ISRIC - World Soil Information (WDC-Soils) has a mission to serve the international community as custodian of global soil data and information, and to increase awareness and understanding of soils in major global issues. With partners we have implemented a server database based on PostgreSQL, known as WoSIS (World Soil Information Service). The aims are to safeguard and subsequently share soil data upon their standardization and harmonization. The data come from disparate sources; conditions for use are stored in WoSIS together with the full data lineage to ensure that data providers are duly acknowledged. In accord with these conditions (licenses), the submitted data is gradually standardized and harmonized, ultimately to make them “comparable as if assessed by a single given (reference) method.” So far, we have focused on point (profile) data, limiting ourselves to the selection of soil analytical and physical properties listed in the GlobalSoilMap (2013) specifications. A wider range of quality-assessed soil data (point, polygon, and grid) will be considered in the future. The present complement of standardized soil profile data is freely accessible at: http://www.isric.org/content/wosis-distribution-set.
Worldwide, soil data are sampled, analyzed and mapped according to a wide range of methods and standards. This paper describes procedures for safeguarding, standardizing resp. harmonizing, and subsequently serving of quality-controlled world soil data (point, polygon, grid) to the international community as developed at ISRIC in the framework of the Data/WoSIS project (Ribeiro et al. 2015).
So far we have focused on processing legacy profile (point) data, limiting ourselves to the soil properties considered in the GlobalSoilMap specifications (GlobalSoilMap 2013). These are: organic carbon, soil pH, soil texture (sand, silt, and clay), coarse fragments (< 2mm), cation exchange capacity, electrical conductivity, bulk density, and water holding capacity. The above properties are commonly considered in digital soil mapping activities aimed at achieving global grid coverage at a high resolution (Arrouays et al. 2014; Hengl et al. 2015).
Soil profile data for consideration in WoSIS were contributed by a wide range of providers. They come in various formats and have been collated following diverse national or international standards as described in the metadata; not all these data may be shared freely. Access rights (licenses), as defined by each data provider, ultimately determine which subsets may freely be served to the international community upon their standardization and harmonization. Therefore, when processing the wealth of contributed data, priority has been given to those datasets having a non-restrictive license (Creative Commons 2016), hereafter referred to as ‘shared data’.
Harmonization, as defined by the Global Soil Partnership (GSP, Baritz et al. 2014), involves ‘providing mechanisms for the collation, analysis and exchange of consistent and comparable global soil data and information’. Areas of harmonization include those related to: a) soil description, classification and mapping, b) soil analyses, c) exchange of digital soil data, and d) interpretations. Such a harmonization is as a prerequisite for the development/ testing of a soil information model that can underpin global soil data interoperability and modelling (Omuto et al. 2012), and requires concerted international collaboration. As indicated, seen the breadth and magnitude of the task, in WoSIS we are presently limiting ourselves to standardizing soil analytical method descriptions for the soil properties considered in the GlobalSoilMap (2013) specifications.
Main steps towards the standardization and harmonization of legacy soil profile data, as developed for WoSIS, are depicted in Figure 1. Upon a broad screening for possible inconsistencies, all ‘shared’ data are imported ‘as is’ into a PostgreSQL database together with the original naming and coding conventions, abbreviations, domains, lineage and data license. Thereby copies of the source materials are safeguarded at ISRIC in the WDC-Soils repository.
Figure 1: Generalized workflow for processing data in WoSIS
Next the source databases are imported into WoSIS, forming the first major step of data standardization (into a single data model). The second step of data standardization, applied to the values for the various soil properties as well as to the naming conventions (e.g. property name or lab method name), is needed to make the data queryable and useable. According to Soil Survey Staff (2011), ‘a soil property is best described by key elements of the (laboratory) procedure applied.’ Similarly, in the WoSIS approach, major characteristics of commonly used methods for determining a given soil property are identified first (Ribeiro et al. 2015). For soil pH, for example, these are the extractant solution (water or salt solution), and in case of salt solutions the salt concentration (molarity), as well as the soil/solution ratio; a further descriptive element is the type of instrument used for the actual laboratory measurement. By using this approach, soil data that have been analyzed using similar analytical procedures may be grouped together. For example, only pH KCl samples determined as if analyzed according to the ISO 10390:2005 standard, namely: solution (KCl), concentration (1M), ratio (1:5) and instrument (glass electrode).
A desired third and final step in the standardization/harmonization process, not yet undertaken in this version of WoSIS, will require data harmonization to make the data comparable that is as if assessed by a single given (reference) method. According to GlobalSoilMap (2013) there is generally no universal equation for converting data from one method to another in all situations. Developing soil type specific equations will require inter-laboratory method comparisons using reference samples (Jankauskas et al, 2006), for example in the framework of the Global Soil Partnership (Baritz et al. 2014).
Status and future developments
The WoSIS server database currently holds data for some 100,000 unique profiles of which some 80,000 are georeferenced within defined limits (corresponding with over 30 million soil records). The number of measured data for each property varies between proﬁles and with depth, generally depending on the purpose of the initial studies. Further, the data lineage strongly determined which data could be standardized with acceptable confidence (as flagged in the database). The resulting data is freely available via WFS (web feature service) at http://www.isric.org/content/wosis-distribution-set; other channels and formats for serving the data are under development at ISRIC.
With a view to achieving global interoperability, WoSIS procedures are developed to align with the frameworks and standards of collaborative international activities such as the Global Soil Partnership, GlobalSoilMap, and Global Earth Observation System of Systems (GEOSS 2012), while ensuring compliance with INSPIRE (2015). An important development has been the consideration of WoSIS in the OGC soil interoperability experiment aimed at developing a markup language for soil data (soilML). WoSIS is an important component of ISRIC’s evolving global spatial data infrastructure (SDI).
Instrumental to enhanced usability and accessibility of data managed in WoSIS will be the continued harmonization of soil property names and values as well as the further standardization of soil analytical method descriptions. Development and testing of such procedures/interfaces will allow for the fulfilment of future demands for global soil information, and enable further incorporation and exchange of soil data shared by third parties through a federated, interoperable system. For example, WoSIS could serve as a central node in the global soil SDI foreseen by the Global Soil Partnership.
WoSIS-related activities are already catalyzing institutional collaboration with institutes in Africa and Latin America. Capacity building and collaboration with (inter)national soil institutes around the world will be essential to create and share ownership of the newly derived soil information as well as the necessary expertise and capacity to further develop and test the system worldwide.
We gratefully acknowledge the contributions and shared knowledge of a steadily growing number of data providers and experts; for details please see http://www.isric.org/content/wosis-cooperating-institutions-and-experts.
The authors declare that they have no competing interests.
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