Appendix E - Level-1 LPGS and NLAPS Product Differences (Historical)
At the start of the mission, two USGS Landsat 7 Level-1 product generation systems, the Landsat Product Generation System (LPGS) and the National Landsat Archive Processing System (NLAPS), were available to create products from ETM+ data that were similar but not identical. LPGS products were ordered through the EOS Data Gateway while Earth Explorer was used to order NLAPS products. Users of data acquired prior to 2004 should be aware of the following differences.
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. After this date, NLAPS processing was changed so that all post April 5, 2004 ETM+ L1G data scales from 1-255 match LPGS processing (the output scale for TM and MSS processing 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 is desired.
E.3 Gain Change Scenes
Earlier, it was reported that the LPGS and NLAPS have different scaling procedures for scenes with gain state changes. This is not the case. Both systems always utilized the low gain LMAX value when scaling the radiance of a scene. LPGS continues to utilize this value as gain settings have changed with modifications to the LTAP.
E.4 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 (see Figure E-1) can be used when the processing system origin is unknown.
Figure E-1. LPGS (left) Did Not Trim Scan Edges like NLAPS (right)
E.5 Map Projection Coordinates
The two systems also used slightly different output projection pixel placement schemes. LPGS products continue to have map coordinates that are pixel center based. NLAPS coordinates assigned coordinates to the upper left corner. These differences are illustrated in Figure E-2 where the upper left corner pixel is depicted for all three ETM+ band resolutions.
Figure E-2. Illustration of Upper Left Corner Pixel Alignment for LPGS versus NLAPS L1G Products
E.6 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 had the SampleFormat tag set to an unsigned integer format. The LPGS does not use the SampleFormat tag for its GeoTIFF formatted products.
E.7 Gimbaled X-band Antenna (GXA) Slewing
Data anomalies that manifest as scan line offsets can occur when the 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 zero fills the affected scans.