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“Honest GIS”: Error and Uncertainty . Longley et al., 1/e, chs. 6 and 15 Longley et al., 2/e, ch. 6 See also GEO 565 Lecture 12 Berry online text. Blinded by Science?. Result of “accurate” scientific measurement Reveal agenda, biases of their creators. GIS databases built from maps

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honest gis error and uncertainty

“Honest GIS”:Error and Uncertainty

Longley et al., 1/e, chs. 6 and 15

Longley et al., 2/e, ch. 6

See also GEO 565 Lecture 12

Berry online text

blinded by science
Blinded by Science?
  • Result of “accurate” scientific measurement
    • Reveal agenda, biases of their creators
  • GIS databases built from maps
    • Not necessarily objective, scientific
    • measurements
  • Impossible to create perfect representation of world
the necessity of fuzziness
The Necessity of “Fuzziness”
  • “It’s not easy to lie with maps, it’s essential...to present a useful and truthful picture, an accurate map must tell white lies.” -- Mark Monmonier
  • distort 3-D world into 2-D abstraction
  • characterize most important aspects of spatial reality
  • portray abstractions (e.g., gradients, contours) as distinct spatial objects
fuzziness cont
Fuzziness (cont.)
  • All GIS subject to uncertainty
  • What the data tell us about the real world
  • Range of possible “truths”
  • Uncertainty affects results of analysis
  • Confidence limits - “plus or minus”
    • Difficult to determine
  • “If it comes from a computer it must be wright”
slide6
Longley et al.,

1/e ch. 6, p. 132

2/e ch. 9, p. 208

uncertainty
Uncertainty
  • Measurements not perfectly accurate
  • Maps distorted to make them readable
    • Lines repositioned
    • 5th St. and railroad through Corvallis at scale of 1:250,000
    • At this scale both objects thinner than map symbols
  • Map is generalized
  • Definitions vague, ambiguous, subjective
  • Landscape has changed over time
assessing the fuzziness
Assessing the Fuzziness
  • positions assumed accurate
  • really just best guess
  • differentiate best guesses from “truth”
  • “shadow map of certainty”
    • where an estimate is likely to be the most accurate
  • tracking error propagation
spread boundary locations to a specified distance zone of transition cells on line are uncertain
Spread boundary locations to a specified distance:Zone of transition, Cells on line are uncertain
based on distance from boundary code cells with probability of correct classification
Based on distance from boundary, code cells with probability of correct classification
slide19
Same thing for Forest mapLinear Function of increasing probabilityCould also use inverse-distance-squared
original overlay of s2 f5 overlay implied 100 certainty shadow map says differently
Original overlay of S2/F5:Overlay implied 100% certaintyShadow map says differently!
towards an honest gis
Towards an “Honest GIS”
  • can map a simple feature location
  • can also map a continuum of certainty
  • model of the propagation of error (when maps are combined)
  • assessing error on continuous surfaces
    • verify performance of interpolation scheme
more strategies
More Strategies
  • Simulation strategy
    • Complex models
    • Describing uncertainty as “a spatially autoregressive model with parameter rho” not helpful
    • How to get message across
  • Many models out there
    • Recent research on modeling uncertainty (NCGIA Intiative 1)
    • Users can’t understand them all
strategies cont
Strategies (cont.)
  • Producer of data must describe uncertainty
    • “RMSE 7 m” (Lab 6, your Mt. Hood DEM)
    • Metadata
  • FGDC - 5 elements
    • Positional accuracy
    • Attribute accuracy
    • Logical consistency (logical rules? polygons close?)
    • Completeness
    • Lineage
strategies cont26
Strategies (cont.)
  • What impact will uncertainty have on results of analysis??

(1) Ignore the issue completely

(2) Describe uncertainty with measures (shadow map or RMSE)

(3) Simulate equally probable versions of data

simulation example try it yourself at http www ncgia ucsb edu ashton demos propagate html
Simulation Example:Try it yourself athttp://www.ncgia.ucsb.edu/~ashton/demos/propagate.html
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