Explore space!
Exploratory Spatial Data Analysis
ESDA
Exploratory
Focus on discovery and assumption-free investigation
Spatial
Patterns and processes that put space and geography at the core
Data Analysis
Statistical techniques for exploration
Questions that ESDA helps…
Answer
Is the variable I’m looking at concentrated over space?
Do similar values tend to be located close by?
Can I identify any particular areas where certain values are clustered?
Ask
What is behind this pattern?
What could be generating the process?
Why do we observe certain clusters over space?
Spatial Autocorrelation
Everything is related to everything else, but near things are more related than distant things.
Waldo Tobler (1970)
Spatial Autocorrelation
Statistical representation of Tobler’s law
Spatial counterpart of traditional correlation
Degree to which similar values are located in similar locations
Two flavours
Positive: similar values \(\rightarrow\) similar location (closeby)
Negative: similar values \(\rightarrow\) disimilar location (further apart)
Two scales
Global: do values tend to be close to other (dis)similar values?
Local: are there any specific parts of a map with an extraordinary concentration of (dis)similar values?
Global Spatial Autocorrelation
“Clustering”
Overall trend where the distribution of values follows a particular pattern over space.
Positive
Similar values close to each other (high-high, low-low)
Negative
Similar values far from each other (high-low)
How to measure it?
Moran Plot
Moran’s I
Formal test of global spatial autocorrelation
Statistically identify the presence of clustering in a variable
Slope of the Moran plot
Local Spatial Autocorrelation
“Clusters”
Pockets of spatial instability
Portions of a map where values are correlated in a particularly strong and specific way
Clusters
High-High
Positive SA of high values (hotspots)
Low-Low
Positive SA of low values (coldspots)
High-Low
Negative SA (spatial outliers)
Low-High
Negative SA (spatial outliers)
LISAs
Local Indicators of Spatial Association
Statistical tests for spatial cluster detection
Compares the observed map with many randomly generated ones to see how likely it is to obtain the observed associations for each location
import esda
