Spatial statistics
Lecture 3
What are spatial statistics
•
•
•
•
Not like traditional, a-spatial or non-spatial statistics
But specific methods that use distance, space, and
spatial relationships as part of the math for their
computations
It is a spatial distribution and pattern analysis tool
– Identifying characteristics of a distribution; tools
used to answer questions like where is the
center, or how are feature distributed around the
center? (Measuring Geographic Distributions)
– Quantifying or describing spatial pattern; are our
features random, clustered, or evenly dispersed
across our study area? (Analyzing Patterns and
mapping clusters)
– Mainly deal with point, line, polygon (vector)
Why use spatial statistics?
– To help assess patterns, trends, and
relationships
• Better understanding of geographic
phenomena
• Pinpoint causes of specific geographic
patterns
• Make decision with high level of confidence
• Summarize the distribution in a single
number
1. Measuring geographic (spatial)
distribution
– Not only crime analysts but
also GIS practitioners in
many research areas, such
as epidemiology,
archaeology, wildlife biology,
and retail analysis, will
benefit from the spatial
statistics tools in ArcGIS 9.
These tools can be easily
modified or extended
because most were written
using the Python scripting
language. The source code
for the statistical tools can be
accessed from ArcToolbox
and serve as samples and
templates for further
customization
Mean center of
population
distribution and
pattern,
Track changes
in the
distribution
Average of x, y
coordinates
Median center
• Identifies the location that minimizes overall Euclidean
distance to the features in a dataset
• While the Mean_Center tool returns a point at the
average X and average Y coordinate for all feature
centroids, the median center uses an iterative algorithm
to find the point that minimizes Euclidean distance to all
features in the dataset.
• Both the Mean_Center and Median Center are measures
of central tendency. The algorithm for the Median Center
tool is less influenced by data outliers.
Distances from each feature centroid to every other feature centroid in the dataset
are calculated and summed. Then the feature associated with the shortest
accumulative distance to all other features (weighted if a weight is specified) is
selected and copied to a newly created output feature class
Central feature
Mean center
How feature disperse around center
Mean center and central feature tools tell about the center of a distribution
But do not tell the overall distribution.
Following tools tell how dispersed our features are around that center
• Standard distance
• Directional distribution (standard
deviational ellipse)
• Linear directional mean
Showing those
locations are
within one
standard
deviation of the
central feature
Showing those
locations are
within one
standard
deviational
ellipse of the
central feature,
in a north-west
to south-east
direction
• The trend of a set of line features is measured by calculating the average
angle of the lines. The statistic used to calculate the trend is known as the
directional mean. While the statistic itself is termed the "directional mean",
it is used to measure either direction (such as hurricanes) or orientation
(faults).
Python Script
2. Analyzing spatial patterns
• Give us ways to measure the degree to which our features
are clustered, dispersed, or randomly distributed across
the study area
• 2.1 Analyzing Patterns
– Global calculations
– Identifies the patterns/overall trends of data
• Are features clustered and what is the overall pattern?
– Spatial Autocorrelation tool
• 2.2 Mapping Cluster
– Local calculations
– Identifies the extent and location of clustering or dispersion
• Where are the clusters (or where are the hot spots)?
– Hot Spot Analysis tool
2.1 Analyzing patterns
•
•
•
•
Average nearest neighbor
High/low clustering
Multi-distance spatial cluster analysis
Spatial autocorrelation
• The Average Nearest Neighbor tool returns
five values: Observed Mean Distance,
Expected Mean Distance, Nearest Neighbor
Index, z-score, and p-value
Nearest neighbor index, >1 (dispersion)
<1 (clustering)
Very sensitive
to the area
How Spatial Autocorrelation: Moran's I (Spatial Statistics) works
• This tool measures spatial autocorrelation (feature similarity) based on
both feature locations and feature values simultaneously. Given a set of
features and an associated attribute, it evaluates whether the pattern
expressed is clustered, dispersed, or random. The tool calculates the
Moran's I Index value and both a Z score and p-value evaluating the
significance of that index. In general, a Moran's Index value near +1.0
indicates clustering while an index value near -1.0 indicates dispersion.
However, without looking at statistical significance you have no basis for
knowing if the observed pattern is just one of many, many possible
versions of random.
• In the case of the Spatial Autocorrelation tool, the null hypothesis states
that "there is no spatial clustering of the values associated with the
geographic features in the study area". When the p-value is small and the
absolute value of the Z score is large enough that it falls outside of the
desired confidence level, the null hypothsis can be rejected. If the index
value is greater than 0, the set of features exhibits a clustered pattern. If
the value is less than 0, the set of features exhibits a dispersed pattern.
•
Z score is a measure of standard
deviation. If you have σ is (-1.96,
1.96), z score is falling between
them, you are seeing a pattern of
random pattern. If z score falls
outside, like -2.5 or 5.4, then you
have a pattern that’s too unusual
to be a pattern of random
chance
2.2 Mapping cluster
• Cluster and outlier analysis
• Hot spot analysis
Example for park-served
population (congestion)
Red:
Gi_Z-score (>1.96), with p<0.05
Red: Moran’s I_Z-score (>1.96), with p<0.05
Blue: Moran’s I_Z-score (<-1.96), with p<0.05
Source: Yunbo Bi’s Master’s thesis, 2012
references
• “understanding Spatial Statistics in ArcGIS 9” by
Sandi Schaefer and Lauren Scott.
• ArcGIS desktop help