Abstract
Accurate, consistent and timely cropland information over large areas is critical to solve food security issues. To predict and respond to food insecurity, global cropland products are readily available from coarse and medium spatial resolution earth observation data. However, while the use of satellite imagery has great potential to identify cropland areas and their specific types, the full potential of this imagery has yet to be realized due to variability of croplands in different regions. Despite recent calls for statistically robust and transparent accuracy assessment, more attention regarding the accuracy assessment of large area cropland maps is still needed. To conduct a valid assessment of cropland maps, different strategies, issues and constraints need to be addressed depending upon various conditions present in each continent. This study specifically focused on dealing with some specific issues encountered when assessing the cropland extent of North America (confined to the United States), Africa and Australia. The process of accuracy assessment was performed using a simple random sampling design employed within defined strata (i.e., Agro-Ecological Zones (AEZ’s) for the US and Africa and a buffer zone approach around the cropland areas of Australia. Continent-specific sample analysis was performed to ensure that an appropriate reference data set was used to generate a valid error matrix indicative of the actual cropland proportion. Each accuracy assessment was performed within the homogenous regions (i.e., strata) of different continents using different sources of reference data to produce rigorous and valid accuracy results. The results indicate that continent-specific modified assessments performed for the three selected continents demonstrate that the accuracy assessment can be easily accomplished for a large area such as the US that has extensive availability of reference data while more modifications were needed in the sampling design for other continents that had little to no reference data and other constraints. Each continent provided its own unique challenges and opportunities. Therefore, this paper describes a tale of these three continents providing recommendations to adapt accuracy assessment strategies and methodologies for validating global cropland extent maps.
Highlights
Accurate and consistent cropland information is crucial to answer the issues of global food security to make future policy, investment and logistical decisions [1,2,3,4,5,6,7]
The results indicate that continent-specific modified assessments performed for the three selected continents demonstrate that the accuracy assessment can be accomplished for a large area such as the United States (US) that has extensive availability of reference data while more modifications were needed in the sampling design for other continents that had little to no reference data and other constraints
The results of the accuracy assessment process that was performed including specific issues and constraints observed in the cropland maps are presented in the following three major components for each of the three selected continents: (1) Stratification: dividing each continent into homogeneous regions; (2) Collecting reference data and sampling design: developing continent specific procedures for effective and valid collection of reference data using an appropriate sampling design; and (3)
Summary
Accurate and consistent cropland information is crucial to answer the issues of global food security to make future policy, investment and logistical decisions [1,2,3,4,5,6,7]. Remote sensing has been recognized as an extremely effective, economical and feasible approach to create cropland thematic maps over a range of spatial and temporal scales [11,12,13]. Cropland maps such as Global Map of Irrigation Areas (GMIA), Global Map of Rain-fed Areas (GMRCA) [14], Global Monthly Irrigated and Rain-fed Crop Areas (MIRCA2000) [15], Global Rain-fed, Irrigated and Paddy Croplands (GRIPC) [16] and MODIS-Cropland [17] derived from coarse-resolution satellite data are currently some of the main sources of cropland information on a global scale. Global cropland products were generated at 250 m and 30 m spatial resolutions by the NASA MEaSURES
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