Effects of spatial resolution: worked example
This example illustrates how results of an assessment can vary with spatial resolution of input data.
The exposure data are taken from an assessment of the potential health impact of pesticides, in the East Anglia region of Great Britain, in which ambient pesticide exposures were modelled for a 250x250m grid. The effect of spatial scale on the final estimate of health impact is illustrated.
GIS was used to area-weight the modelled exposures so they could be mapped at two additional scales: wards and district level. For comparison, a common classification (Table 1) was applied to the different exposure maps. The total population within each class was computed, from postcode headcount data, and exposure profiles derived (Figure 1). Figure 2 shows the mapped exposures and population densities for each spatial scale.
| Class | Exposure (µg/m3) |
| 1 | 0 - 0.002 |
| 2 | >= 0.002 - 0.006 |
| 3 | >= 0.006 - 0.018 |
| 4 | >= 0.018 - 0.054 |
| 5 | >= 0.054 - 0.162 |
| 6 | >= 0.162 - 0.486 |
| 7 | >= 0.486 - 1.45 |
| 8 | >= 1.45 |
Table 1. Exposure classification
Figure 1. Pesticide exposure and population density at different spatial scales
Figure 2. Pesticide exposure distribution at 250m, ward and district level
On the basis of the estimated exposures, the expected number of cases attributable to pesticide exposure for a hypothetical disease was calculated for the whole of East Anglia. The base prevalence for the hypothetical disease was assumed to be 15 per 100,000. The following formula for attributable burden was used:
AB = (RR – 1 / RR) * P * F
Where: RR is the relative risk, P the base prevalence for the disease, and F the fraction of the exposed population.
Two exposure scenarios are presented:
- The first scenario is for a binary exposure, in which classes 1+2 (< 0.006 µg/m3) are the non-exposed group and classes 3 and above (>= 0.006 µg/m3) represent the exposed. The RR for the exposed group is 1.09.
- The second scenario includes the same non-exposed group, plus three exposure categories based on concentration.
- Low exposure includes classes 3+4 (>= 0.006 - 0.054 µg/m3) with a RR of 1.09
- Medium exposure includes classes 5+6 (>= 0.054 - 0.486 µg/m3) with a RR of 1.12
- High exposure is classes 7+8 (>= 0.486 µg/m3) with a RR of 1.17.
The results for these two scenarios for the different spatial scales are presented in Figure 3.
Figure 3. Expected cases of hypothetical disease due to pesticide exposure calculated at three spatial scales
print version