Using satellite imagery for counting crop acreage
A conversation with Lieven Claessens, of the International Potato Center
Let’s say you have a wonder food—or anyway, a crop you’re promoting or tracking. Other than a laborious foot trek, how do you know how much of it people are really planting in a region–especially if farmers grow a complex mosaic of crops?
Yes, district officers estimate acreage devoted to various items. But how good are those estimations?
CIP, the International Potato Centre, promotes sweet potato for its high Vitamin A content and other nutrients. CIP wanted to know how much sweet potato was being planted by farmers in eastern Uganda.
So they developed a way to use satellite imagery to peer closely into fields. Until recently, most thought it wouldn’t be possible to differentiate food crops in a complex farming system. But for this project researchers tuned the satellite cameras to pick up not what the human eye would see–a tangle of green—but wavelengths we can’t see, mostly “infrared.”
Satellite images of infrared are similar the infrared film you (or more likely your parents) may have experimented with in the ’60s or ’70s– trees and grass came out in reds and violets.
Each species reflects its own “signature” colour. Using a hand-held sensor, researchers figure out what that colour is, for, say, sweet potato, as well as crops planted next to or near it, so they can differentiate them. Then they check for those colours in the satellite images.
The result? According to CIP researcher Lieven Claessens,“We discovered that only 63 percent of sweet potatoes in the field showed up in the national statistics. In other words, sweet potatoes were undercounted by nearly 40 percent.”
Improving the accuracy of counting by such a large percentage could be a boon to agriculture researchers worldwide. One more win for “the eye in the sky.”