The two USGS Landsat 7 Level 1 product generation systems, LPGS and the National Landsat Archive Processing System (NLAPS), create products that are similar but not identical. LPGS products are ordered through the EOS Data Gateway while EarthExplorer is used to order NLAPS products. Users should be aware of the following differences.
Scaling
After radiometric correction the LPGS system scales the output values
of the ETM+ 1G image data within the range of 1-255. The zero value
is reserved
for fill data. NLAPS, prior to April 5, 2004, scaled
the 1G image data from 0-255 and also used 0 for fill data. NLAPS now
scales (post April 5, 2004) ETM+ 1G data from 1-255 to match LPGS (the
output scale for TM and MSS remains unchanged (0-255)) . This
change was made to assist in identifying the potential gaps for SLC-off
products.
Users must be cognizant of which system
was used and the date of processing
if a
conversion back to radiance, as described in Chapter 11, is desired.
Gain Change Scenes
Earlier, it was reported that the LPGS and NLAPS have different scaling
procedures for scenes with gain state changes. This isn't the case. Both
systems always utilize the low gain LMAX value when scaling the radiance
of a scene.
Edge Trimming
The two systems also treat the scenes scan line edges differently. NLAPS
trims the band offsets while LPGS does not. This distinguishing characteristic
(Figure 12.1) can be used when system origin is unknown.
Figure 12.1 The LPGS (left) does not trim scan edges like NLAPS (right).
Map Projection Coordinates
The two systems also use slightly different output projection pixel placement
schemes. LPGS products have map coordinates that are pixel center based.
NLAPS coordinates assigns coordinates to the upper left corner. These
differences are illustrated in figure 12.2 where the upper left corner
pixel is depicted for all three band resolutions.
Figure 12.2. Upper left corner pixel alignment for LPGS versus NLAPS
1G products.
GeoTIFF Header
There is also a difference in the TIFF header for GeoTIFF formatted products.
Both systems have the BitsPerSample tag set to 8. The NLAPS product also
has the SampleFormat tag set to unsigned integer format. The LPGS does
not use the SampleFormat tag for its GeoTIFF formatted products.
GXA Slewing
Data anomalies that manifest as scan line offsets can occur when the Gimbaled
X-band Antenna (GXA) is redirected from one ground station to another.
Level 1 processing algorithms attempt to correct this problem but are
not always successful. When not, LPGS estimates an end-of-line (EOL) and
processes the damaged scans. The scans will be offset but available. NLAPS
assumes an absent EOL means no data and will zero fill the affected scans.
