Geog 458: Map Sources and Errors

1
Geog 458Map Sources and Errors

• January 20, 2006
• Data Storage and Editing

2
Outlines

• Importance of data editing
• Detecting and editing different types of errors
• Entity errors
• Attribute errors
• Combining data of different sources
• Transformation
• Rubber-sheeting
• Metadata content standards

3
1. Importance of data editing

• Data input necessarily involves diverse errors
• Primary data capture positional accuracy of GPS
  receivers, geometric distortion of air photo,
  malfunction of electronic scanners
• Secondary data capture a map in itself the model
  of reality digitizing with no proper use of
  editing tool (e.g. snapping), vectorization
  without postprocessing, georeferencing with
  imprecise control points
• Data transfer information loss caused by (1)
  file conversion (2) lack of communication (no
  metadata) (3) ignorance of GIS users
• Data editing can determine whether to cut off
  chains of imperfection (from data input, to
  analysis, and decision-making)
• Data editing is the process for detecting and
  eliminating errors inherent in data input, and
  avoiding error-prone analysis that may lead to
  wrongfully informed decision

4
2. Detecting errors in a single dataset

• Reading (DeMers chapter 6) distinguishes between
  entity (spatial) and attribute (non-spatial)
  errors
• Entity errors
• Node errors dangling nodes, pseudo nodes
• Polygon errors sliver, incorrect label point
• Attribute errors
• Incomplete values
• Incorrect values

5
Node

• What is node?
• A node is an endpoint of an arc. The from-node is
  the first vertex in the arc the to-node is the
  last vertex. Arc-node relation defines
  connectivity
• Node types
• Normal
• Dangling
• Pseudo

6
Node errors

• Dangling and pseudo nodes often identify
  automation errors however they can also be valid
  components of a feature
• Dangling node, if not legitimate (e.g.
  cul-de-sac) may indicate errors
• Pseudo node, if not legitimate (e.g. island,
  attribute change) may indicate errors
• How do I check node errors?
• GIS provides functionalities for labeling
  potential node errors
• But most of error detection requires visual
  inspection
• How do I fix node errors?
• Interactive editing in a proper snapping
  environment Editor toolbar
• Automated data cleaning with a reasonable
  tolerance level Topology toolbar

7
Displaying potential node errors

• Dangling nodes

• Pseudo nodes

8
Node errors

• Why do I have to fix node errors?
• Because it ensures correct topology that forms
  the basis for further analysis
• It validates connectivity
• If the data is used for network operations such
  as hydrological model or routing
• It validates contiguity
• If the data is used for spatial overlay such as
  point-in-polygon, intersection between line and
  polygon

9
Polygon errors

• Most common polygon errors
• Missing or multiple label points in arc/info
  coverage
• Sliver polygon
• How do I identify polygon errors?
• Label error labelerrors in arc/info workstation
• Sliver polygon select areas where its area is
  less than reasonably small value
• How do I fix polygon errors?
• Label error createlabels in arc/info workstation
• Sliver polygon merge silver polygon to
  neighboring polygon (use eliminate in arc/info
  workstation)

10
Attribute errors

• Missing attribute values
• Compare the unique list of values to master list
• Check -9999 (use query builder or selection tool)
• Incorrect attribute values
• Typing errors spelling check
• Domain check compare the list of values to a
  range of possible values
• Land use code should be within a certain range
• Longitude in decimal degree should be minus for
  Seattle
• Some values are determined by other attributes
• Zip code and State
• County name and FIPS code
• Validate values using spatial overlay
• Seattle-based company should be located in
  Northwestern US

11
Checking topology errors in geodatabase

• Define topology rules
• Make an edit that participates in topology using
  topology editor toolbar
• Then you can validate the entire topology
• Error inspector will list which edit violates
  rules
• Based on output from error inspector, topology
  errors can be corrected or made as an exception
  if errors are not errors indeed (e.g.
  cul-de-sacs, attribute change in the road)

12
3. Combining data of different sources

• When you work on data of different sources and
  they are not well aligned spatially even after
  projection change, it is necessary to make
  spatial adjustments of a layer to the layer with
  a higher accuracy
• Two kinds of spatial adjustments transformation
  and rubber-sheeting
• Transformation converts data from one coordinate
  system to another can be used to shift your data
  within a coordinate system (shift, rotate,
  enlargement and so on)
• Rubber-sheeting is usually used to correct for
  geometric distortions they may be introduced by
  imperfect registration in map compilation, lack
  of geodetic control in source data, and so on.
• Spatial adjustment allows you to improve data
  quality

13
Transformation
Source control point
Destination control point
? Displacement links
14
Rubber-sheeting

• Rubber-sheeting adjusts source layer (dashed
  line) to target layer (solid line) based on
  displacement links
• Source layer is adjusted to target layer by
  setting snapping properties

Edge-matching
15
4. Metadata Content StandardSection 2 Data
Quality
16
Data Quality Components

• Positional accuracy how much is position on data
  in hand deviated from data with higher accuracy
  (need well-defined point, geodetic control, road
  intersection or data from survey)?
• Attribute accuracy how much is attribute on data
  in hand deviated from data with higher accuracy?
• Logical consistency is data topologically clean?
  
• Completeness are geographic features exhaustive
  within study area? Does attribute contain all of
  information needed?

17
Questions

• In the readings, common error types are
  identified as follows. Identify data quality
  components P, A, L, C for each.
• Is there any missing geographic features in data
  entered?
• Are all entities within registration marks?
• Do all polygons have a label point?
• Do all entities have the right location?
• Do all entities have the right shape and size?
• Are all attributes values entered?
• Are nodes connected to arcs?
• Does polygon has any sliver?