Analytical Capabilities - GIS and Mapping - Lecture Notes, Study notes of Geology

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Spatial Analysis and Modeling

Resources

Analysis Components

• analyses may be applied to (use as input):
  • tabular attribute data
  • spatial data/layers
  • combination of spatial and tabular
• results may be displayed as (produce as output):
  • table subsets, table combinations, highlighted records (rows), new variables (columns)
  • charts
  • maps/map features: » highlights on existing themes » new themes/layers
  • combination

Advanced and Specialized Applications:

in comparison to basic applications

Most ‘ basic’ analyses are used to create descriptive models of the world, that is, representations of reality as it exists. Most ‘ advanced’ analyses involve creating a new conceptual output layer, or in some cases table(s) or chart(s), the values of which are some transformation of the values in the descriptive input layer. e.g. slope or aspect layer Most ‘ specialized ’ applications involve using GIS capabilities to create a predictive model of a real world process, that is, a model capable of reproducing processes and/or making predictions or projections as to how the world might appear. e.g. fire spread model, traffic projections

Analysis Options: Basic

(Table of Contents)

• Spatial Operations Vector
  • centroid determination
  • spatial measurement
  • buffer analysis
  • spatial aggregation » redistricting » regionalization » classification
  • Spatial overlays and joins Raster
  • neighborhood analysis/spatial filtering
  • Raster modeling

• Attribute Operations

• record selection » tabular via SQL » ‘information clicking’ with cursor
• variable recoding
• record aggregation
• general statistical

analysis

• table relates and joins geostatistics

Spatial operations:

Centroid or Mean Center

• balancing point for a spatial distribution
  • point representation for a polygon--analogous to the mean
  • single point summary for a distribution (point or polygon)
  • can be weighted by ‘magnitude’ at each point (analogous to weighted mean)
  • minimizes squared distances to other points, thus ‘distant’ points have bigger influence than close points ( Oregon births more impact than Kansas births!)
  • is not the point of “minimum aggregate travel”--this would minimize distances (not their square) and can only be identified by approximation.
• useful for
  • summarizing change over time in a distribution (e.g US pop. centroid every 10 years)
  • placing labels for polygons
• for weird-shaped polygons, centroid may not lie within polygon centroid outside polygon X X n Y Y n i i n i i n = = = = ∑ ∑ 1 1 , Note: many ArcView applications calculate only a “psuedo” centroid: the coordinates of the bounding box (the extent) of the polygon

Spatial measurements:

• distance measures
  • between points
  • from point or raster to polygon or zone boundary
  • between polygon centroids
• polygon area
• polygon perimeter
• polygon shape
• volume calculation
  • e.g. for earth moving, reservoirs
• direction determination
  • e.g. for smoke plumes

Spatial operations:

Spatial Measurement

Comments:

• possible distance metrics:
  • straight line/airline
  • city block/manhattan metric
  • distance thru network
  • time/friction thru network
• shape often measured by:
• Projection affects values!!! perimeter area x 3. = 1.0 for circle = 1.13 for square = large number for irregular polygon ArcGIS 8.1 geodatabases contain automatic variables: shape.length: line length or polygon perimeter shape.area: polygon area (Much easier than AV 3.2—see appendix!) Perimeter to area ratio differs

Spatial Operations: buffer zones

• region within ‘x’ distance units
• buffer any object: point, line or polygon
• use multiple buffers at progressively greater distances to show gradation
• may define a ‘friction’ or ‘cost’ layer so that spread is not linear with distance
• Implement in Arcview 3.2 with Theme/Create buffers in ArcGIS 8 with Tools/Buffer Wizard

Examples

• 200 foot buffer around property where zoning change requested
• 100 ft buffer from stream center line limiting development
• 3 mile zone beyond city boundary showing ETJ (extra territorial jurisdiction)
• use to define (or exclude) areas as options (e.g for retail site) or for further analysis
• in conjunction with ‘friction layer’, simulate spread of fire polygon buffer line buffer point buffers Note: only one layer is involved, but the buffer can be output as a new layer