CHAPTER 11 SECTIONS > Level 0R Product | Level 1R Product | Level 1G Product | Level 1 Differences

11.3 Level 1G Product

The 1G product available to users from EROS is a radiometrically and geometrically corrected Level 0R image. The correction algorithms employed model the spacecraft and sensor using data generated by onboard computers during imaging events. Primary inputs are the PCD, which includes the attitude and ephemeris profiles, the definitive ephemeris (if available) and the MSCD. Refined parameters from the CPF, ground control points and a digital elevation model are also used to improve the overall geometric fidelity of the standard level-one terrain-corrected (L1T) product.

The L1T correction process utilizes both ground control points (GCP) and digital elevation models (DEM) to attain absolute geodetic accuracy. The WGS84 ellipsoid is employed as the Earth model for the Universal Transverse Mercator (UTM) coordinate transformation. Associated with the UTM projection is a unique set of projection parameters that flow from the USGS General Cartographic Transformation Package. The end result is a geometrically rectified product free from distortions related to the sensor (e.g. jitter, view angle effects), satellite (e.g. attitude deviations from nominal), and Earth (e.g. rotation, curvature, relief).

Geodetic accuracy of the L1T product depends on the accuracy of the GCPs and the resolution of the DEM used*. The 2005 Global Land Survey is used as the source for GCPs while the primary terrain data is the Shuttle Radar Topographic Mission DEM. Scenes that have a quality scores of 99 and less than 40 percent cloud cover are automatically processed, and any archived scene, regardless of cloud cover, can be ordered through one of two EROS web portals (Product Ordering.)

During L1T processing the 0R image data undergoes two-dimensional resampling according to the following set of parameters:

• Correction level - L1T*
• Pixel Size - 15, 30, 60 meters for panchromatic, VNIR/SWIR, and thermal
• Resampling kernel - Cubic Convolution (CC)
• Map projection - UTM
  with Polar Stereographic projection used for Antarctica scenes
• Ellipsoid - WGS84
• Image orientation - north up
• Output format - GeoTIFF
• File transfer protocol (FTP) download only

Figure 11.3 - Landsat 7 Resampling Kernel (CC) Versus Other Methods

The Landsat 7 level 1G product projection parameters are listed in Table 12.1. Most projections do not require 13 parameters as evidenced by the empty table cells. Parameter definitions are listed in Table 12.2.

During 1G product rendering image pixels are converted to units of absolute radiance using 32 bit floating point calculations. Pixel values are then scaled to byte values prior to media output. The following equation is used to convert DN's in a 1G product back to radiance units:

L_λ = Grescale * QCAL + Brescale

which is also expressed as:

L_λ = ((LMAX_λ - LMIN_λ)/(QCALMAX-QCALMIN)) * (QCAL-QCALMIN) + LMIN_λ

The LMINs and LMAXs are the spectral radiances for each band at digital numbers 0 or 1 and 255 (i.e QCALMIN, QCALMAX), respectively. LPGS used 1 for QCALMIN while NLAPS used 0 for QCALMIN for data products processed before April 5, 2004. NLAPS from that date now uses 1 for the QCALMIN value. Other product differences exist as well. One LMIN/LMAX set exists for each gain state. These values will change slowly over time as the ETM+ detectors lose responsivity. Table 11.2 lists two sets of LMINs and LMAXs. The first set should be used for both LPGS and NLAPS 1G products created before July 1, 2000 and the second set for 1G products created after July 1, 2000. Please note the distinction between acquisition and processing dates. Use of the appropriate LMINs and LMAXs will ensure accurate conversion to radiance units. Note for band 6: A bias was found in the pre-launch calibration by a team of independent investigators post launch. This was corrected for in the LPGS processing system beginning Dec 20, 2000. For data processed before this, the image radiances given by the above transform are 0.31 w/m2 ster um too high. See the official announcement for more details. Note for the Multispectral Scanner (MSS), Thematic Mapper (TM), and Advanced Land Imager (ALI) sensors: the required radiometry constants are tabulated in this PDF file.

For relatively clear Landsat scenes, a reduction in between-scene variability can be achieved through a normalization for solar irradiance by converting spectral radiance, as calculated above, to planetary reflectance or albedo. This combined surface and atmospheric reflectance of the Earth is computed with the following formula:

Where:
	= Unitless planetary reflectance
	= Spectral radiance at the sensor's aperture
	= Earth-Sun distance in astronomical units from an Excel file or interpolated from values listed in Table 11.4
	= Mean solar exoatmospheric irradiances from Table 11.3
	= Solar zenith angle in degrees

ETM+ Band 6 imagery can also be converted from spectral radiance (as described above) to a more physically useful variable. This is the effective at-satellite temperatures of the viewed Earth-atmosphere system under an assumption of unity emmissivity and using pre-launch calibration constants listed in Table 11.5. The conversion formula is:

The same two 1R options exist for users when defining the size or spatial extent of a Landsat level 1G product ordered from the LP-DAAC.

• Standard Worldwide Reference System (WRS) Scene. The standard WRS scene, as defined above for the 0R product, can be ordered in 1G form. Partial scenes that may exist at the beginning and end of subintervals may be also be ordered.



• Partial Subinterval. A partial subinterval can also be ordered in 1G form. Unlike the 0R product the 1G is limited to a maximum of 3 WRS scenes in size. The variably sized 1G product can float or be positioned at any scan line starting point within a subinterval. Alternatively, the product can be defined by up to three contiguous WRS locations.

The 1G product ordered from the LP-DAAC consists of the corrected image files and descriptive metadata. All other ancillary files delivered with the 0R and 1R products are not included. A user may order a subset of the available bands which affects the actual file count in a 1G product.

The 1G product can be packaged into one of following user-specified output formats:

• Hierarchical Data Format. The HDF packaging format used for the 0R and 1R products is also used for structuring the 1G. The design employs external elements for the band files and metadata. These are standalone files that are referenced via tags and pointers residing in an HDF directory. External elements provide users with two processing options - exploit the NCSA HDF libraries for data access or process the data files directly using homegrown code.

  The number of files comprising an HDF-formatted 1G product will vary according to the number of bands ordered. A product with a full band complement has 11 files - the HDF directory, a metadata file, and a separate file for each band. The HDF directory and metadata files are always present regardless of bands ordered. Please refer to the Landsat 7 0R Distribution Product Data Format Control Book, Volume 5 (PDF) for details regarding band file specifics. The 1R metadata file description can be found in the ESDIS Level 1 Product Generation System Output Files DFCB (PDF).

  The HDF format can be specified for any type of 1G product ordered from the LP-DAAC.




• Fast. The Fast Format was originally developed by EOSAT as a means for quickly accessing Landsat 4 and 5 image data. Its structure is straightforwardly simple. Each band is self contained in its own file (i.e external element style). A header file containing three records accompanies the image data. The three records in order of appearance are labeled administrative, radiometric, and geometric respectively. Sensor specific information is placed in the administrative record, gains and biases can be found in the radiometric record while projection information and image coordinates are stored in the geometric record. A single header file along with the image files constitute the Fast product.

  A derivative of the Fast Format (Fast-L7) used by EOSAT for Landsat (FAST-B) and Indian Remote Sensing products (Fast-C) was created for Landsat 7. Several differences are worth noting. File names are now included in the administrative record which allows for direct file access. A separate header file now accompanies the panchromatic, thermal and VNIR/SWIR band groups for Landsat 7. For Fast-B and Fast-C all bands were resampled to a common grid cell size thus permitting a single header file. In all likelihood each of the band groups for Landsat 7 will be resampled to a common resolution (i.e. 15, 30, & 60 meters) thus requiring a distinct header file for each.

  All critical fields required for product ingest were left unchanged in the Fast L-7A Format. As a consequence Heritage Fast readers residing on user systems can be used for the Landsat 7 Fast formatted product. A full layout of the Fast L-7A Format can be found in the ESDIS Level 1 Product Generation system Output Files DFCB.

  The Fast-L7 format supports all variations of the 1G product.




• GeoTIFF. Geographic tagged image file format (GeoTIFF) is based on Adobe's TIFF - a self-describing format developed to exchange raster images such as clipart, logotypes, and scanned images between applications and computer platforms. Today, the TIFF image file format is used to store and transfer digital satellite imagery, scanned aerial photos, elevation models, and output from digital cameras. TIFF is the only full-featured format in the public domain, capable of supporting compression, tiling, and extension to include geographic metadata.

  The TIFF file consists of a number of label (tags) which describe certain properties of the file (such as gray levels, color table, byte format, compression size). After the initial tags comes the image data which may be interrupted by more descriptive tags. GeoTIFF refers to TIFF files which have geographic (or cartographic) data embedded as tags within the TIFF file. The geographic data can then be used to position the image in the correct location and geometry on the screen of a geographic information display.

  Baseline TIFF image types can be bilevel, greyscale, palette color, and full color (24 bit). For simplicity's sake the grayscale model was implemented for the Landsat 7 GeoTIFF product. Under this implementation each ordered band is delivered as its own 8 bit greyscale GeoTIFF image. A standard WRS scene possessing the full band complement would thus be comprised of nine separate GeoTIFF images or files. No other files accompany the product. For detailed information regarding the Landsat 7 GeoTIFF implementation please refer to the ESDIS Level 1 Product Generation system Output Files DFCB (PDF). For GeoTIFF details, please download the GeoTIFF Format Specification (PDF) or visit this web site.

  At the present time GeoTIFF format cannot be used for the Space Oblique Mercator and Oblique Mercator projections. Products projected into these reference systems must be formatted using HDF or Fast-L7.

An instrument malfunction occurred onboard Landsat 7 on May 31, 2003. The problem was caused by failure of the Scan Line Corrector (SLC), which compensates for the forward motion of the satellite. Subsequent efforts to recover the SLC have not been successful, and the problem is permanent.

The Landsat 7 Enhanced Thematic Mapper Plus (ETM+) is still capable of acquiring useful image data with the SLC turned off, particularly within the central portion of any given scene. Landsat 7 ETM+ will therefore continue to acquire image data in the "SLC-off" mode.

EDC has recently released several Landsat 7 ETM+ SLC-off data products. The first, a gap-present product became available on October 22, 2003. This product release includes all image data acquired by Landsat 7 in SLC-off mode from July 14, 2003 to present, excluding a 2-week interval from 9/3/03 to 9/17/03.

The center of a gap-present SLC-off data product is very similar in quality to previous Landsat 7 data. However, the scene's edges will contain alternating scan lines of missing data (Level 1G) or duplicated data (Level 0Rp or L1R). The precise location of the affected scan lines will vary from scene to scene, and these gaps will not be visible on the browse image preview when ordering SLC-off data. A preliminary report regarding the utility of Landsat 7 SLC-off data is available in PDF form. This report includes input from scientists affiliated with the USGS, NASA, and the Landsat 7 Science Team.

The gap-present SLC-off data product is available as a single scene entity in Level 1G terrain corrected (L1T) form. As of November, 2008, the USGS offers all archived Landsat scenes to the public at no charge. Newly acquired Landsat 7 ETM+ SLC-off and Landsat 5 TM images with less than 40 percent cloud cover are automatically processed and made freely available for immediate download. SLC-off data products can be searched and ordered via the Earth Explorer, and Global Visualization L7 Image Browser.

The second product now being offered (as of May 10, 2004) is in 1G form and has the gap areas filled with Landsat 7 data acquired at a similar time of year and prior to the SLC failure. The two scenes are geometrically registered, and a histogram matching technique is applied to the fill pixels which provides the best-expected radiance values for the missing data.

Figure 11.4 - Top image: pre-SLC anomaly, middle of image. Middle image: scene after SLC anomaly. Bottom image: scene after SLC anomaly with data gaps filled.

The USGS, in conjunction with NASA, is continuing to research other methods of providing merged data products and will continue to provide information resulting from this work as it becomes available. Various methodoligies have been examined to fill the data gaps with observations acquired during prior or later than the target scene of interest. An exampled of a gap-filled product is illustrated in Figure 11.4.

If available, the Landsat 7 definitive ephemeris is used for geometrically correcting ETM+ data. Definitive ephemeris substantially improves the positional accuracy of the Level 1G product over predicted ephemeris.

An ephemeris is a set of data that provides the assigned places of a celestial body (including a manmade satellite) for regular intervals. In the context of Landsat 7, ephemeris data shows the position and velocity of the spacecraft at the time imagery is collected. The position and velocity information are used during product generation.

The Landsat 7 Mission Operations Center receives tracking data on a daily basis that shows the position and velocity of the Landsat 7 spacecraft. This information comes from the three US operated ground-receiving stations and is augmented by similar data from NASA's Tracking and Data Relay Satellites. The Flight Operations Team processes this information to produce a refined or "definitive" ephemeris that shows the position and velocity of Landsat 7 in one minute intervals. Tracking data are used to compute the actual spacecraft position and velocity for the last 61 hours and to predict these values for the next 72 hours. The predicted ephemeris data are uploaded to the spacecraft daily. On-board software interpolates from this data to generate the positional information contained in the Payload Correction Data (PCD).

Engineers with the Landsat Program have completed a predicted versus definitive ephemeris analysis. Comparisons to ground control points demonstrate the definitive ephemeris is, in fact, reliably more accurate than the predicted ephemeris. Geometric accuracy on the order of 30-50 (1 sigma) meters, excluding terrain effects, can be achieved when the definitive ephemeris is used to process the data. Level 1G products produced after March 29, 2000 use definitive ephemeris if available. The .MTL field "ephemeris_type" in the product metadata files identifies whether a product was created with definitive or predicted ephemeris. Daily definitive ephemeris profiles have been archived since June 29, 1999 and are available for downloading.

The LMIN's and LMAX's are a representation of how the output Landsat ETM+ Level 1G data products are scaled in radiance units. The LMIN corresponds to the radiance at the minimum quantized and calibrated data digital number (QCALMIN), which is typically "1" or "0" and LMAX corresponds to the radiance at the maximum quantized and calibrated data digital number(QCALMAX), typically "255".

Reflective bands:

The LMIN's are set so that a "zero radiance" scene will still be on scale in the 8 bit output product, even with sensor noise included. LMIN should result in "zero " radiance being about 5 DN in low gain and 7.5 DN in high gain. The LMAX's are set so that LMAX corresponds to slightly less than the saturation radiance of the most sensitive detector. This is done so that in the output product all "pixels" saturate at the same radiance. Currently the LMAX is set to be 0.99 of the pre-launch saturation radiance of the most sensitive detector in each band.

Normally, there is no need to change the LMIN's or LMAX's, unless something changes drastically on the instrument. If the sensitivity of the instrument increases, which is not expected, there is no need to change the LMIN and LMAX values. If the sensitivity decreases, the LMAX values can be increased which in turn increases the usable dynamic range of the product (this will not occur unless the change is large). The changes that have taken place to date have been mostly due to the adoption of "improved" pre-launch gains for the instrument that have, in effect, "increased" its sensitivity. The Landsat Project Science Office also detected a few errors in the original numbers, which were corrected.