Soil and Landscape Grid National Soil Attribute Maps - Rock outcrop occurrence (3" resolution) - Release 1 

The map gives a modelled estimate (probability) of the spatial distribution of rock outcroppings across Australia.

This product was produced in the development of the updated soil thickness map of Australia, details of which are published in Malone and Searle (2020; https://doi.org/10.1016/j.geoderma.2020.114579). This product is the output from Model 1 of aforementioned paper and uses the Rock Properties database provided by Geoscience Australia which gives the locations of sampled rock outcrops across Australia (http://www.ga.gov.au/scientific-topics/disciplines/geophysics/rock-properties). Filtering this dataset resulted in 14616 rock outcrop locations within areas where relief >300 m. A machine learning model was used to find relationships between observed data and associated environmental covariate data to inform the mapping of rock outcrop occurrence across Australia.

Detailed information about the Soil and Landscape Grid of Australia can be found at - SLGA

• Attribute Definition: Probability of rock outcrops;
• Units: 0-1;
• Period (temporal coverage; approximately): 1950-2021;
• Spatial resolution: 3 arc seconds (approx 90m);
• Total number of gridded maps for this attribute: 1;
• Number of pixels with coverage per layer: 2007M (49200 * 40800);
• Data license : Creative Commons Attribution 4.0 (CC BY);
• Target data standard: GlobalSoilMap specifications;
• Format: Cloud Optimised GeoTIFF;

 

We at TERN acknowledge the Traditional Owners and Custodians throughout Australia, New Zealand and all nations. We honour their profound connections to land, water, biodiversity and culture and pay our respects to their Elders past, present and emerging. This work was jointly funded by CSIRO, Terrestrial Ecosystem Research Network (TERN) and the Australian Government through the National Collaborative Research Infrastructure Strategy (NCRIS).
We are grateful to the custodians of the soil site data in each state and territory for providing access to the soil site data, and all of the organisations listed as collaborating agencies for their significant contributions to the project and its outcomes. 

The map gives a modelled estimate (probability) of the spatial distribution of rock outcroppings across Australia.
The aim is to operate an open national capability that provides access to verified, science-quality land surface dynamics data and soils information layers, plus high-end data analytics tools that integrated with other TERN observations can meet the needs of ecosystem researchers and actionable information for policy makers and natural resource managers. 

The modelling and mapping of rock outcrop occurrence was performed as part of efforts to update and improve modelling of soil thickness across the Australia. Following is the description of method and further details of this work.

Rather than fitting a single model of soil thicknesses we went for a nuanced approach which entailed three separate models for:

Model 1. Predicting the occurrence of rock outcrops.
Model 2. Predicting the thickness of soils within the 0-2 m range
Model 3. Predicting the occurrence of deep soils (soils greater than 2 m thick).
Models 1 and 3 used the categorical model variant of the Ranger RF which was preceded by distinguishing; for Model 1, the observations that were deemed as rock outcrops from soils. And for Model 3, distinguishing soils that were less than 2 m thick (and not rock outcrops) from soils greater than 2 m thick. Ultimately both Models 1 and 3 were binary categorical models. 50 repeats of 5-fold CV (cross-validation) iterations of the Ranger RF model were run for each Model variant.

Model 2 used the regression form of the random forest model. After removing from the total data set the observations that were regarded as rock outcrops and soil greater than 2 m, there were 111,302 observations available. Of these, 67,698 had explicitly defined soil thickness values. The remaining 43,604 were right-censored data and were treated as follows. For each repeated 5-fold iteration, prior to splitting the data in calibration and validation datasets, values from a beta function were drawn at random of length 43,604. This value (between 0 and 1) was multiplied by the censored value soil thickness and then added to this same value, creating a simulated pseudo-soil thickness. Once the simulated data were combined with actual soil thickness data, the values were square-root transformed to approximate a normal distribution. Ranger RF modelling proceeded after optimising the Hyperparameter settings as described above for the categorical modelling. Like the categorical modelling, 50 repeated 5-fold CV iterations were computed.

All three model approaches were integrated via a simple ‘if-then’ pixel-based procedure. At each pixel, if Model 1 indicated the presence of rock outcrops 45 times or more out of 50 (90% of resampling iterations), the estimated soil thickness was estimated as rock outcrop, or effectively 0 cm. Similarly, for Model 3 which was the model based on prediction of deep soils (soils >2 m deep). In no situations did we encounter both Models 1 and 3 predict in the positive on 90% or more occasions simultaneously. If Model 1 or 3 did not predict in the positive in 90% of iterations, the prediction outputs of Model 2 were used.

After model integration, we derived a set of soil thickness exceedance probability mapping outputs. These were derived simply by assessing the empirical probabilities (at each pixel) and then tallying the number of occasions the estimated soil depth exceeded given threshold depths of 10 cm, 50 cm, 100 cm, and 150 cm. This tallied number was divided by 50 to give an exceedance probability for each threshold depth.

All processing for the generation of these products was undertaken using the R programming language (R Core Team, 2020).

• Code - https://github.com/AusSoilsDSM/SLGA
• Observation data - https://esoil.io/TERNLandscapes/Public/Pages/SoilDataFederator/SoilDataFederator.html
• Covariate rasters - https://esoil.io/TERNLandscapes/Public/Pages/SLGA/GetData-COGSDataStore.html