1. What’s the Point? Working with 0-D Spatial Data in ArcGIS
Advanced GIS Workshop
April 28-29, 2012
Antioch University
New England
Chris Brehme
Keene State College

2. Getting Centered: Measures of Centrality
Central Feature
Most centrally located of existing points
Mean Center or Centroid
Sum the X values and Y values; average of each = X,Y of centroid
Weighted Centroid
Same as centroid, except 1 or more values are weighted
Median Center or Manhattan Median
Evenly divide the points N & S with a line, and evenly divide the points E & W with a line; intersection of lines = median center

3. Dispersion Analysis Standard Distance Standard Deviational Ellipse
Analogous to standard deviation
Represented graphically as circles on a 2-D scatter plot
Standard Deviational Ellipse
Captures Directional Bias
Measures Dispersion along 2 axes of a point distribution
Calculate Home Range of an animal

4. Point Pattern Analysis
Analysis of spatial properties of points rather than a single summary measure (e.g. centroid, etc.)
Two primary approaches:
Quadrat Analysis
Point Density approach
based on observing the frequency distribution or density of points within a set of grid squares.
Nearest Neighbor Analysis
Point Interaction Approach
based on distances of points from one another

5. Quadrat Analysis in ArcGIS
Generate Random Points
Create Fishnet
Spatial Join to Count Points in each Quadrat
Map Point Density

9. Nearest Neighbor Index
Advantages
takes into account distance
No quadrat size problem to be concerned with
Disadvantages
highly dependent on the size and shape of study area
based on only the mean distance
Doesn’t incorporate local variations
Based on point location only and doesn’t incorporate magnitude of phenomena at that point

10. Output of ArcGIS Nearest Neighbor Tool:

11. Other Measures Point Distance
Calculate distance from one set of points to another

12. Other Measures Spatial Autocorrelation Measures:
- Join Count Statistic
- Moran’s I
- Geary’s C ratio
- General (Getis-Ord) G

13. Interpolation: Thiessen Polygons
Points connected with (invisible) lines
Polygon edges are drawn perpendicular to these
Image from GISCommons.org

14. Interpolation: IDW
Image from Paul Bolstad, GIS Fundamentals 3rd Edition, 2010

15. IDW in ArcGIS Z-value field (value to estimate)
Output cell size (the size of the grid cell)
Power (higher values emphasize nearer points)
Search Radius (could lead to No Data values if no points are within the radius)
# of Points (interacts with search radius option)

16. Try it Yourself!
Point Pattern Analysis