A novel deep convolutional neural network approach for large area
satellite time series land cover classification
Abstract
The state of the practice for large area land cover mapping is based on
the application of supervised classifiers to multi-temporal
multi-spectral optical wavelength data. Deep learning approaches have
been developed but are typically applied to individual images with less
research on their application to satellite time series. The recent
availability of 30 m Landsat analysis ready data (ARD) has significantly
increased the ease of using Landsat data. Irregular gaps in the Landsat
image time series reduce the easy application of deep learning to time
series. This study proposes a novel solution based on a two dimensional
(2-D) array (one spectral dimension and one temporal dimension) derived
at each ARD pixel time series and using the 2-D convolutional neural
network (CNN) deep learning algorithm. Classification results are
presented for all of the Conterminous United States (CONUS) considering
a year of Landsat 5 and 7 data. The 30 m USGS National Land Cover
Database (NLCD) (15 classes after filtering) and USDA Crop Data Layer
(CDL) (22 classes after filtering) products are filtered conservatively
across the CONUS and sampled to construct >3.31 and
>0.48 million training pixels respectively defined with
reliable accuracy and reduced spatial autocorrelation. The CNN and a
conventional Random Forest (RF) classifier are trained using 10%, 50%
and 90% of the training samples, and used to classify the remaining
samples. Classification experiments are undertaken independently using
the NLCD and CDL training data. Different CNN structures with different
learnable coefficients are used and the accuracy results compared with
conventional RF results. The main findings were (1) application of the
CNN with different structures provided only about 1% accuracy
difference, the optimal CNN structure was dependent on the number of
training samples, and increasing the number of CNN learnable
coefficients beyond the number of training samples was not helpful; (2)
although the CNN training time was up to two orders of magnitude slower
than the RF, the classification time was an order of magnitude faster,
(3) the CNN provided 2-5% higher accuracy than the RF which is notable
given the large number of classes and that the overall classification
accuracies were >80%.