and sources of error l.jpg
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


Slide9 l.jpg

Age of data

Northallerton circa 1999

Northallerton circa 1867

GEOG2750 – Earth Observation and GIS of the Physical Environment


Slide10 l.jpg

Global DEM

National DEM

European DEM

Scale of data

Local DEM

GEOG2750 – Earth Observation and GIS of the Physical Environment


Digitiser error l.jpg
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


Slide12 l.jpg

Regular shift

original

digitised

GEOG2750 – Earth Observation and GIS of the Physical Environment


Slide13 l.jpg

Distortion and edge-effects

original

digitised

GEOG2750 – Earth Observation and GIS of the Physical Environment


Slide14 l.jpg

Systematic and random errors

original

digitised

GEOG2750 – Earth Observation and GIS of the Physical Environment


Slide15 l.jpg

Obvious and hidden errors

original

digitised

GEOG2750 – Earth Observation and GIS of the Physical Environment


Vector to raster conversion error l.jpg
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


Slide17 l.jpg

Effects of raster size

Fine raster

Coarse raster

GEOG2750 – Earth Observation and GIS of the Physical Environment


Slide18 l.jpg

Effects of grid orientation

Original

Original raster

Tilted

Shifted

GEOG2750 – Earth Observation and GIS of the Physical Environment


Attribute uncertainty l.jpg
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


Slide20 l.jpg

Imprecise and vague

505.9

500

500-510

238.4

240

230-240

GEOG2750 – Earth Observation and GIS of the Physical Environment


Slide21 l.jpg

Mixed up

505.9

238.4

238.4

505.9

GEOG2750 – Earth Observation and GIS of the Physical Environment


Slide22 l.jpg

Just plain wrong...!

505.9

100.3

238.4

982.3

GEOG2750 – Earth Observation and GIS of the Physical Environment


Generalisation l.jpg
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


Slide24 l.jpg

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


Question25 l.jpg
Question…

  • An appreciation of error and uncertainty is important because…

GEOG2750 – Earth Observation and GIS of the Physical Environment


Handling error and uncertainty l.jpg
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


Question27 l.jpg
Question…

  • What error handling facilities are their in proprietary GIS packages like ArcGIS?

GEOG2750 – Earth Observation and GIS of the Physical Environment


Basic error handling l.jpg
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


Question29 l.jpg
Question…

  • How can error be communicated to end users?

GEOG2750 – Earth Observation and GIS of the Physical Environment


Quantifying error l.jpg
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


Conclusions l.jpg
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


Practical l.jpg
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


Practical33 l.jpg
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


Learning outcomes l.jpg
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


Useful web links l.jpg
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


Next week l.jpg
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


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