GIS can do anything sadly and often badly

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GIS can do anything (sadly and often badly)
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Data quality and errors (and uncertainty) in GIS

• GIS data are capable of more precision with
  digital methods but
• Errors occur in GIS data as often or more
  frequently than in traditional map products, as
  data may be generated from maps and thus can only
  add more error.
• Types of error spatial or attributes
• Sources of error instruments, human, change
• The 'errors' that can occur during the four
  components of GIS
• a. Input b. Database management    c.
  Analysis   d. Output
• Some see stages of error between converting
  reality to GIS display
•   Real world -gt our conception of it -gt
  measurement -gt analysis -gt display

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1. Types of Errors (spatial or attribute)
a. Positional accuracy The hardcopy rule is to
be within the width of a line 0.5mm. e.g. at a
scale of 150,000, error should be no more than
25 metres on the ground at 1250,000 error must
be lt125 metres. Positional accuracy can be
measured in Root Mean Square Error (RMS)  a
measure of the distance between the true and
estimated location RMS is the square root of the
sum of the squared errors in x and y b.
Attribute accuracy Classification and measurement
may be rated in terms of correct, with a
standard of e.g. 80. It may not be 100 due to
the uncertain nature of some features such as
vegetation boundaries (unless man-modified).  On
a map and database, items such as forest types
are grouped and placed within a boundary, which
may be uncertain. UNBC example For both spatial
and attribute data, digital systems enable high
precision (decimal places), but no guarantee of
more accuracy.
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2. Sources / Causes errors before GIS
a. Instrument inaccuracies satellite / GPS /
surveying    (spatial) similarly for (attribute)
measuring instruments b. Human
processing mis-interpretation (e.g. photos)
spatial and attribute effects of scale change and
generalization effects of classification -
nominal (categorical) /  ordinal /
interval    different groups and jurisdictions
using variable classifications c. Actual
changes (out of date) Gradual 'natural'
changes river courses, glacier recession
Catastrophic change fires, floods, landslides
Seasonal and daily changes lake / sea / river
levels Man-made urban development, new roads,
cut blocks Attribute change forest growth
(height etc.), road surfacing ..need for
documentation of date and collection methods in
metadata
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TRIM data and orthophoto near Trutch
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3. GIS processing errors
a. Input Digitizing human error and the width
of a line Dangling nodes (connected to only one
arc) permissible in arc themes Pseudo-nodes
(connected to one or two arcs) - permissible in
island arcs, and where attributes change, e.g.
road becomes paved from dirt or vice versa.
Topology is needed for GIS analysis Topology
the spatial relationships between geographic
features. It should not to be confused with
topography, the form of the land.
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Digitizing errors
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b. Database management

• Data precision too little, too much
• Missing values
• Metadata how / when were data gathered etc..
• Units of measurement

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c. Data analysis
Interpolation of point data into lines /
surfaces e.g. TIN / contours. Overlay of
layers, digitized separately from different
sources or scales, e.g. soils and vegetation.
They have common borders, but slight differences
cause 'slivers'. The compounding effects of
processing and analysis of multiple layers for
example, if two layers each have correctness of
90, the accuracy of the resulting overlay is
around 81. Inappropriate or inadequate inputs
for models local example Dubious
classifications example
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Inappropriate analysis
Trail gradients
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Yikes those class boundaries
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More garbage classes
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d. Data output

• colour palettes colours don't match from screen
  to Printer
• Scale changes - level of detail (generalisation)
• Beware of using software design defaults ....
• Scale bars- round numbers, logically subdivided,
  in logical units .. Beware of scale
  statements
• Legend items and design   e.g. Zorro lines  
  (see below)

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Yikes scale bar, zorro legend, north arrow ?
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Review

• Things you should consider after finishing this
  lecture
• Computer data have as many or more errors than
  printed maps
• The difference between accuracy and precision
• The effects of scale and generalization
• Lack of documentation - the need for metadata
• Age and date of GIS data (relative to rate of
  change)
• Effect of area jurisdictions - e.g provincial and
  federal differences
• The challenge of a large province and country
• Summary (primer) http//gis.nic.in/gisprimer/er
  rors.html
• Geographers Craft http//www.colorado.edu/geogra
  phy/gcraft/notes/error/error_f.html

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