Environmental Covariate Data for Spatial Prediction of Soil Properties for the Busia Area, Kenya

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By Joshua Minai1, Darrell Schulze1

Purdue University

Environmental covariates were carefully selected to represent factors of soil formation: climate, relief, organisms, and time.

Version 1.0 - published on 18 Nov 2019 doi:10.4231/7F9R-4W74 - cite this Content may change until committed to the archive on 18 Dec 2019

Licensed under CC0 1.0 Universal


A raster stack of 23 environmental covariates primarily based on remote sensing for predicting soil properties. These covariates were selected to represent the factors of soil formation and included the following. (1) The 1 Arc-Second (30 m) NASA shuttle Radar Topographic Mission (SRTM) global elevation dataset was downloaded from the USGS Earth Explorer (USGS, 2017a). (2) Eight terrain attributes (TAs), namely, Multiresolution Index of Valley Bottom Flatness (MRVBF), Multiresolution Ridge Top Flatness (MRRTF), plan curvature, profile curvature, relief intensity, slope, topographic position index (TPI), and SAGA wetness index (TWI), all based on the 30 m elevation dataset. (3) Annual average precipitation and temperature derived from monthly averages over the years 1970 – 2000 obtained from WorldClim ver. 2 at https://www.worldclim.org, all at a spatial resolution of ~1 km (Fick and Hijmans, 2017). (4) All eleven bands from Landsat 8 OLI (Operational Land Imager) and TIRS (Thermal Infrared Sensor) level 2 images of identification number LC08_L1TP_170060_20171226_20180103_01_T1 of path 170 and row 60 with an acquisition date of 26 December 2017 were downloaded from the USGS Earth Explorer (USGS, 2017b). These two sensors provide seasonal coverage of the global landmass at a spatial resolution of 30 m for the visible, near infrared, and short wavelength infrared; 100 m for the thermal infrared sensor; and 15 m for the panchromatic band. (5) The normalized difference vegetation index (NDVI) was calculated from the Landsat 8 level two data as a proxy for the vegetation (organisms) soil forming factor. 

These twenty-three environmental covariates were projected to the WGS84 Web Mercator (Auxiliary Sphere) coordinate system. Climate and Landsat data were subjected to bilinear interpolation resampling technique in ArcMap to a resolution of 30 m.

USGS. (2017a). NASA Shuttle Radar Topography Mission (SRTM) Global 1 arc second dataset (SRTMGL1), Digital Elevation Version 3. URL: https://earthexplorer.usgs.gov/

USGS. (2017b). Landsat 8 OLI aerial imagery Level 1. URL: https://earthexplorer.usgs.gov/

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