PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
July 2015
523
Sensor Artefacts
.
The SLC of the Landsat-7
Enhanced Thematic Mapper Plus (ETM+)
instrument failed on May 31, 2003
(Gutman et al., 2008). Hence, the second
greatest challenge in the GLS 2005 and
2010 collections is either avoiding or “gap-
filling” the SLC-Off null values, which cover
approximately 22% of each image acquired
after the malfunction. The original GLS
collection avoided the issue by preferentially
selecting Landsat-5 Thematic Mapper (TM)
images, when available, and filling ETM+
gaps by compositing data from other images
from the same WRS-2 scene when they were
not available. However, gap-filling has led to
phenological and bidirectional artefacts in the
composited data products (Storey et al., 2005;
Gutman et al., 2013), and the composited
images do not represent a single point in
time. Repatriation efforts to gather Landsat
images from international receiving stations
continued after collection of the GLS, and so
for the GLS+ we queried the expanded USGS
archive and replaced the SLC-Off and gap-
filled ETM+ images with TM scenes when
such replacements had become available.
Phenology.
Although
“peak-season”
phenology was a criterion for selecting the
GLS, we found that many of the scenes were
not phenologically suitable and could lead to
spurious identification of forest cover changes
(Townshend et al., 2012; Kim et al., 2011).
An assessment of each image’s temporal
proximity to the season of maximum NDVI
revealed that 3515 images from the GLS1990
(Figure 2 top), 441 images from the GLS2000
(Figure 2 middle) and 574 scenes from
the GLS2005 (Figure 2 bottom) needed to
be replaced. Phenologically appropriate
substitutes were found and incorporated into
the GLS+ (Kim et al., 2014).
Figure 1. Landsat GLS2000 images acquired in Colombia (in order left to right p004r057, p003r058, p003r057, p004r058). All the scenes had zero cloud cover
reported in their metadata file.
Figure 2. Global coverage of replacement images for GLS1990 (top), GLS2000 (middle), and
GLS2005 (bottom). Images were replaced with L1T data as needed where possible.
