Geography Reference
In-Depth Information
Intercept
Value
High :
81.346
Low :
28.921
N
Inland water
0
20
40 km
Figure 8.9
GWR intercept: elevation against July 2006 precipitation
amount in Northern Ireland.
GWR can be conducted at any location, thus a GWR model can be i tted at a location
where there is no observation. In the following example, GWR model parameters
were obtained on a 1-km grid and this makes visualization of spatial variation in
parameters easier than when parameter estimates are made at the locations of obser-
vations only. Figure 8.9 shows the mapped GWR intercept values, Figure 8.10 shows
the slope coei cients, and Figure 8.11 shows the coei cients of determination (
r
2
;
here computed outside of GWR 3.0, as that package does not return standard GW
r
2
values). Note that the number of decimal places is kept at three for all of the following
i gures since the values for the slope and
r
2
are small.
Clearly, there is much variation in the nature and the strength of the relationship
between elevation and precipitation amount in Northern Ireland. Values of the GWR
intercept (Figure 8.9) are largest (and larger than the global model intercept of
52.203 mm) in parts of the south west, parts of the mid east (south and west of Belfast),
and the south east. Where the intercept is large and the slope has a negative or small
positive value, this indicates that, in such areas, precipitation amounts are large (in
proportion to the size of the intercept), irrespective of the elevation. Comparison of
some areas in Figure 8.9 (GWR intercept) with Figure 8.10 (GWR slope) shows that
some areas (e.g. in parts of the south west) fuli l these criteria. h e GWR slope values
(Figure 8.10) are largest in parts of the far west, the Ards Peninsula (south and west of
Belfast), and some other areas, most notably to the south and west of Lough Neagh.
Where the GWR
r
2
(Figure 8.11) is also large, this suggests that elevations and precipi-
tation amounts are strongly related (i.e. a large slope indicates a large increase in
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