Landsat Missions

LPGS and NLAPS Processing Systems Comparison

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Historically, two processing systems generated the USGS Landsat standard data products.

  • Level-1 Product Generation System (LPGS) - Currently, all Landsat data is processed through LPGS. LPGS metadata is contained in a MTL.txt file, which accompanies all data files.
  • National Land Archive Production System (NLAPS) - Some available Landsat 4 and Landsat 5 data were processed through NLAPS. NLAPS metadata is contained in a .WO file, which accompanies all data files. A listing of NLAPS-processed scenes can be found at L4-5TM_NLAPS.xlsx

While the products generated by the LPGS and NLAPS systems are similar, there are geometric, radiometric, and data format differences:

Geometric Differences

Formerly, NLAPS and LPGS used different methods to align the bands: LPGS aligned the bands to the center of each pixel, and NLAPS aligned bands to the edge of each pixel. As of December 2008, both systems align the bands to the center of each pixel.

The center of the 1, 1 pixel is the X, Y map location listed in the header records of the images. The LPGS system centers all bands about a common center X, Y location. The centers are aligned but the edges are not. Prior to December 2008, the NLAPS edges aligned all bands but lost center point consistency. Figures 1a and 1b illustrate the differences in the corner point location handling of the systems. The large box is the thermal pixel at 60 m. The middle box is the visible and near-IR pixel at 30 m. The smallest box is the panchromatic pixel at 15 m (Landsat 7).

Upper Left Coordinates of LPGS and NLAPS Products as of December 2008.
Figure 1a. Upper Left Coordinates of LPGS and NLAPS Products as of December 2008.

Upper Left Coordinates NLAPS Products prior to December 2008.

Figure 1b. Upper Left Coordinates NLAPS Products prior to December 2008.

The upper left coordinates of the pixels vary for products produced by LPGS between the thermal, reflective, and panchromatic bands. For NLAPS products, the upper left coordinates of each band remained the same, but center point consistency was lost.

NLAPS and LPGS resampling methods are not identical. The two systems use different ephemeris files as well as different algorithms to process quaternion, gyro, and gyro drift data. This may cause some slight additional differences between the two products.

This paper discusses the Geometric differences that existed between LPGS and NLAPS products in depth. - .pdf (248KB)

Radiometric Differences

Since April 5, 2004, (Release 4.3.0e14), scenes processed by NLAPS are scaled the same as LPGS-processed scenes.

Scenes processed after April 5, 2004: Both LPGS and NLAPS scale Level-1 products to a range of 1-254. DN values of 0 are reserved for scan gap and flag fill. DN values of 255 are reserved for saturation.

Scenes processed prior to April 5, 2004: LPGS scaled Level-1 products to a range of 1-254 with values of 0 and 255 set aside to flag fill and high saturation, respectively. NLAPS scaled images from 0 to 254, with a value of 0 DN used for both fill and the lowest radiance value in the scene. A comparison of LPGS versus NLAPS products results in a gain ratio of 254/255 and a bias of 1. This equates to a 0.4 percent scaling difference between the two processing systems, which is eliminated if the L1 data are converted to radiance.

Example of LPGS / NLAPS Radiometric Comparison Data

Figure 2. Example of LPGS / NLAPS Radiometric Comparison Data

This paper discusses the radiometric differences between LPGS and NLAPS products in depth. - .pdf (1021KB)

Data Format Differences

GeoTIFF products have a difference in the tiff header. Both systems have the BitsPerSample tag set to 8. The NLAPS product also has the SampleFormat tag set to unsigned integer format. LPGS does not use the SampleFormat tag in its GeoTIFF products, even though they are also unsigned integer format.

The two systems handle the Gimbaled X-Band Antenna (GXA) slewing differently. GXA anomalies appear as scan line offsets or extensions and can occur when the GXA redirects itself from one ground station to another. Level-1 processing may correct this problem in some instances. LPGS estimates an 'End-of-Line' (EOL) and processes the scans with that estimated number. The data are offset but are available. NLAPS defines no EOL as 'no data,' and zero-fills the affected scans.

The 60 meter and 15 meter NLAPS and LPGS products may have different image sizes. In general, the LPGS product contains one more line and sample than the corresponding NLAPS product for Bands 6 Low (L), 6 High (H), and 8. The 30 meter bands are a consistent size on both L1 processing systems.

 

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