Developing a method to map coconut agrosystems from high-resolution satellite images
Abstract
Our study aims at developing a generalizable method to exploit high resolution satellite images
(VHR) for mapping coconut-based agrosystems, differentiating them from oil palm agrosystems.
We compared two methods of land use classification. The first one is similar to that described by
Teina (2009), based on spectral analysis and watershed segmentation, which we simplified by
using the NDVI vegetation index. The second one is the semi-automatic classification based on
texture analysis (PAPRI method of Borne, 1990). These methods were tested in two different
environments: Vanua Lava (Vanuatu; heterogeneous landscape, very ancient plantations) and
Ivory Coast (Marc Delorme Research Station, monoculture, regular spacing, oil palm plantations);
and their results were evaluated against manually digitized photo-interpretation maps.
In both situations, the PAPRI method produced better results than that of Teina (global kappa of
0.60 vs. 0.40). Spectral signatures do not allow a sufficiently accurate mapping of coconut and do
not differentiate it from oil palm, despite their different NDVI signatures. The PAPRI method
differentiates productive coconut from mixed plantations and other vegetation, either high or low
(70% accuracy). In both situations, Teina’s method allows counting 65% of the coconut trees
when they are well spaced. To increase the method accuracy, we suggest (1) field surveys (for
small scale studies) and/or finer image resolution, allowing a high precision in manual mapping
with a better discrimination between coconut and oil palm, thus limiting the proportion of mixed
pixels. (2) A phenological monitoring could improve the distinction between coconut and oil palm
agrosystems. (3) Hyper-spectral images should allow extracting more precisely the respective
signatures of both species. Another possibility would be (4) an object-oriented analysis as
proposed by the eCognition software. Finally, (5) coupling the Lidar system with watershed
analysis would allow a better characterization of coconut varietal types.
Origin : Files produced by the author(s)