It is generally assumed that increased map precision (ie map unit homogeneity) and map
purity (map unit accuracy) should result from increasing the scale of mapping of the soil
resource, since it should enable a more intricate breakdown of the landscape into
landform facet based units. This study compared the predictive success of a 1:10K scale
soil association map with the 1:25K and 1:lOOK scale soil landscape maps within the
Birrigai area of the Paddy's river catchment, south west of Canberra, A.C.T. The 1:25K
and the 1:lOOK scale soil landscape maps were also evaluated in a second larger
evaluation area in the Paddy's river catchment which allowed more of the larger soil
landscape map units to be evaluated. The 1:25K scale soil map was produced by
another author for the A.C.T Government, and was surveyed at a substantially lower
survey intensity than that for the 1:lOOK and 1:10K scale soil maps (ie only 0.05
observation sites / cm2 of published map). These maps were evaluated using a set of
randomly located independent evaluation sites in each evaluation area, and from these
calculating and comparing standard Marsman & de Gruijter(1986) measures of Map
Purity.
The strength of soil-landscape relationships within this catchment were determined from
a Fixed One Way Analysis of Variance, and from more simplistic graphical
comparisons of the means and standard deviations of the discrete soil data within these
landform based map units. Soil-landscape relationships for the Nominal scale soil data
(ie class type data) were evaluated by comparing the Marsman & de Gruijter(1986)
Homogeneity index ratings among the soil map units. Intensive survey traverses were
also carried out in selected soil landscapes to further evaluate the strength of soil
landscapes present.
The results revealed obvious improvements in map quality associated with increasing
map scale from 1:100,000 to 1:10,000, and these included increases in the predictive
success (Map Purity), reductions in the extent of map unit impurities, and planning
advantages associated with having individual land facets delineated on the 1:10,000
scale map. The respectable purity ratings achieved by the 1:100,000 scale soil
landscape map (ie average purity rating of 63%) was largely attributed to the flexibility
of the "soil material" approach to soil landscape mapping. The relatively poor
performance of the 1:25K consultancy soil landscape map demonstrated the fact that;
any benefit gained from the improved intricacy in the representation of map unit
delineation's with increased mapping scale, will be drastically reduced if it is not
matched by an associated increase in the intensity of field investigations.
Evaluations of the soil-landscape relationships found that the land facets of the Paddy's
river catchment generally failed to delineate areas that were both uniform and unique in
respect of their soil properties. Soil-landscape relationships were instead found to be
quite complex, applying to only certain land facets, and in regards to only certain soil
properties. Soil maps with units based on landsurface features were recommended on
the basis of the importance of other landscape factors other than soils to land capability
ratings, as well as on the useability of such maps. This study recommended the
adoption of a" >2 detailed soil profile observations / land facet in each map unit"
mapping standard to ensure a reasonable estimate of the variability and modal soil
conditions present, as well as a reliable confirmation of the perceived soil-landscape
relationships. The error usually associated with small scale mapping was effectively
reduced by rapid ground truthing, involving driving along the major roads dissecting the
map area and making brief observations of soil exposures on road batters, despite the
bias of the road network making such mapping improvements uneven across the map.
The major point to come from this study was the re-emphasising of the point that soil
spatial variability has to be accepted as a "real landscape attribute" which needs to be
accurately described and communicated to land users, and must not be considered as
some sort of soil mapping failure. The fact that individual facets of the landscape
rarely coincide with unique pockets of uniform and unique soils and soil properties must
be considered simply an on the ground reality of nature, and not some mapping failure.
It was thought that since other landscape factors (eg hillslope gradient) most often
dominate the determination of land use suitability and capability, it is better to
effectively describe the range and modal state of the soil conditions within such facets,
then to attempt to extrapolate possible soil boundaries using geostatistical techniques
which cut across such land facets, and mayor may not correlate with real groupings of
soil properties, depending on the spatial resolution of the soil variability distribution in
the landscape. Even so the results of this investigation do put the validity of the
physiographic terrain class mapping model as a predictor of soil traits under question, at
least for the more complex landscape settings.
| Identifer | oai:union.ndltd.org:ADTP/234124 |
| Date | January 1997 |
| Creators | Eldridge, Simon Michael, n/a |
| Publisher | University of Canberra. n/a |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Simon Michael Eldridge |
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