Terrain

Elevation, slope, aspect and topography (e.g., drainage structure) play important roles in influencing the macro and micro conditions that affect the feasibility and productivity of agriculture. Beyond the terrain conditions at the specific locations of agricultural production, terrain exerts a strong influence on the connectivity of locations via road, rail and river transportation networks. Terrain conditions do therefore help shape the degree of integration or isolation of farm households with respect to markets and services. In the context of HarvestChoice, digital elevation data are primarily used as input layers into the generation of maps and spatial databases of agreocological zones (AEZs) and market access.

Elevation dataset comparison

In developing agreocological zones, elevation is used to distinguish highland and lowland conditions within similar major climate regions. Certain crops (and associated pests and disease) are better adapted to, say, cooler tropical areas. Thus, by sub-dividing the agroecological zones into highland (cool) and lowland (warm) conditions we are better able to assess the production potential of these lands for different types of (rainfed) agriculture.¹

Slope is a key component of the market access surfaces in that it provides a gradient measure that is used to modify the time needed to travel across the roads and paths. Areas with a greater slope are harder to traverse and thus will require more time to travel to the target market centers.

The primary source of digital terrain data used by HarvestChoice is the GTOPO30dataset which is available globally at a resolution of 30 arc seconds (approximately 1km at the equator). This dataset provides information on elevation, slope and aspect that also underpin the Hydro1kspatial database of streams and watersheds.

For future analysis HarvestChoice will increasingly make use of the SRTM 90 Digital Elevationdata. This finer resolution dataset (90m at the equator) represents a significant advance in terms of reliable terrain assessment compared to prior global/regional elevation data products. Enhanced SRTM 90 Ver. 4data (quality improvement through void filling for elevation and hydrological modeling purposes, and reformatted for easier use by GIS analysts in 5 degree x 5 degree tiles) will be taken from the website of the CGIAR's Consortium on Spatial Information(CSI).

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Notes
¹ Agroecological zones offer a consistent but coarse means of assessing production potential (e.g. for rapid area screening), and HarvestChoice uses additional tools for more complete assessments of production potential. In particular, we use crop growth (simulation) models on a grid-cell basis such that the yield potential of individual crops are assessed using a wide range of cell-specific climate and soil attributes. In such approaches the effects of terrain are largely captured through the direct use of observed or predicted weather variables (such as maximum and minimum temperature).