Landsat Missions

Two separate processing systems generate U.S. Geological Survey (USGS) Landsat 7 Level 1 products:

While the products generated by the LPGS and NLAPS systems are very similar, users should be aware that several differences exist. These differences fall into three general categories:

Geometric Differences

NLAPS trims the edges of ETM+ scenes. LPGS products contain all image data sampled, including the jagged scene edges and IC shutter intrusion at the end of each scan.

Figure 1. Scene edge of the NLAPS product (left) has been trimmed, whereas the LPGS product (right) has not been trimmed.

Figure 1. Scene Edges of an NLAPS and an LPGS Product

NLAPS and LPGS use different methods to align the bands. LPGS aligns the bands to the center of each pixel. NLAPS aligns bands to the edge 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. The NLAPS edges align all bands but lose center point consistency. Figure 2 and Figure 3 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.

Figure 2. Upper Left Coordinates of LPGS and NLAPS Products

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 remain the same, but center point consistency is 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 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.

Figure 4: Example of LPGS / NLAPS Radiometric Comparison Data

Figure 3. 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.

LPGS and NLAPS Processing Systems Comparison