MEASURING DIFFERENCES BETWEEN OVERLAPPING BUT UNPAIRED SPATIAL SURVEYS USING A GEOGRAPHIC INFORMATION-SYSTEM

To compare the results from two overlapping but unpaired synoptic surv eys, a method using a geographic information system is proposed. The n ew method uses a GIS to create (i) minimum convex polygons (MCP) enclo sing each set of survey data and (ii) Voronoi tesselations assigning a rea weights to each datum. The pairs of MCP and Voronoi maps are overl aid to produce a mosaic of polygons each with one datum assigned from each survey. The differences between the pairs weighted by the polygon areas provides the basis for statistical testing. Area-weighted means and variances of paired differences are computed and a z-statistic me asures the significance of differences for the whole intersection area . A cross-product autocorrelation statistic provides an assessment of the spatial distribution of differences. Alternate, conventional metho ds are compared with the new method: analysis of variance (ANOVA), ana lysis of covariance (ANCOVA), and contour overlays. The four methods w ere applied to the comparison of two macrobenthic surveys conducted in Lake Erie. Measures for five different taxa were examined. The ANOVA and ANCOVA methods found many significant differences between surveys. Both methods were judged inappropriate as sampling data are not expec ted to be drawn from normally-distributed populations in spatial surve ys. Differences between surveys were detected but were difficult to as sess using contour overlays. The new method only found a significant d ifference in one taxon, an expected difference as members of that taxo n were absent from most sites in one survey.