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Landsat Status: ETM+ Post Scan Line Corrector Product Usability

Landsat Status: ETM+ Post Scan Line Corrector Product Usability. Jim Lacasse Landsat Data Acquisition Manager USGS EROS Data Center, Sioux Falls, SD jmlacasse@usgs.gov. Landsat Status: ETM+ Post Scan Line Corrector Product Usability. Background on the ETM+ instrument problem

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Landsat Status: ETM+ Post Scan Line Corrector Product Usability

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  1. Landsat Status: ETM+ Post Scan Line Corrector Product Usability Jim Lacasse Landsat Data Acquisition Manager USGS EROS Data Center, Sioux Falls, SD jmlacasse@usgs.gov IGARSS 2004 Symposium Anchorage, Alaska

  2. Landsat Status: ETM+ Post Scan Line Corrector Product Usability • Background on the ETM+ instrument problem • Plans the USGS has implemented to reduce the impact of the SLC failure • ETM+ SLC-off Products Available • Preliminary assessments of new products • User feedback IGARSS 2004 Symposium Anchorage, Alaska

  3. Background on the ETM+ instrument problem IGARSS 2004 Symposium Anchorage, Alaska

  4. Duplicate Imagery Scan #129 (FWD) Scan #130 (REV) Detector Noise ETM+ Scan Line Corrector (SLC) Failure • On 31 May 2003 (Path 76, row 13: Day 151), image artifacts indicated a possible anomaly in the instrument. Resulting investigation by the instrument manufacturer, flight and ground systems staff and government management indicated that a mechanical failure had occurred and that the instrument and the data it would produce were permanently affected. IGARSS 2004 Symposium Anchorage, Alaska

  5. The SLC Assembly • Scan Line Corrector (SLC) is a subcomponent within the Enhanced Thematic Mapper Plus (ETM+) • SLC is an electromechanical device that rotates secondary mirrors through an angular range of +/-1 deg during a single scan of the primary mirror; corrects the image data for forward motion of the spacecraft. IGARSS 2004 Symposium Anchorage, Alaska

  6. The SLC Function • The SLC compensates for forward motion of the spacecraft which is over 16,000 mph • Upper image shows overlap/underlap without compensation (SLC not operating) • Bottom image shows corrected field of view with SLC working IGARSS 2004 Symposium Anchorage, Alaska

  7. Plans the USGS has implemented to reduce the impact of the SLC failure IGARSS 2004 Symposium Anchorage, Alaska

  8. Decision to continue the Landsat 7 Mission • Shortly after the SLC failed, prototype Level-1 products were generated with the goal of making an initial assessment as to the scientific and operational usability of the SLC-off data products. • Past members of the Landsat 7 Science Team and operational users were provided with samples of these products to make an assessment of how the degraded data affected their particular application and whether the product will affect their future use of Landsat 7 ETM+ data. IGARSS 2004 Symposium Anchorage, Alaska

  9. Level-1G SLC-off Preliminary Data Usability Assessments • The participating users represented a cross-section of the Landsat user community including a number of research areas and some operational users. • Disciplines included geography, agriculture, forestry, rangeland ecosystems, glaciology and ice cap monitoring, ecological remote sensing, phenological characterization, coastal/oceanographic remote sensing and coral reef monitoring, tropical forest monitoring, water quality monitoring, remote sensing methodology and techniques development, and global change monitoring IGARSS 2004 Symposium Anchorage, Alaska

  10. Results of Preliminary Data Usability Assessments (1 of 3) General Findings: • It is possible to generate data products containing the anomaly that retain their geometric and radiometric fidelity although there will be visible data gaps in the imagery. • The anomaly does negatively impact the usability of the data although many applications are still possible with these anomalous data. • The degree to which this limits their usefulness is dependant on the scientific application. The users and applications of these products fall into one of the two general categories: IGARSS 2004 Symposium Anchorage, Alaska

  11. Results of Preliminary Data Usability Assessments (2 of 3) • Those for whom the anomaly is a small or tolerable impact; the data still retain solid usability with only small consequences or adaptations. • large area monitoring. • monitoring tropical deforestation, qualitative assessments of crops, land cover change, and Global Change detection and monitoring • primary beneficiaries of the Long Term Acquisition Plan (LTAP) initiated with Landsat 7. • represent the largest portion of Landsat 7 data sales, the bulk customers of hundreds or thousands of scenes over time. • strongly urged returning to LTAP-like global data collection, the anomaly notwithstanding. IGARSS 2004 Symposium Anchorage, Alaska

  12. Results of Preliminary Data Usability Assessments (3 of 3) • Those for whom the anomaly is a major impact; the data have little or no remaining usefulness. • Applications that require complete and detailed data over specific areas are the most affected by this anomaly. • detailed mapping or monitoring small regions that are dependent on complete, reliable coverage of fine details within imagery. • areas where infrequent temporal coverage removes the possibility of extracting missing information repeated coverage of the area. The monitoring of coral reefs was cited as one application that will likely be impossible with anomalous data. These are typically users who extract the maximum information from an individual scene. These users will likely look for other datasets or abandon their applications. • category of the largest number of individual Landsat 7 customers, those who order a handful of scenes or less over time. IGARSS 2004 Symposium Anchorage, Alaska

  13. ETM+ SLC-off Products Available IGARSS 2004 Symposium Anchorage, Alaska

  14. SLC-off Data Characteristics • Same radiometric and geometric precision and accuracy as prior to the SLC failure for remaining 75% of data acquired • Gap widths are regular (systematic) • Gaps from 0 pixels at scene center increasing linearly to 15 pixels at the scene edge in 0-pixel interpolated product • Gaps from 2 pixels at scene center increasing to 14 pixels at the scene edge in a standard Level-1G SLC-off product • Gap locations are random relative to along track motion IGARSS 2004 Symposium Anchorage, Alaska

  15. Level-0R SLC-off • Cost Level-0R SLC-off $200/scene + $90/contiguous scene Scan duplication IGARSS 2004 Symposium Anchorage, Alaska

  16. Level-1G SLC-off Standard and Interpolated Products • Top is Level-1G, SLC-off with 0 pixel interpolation • Lower is Level-1G, SLC-off with 2 pixel interpolation (Standard Level-1G Product) • Cost Level-1G SLC-off $250/scene + $110/contiguous scene ~22km IGARSS 2004 Symposium Anchorage, Alaska

  17. Level-1G SLC-off/SLC-on gap-filled products • Gap-fill generated using ETM+ SLC-on data acquired as close to one year previous as possible • Requires scene-to-scene registration • Clouds and other transient phenomenon (for example fires, floods, water silt) in both SLC-off and SLC-on images are strong factor in final product quality • Cost Level-1G SLC-off is $275/scene IGARSS 2004 Symposium Anchorage, Alaska

  18. SLC-off/SLC-off products • Gap-fill generated using ETM+ SLC-off data acquired when specified by user • Requires scene-to-scene registration • Clouds and other transient phenomenon (for example fires, floods, water silt) in all input images are strong factor in final product quality • Cost Level-1G SLC-off $300/scene IGARSS 2004 Symposium Anchorage, Alaska

  19. L1Gs SLC-on Scene (for registration or fill) L1Gs SLC-Off Scene or Scenes Exclude Data Cloud Mask Create Histograms Histogram Matching Composite Scene Current Image Merging Technique • Local Histogram Matching applied to both SLC-off to SLC-on and SLC-off • Histogram matching is a simple procedure used to correct striping or banding in imagery, and it can be applied in a similar fashion to fill the scan gaps in SLC-Off data. A transfer function is calculated that converts the radiometric values of one scene into equivalent radiometric values of a second scene. The transformed data are used to fill the gaps. IGARSS 2004 Symposium Anchorage, Alaska

  20. Potential New SLC-off Products • The upper left image demonstrates a gap-filling technique that uses the Regression-Tree method to estimate missing pixel DN values. • The lower right image demonstrates the application of an image-segmentation approach that is effectively an advanced method of DN interpolation. IGARSS 2004 Symposium Anchorage, Alaska

  21. L1Gs Anniversary Scene L1Gs SLC-Off Scene Exclude Data Cloud Mask Collect Random Training Points Cubist Develop Regression Tree Model Evaluate Model Composite Scene Regression Tree Image Merging Technique • The method is “trainable.” Theoretically it can be tuned for specific seasons or land cover types. • The method is insensitive to transients. Clouds, silt, snow, etc are modeled or discarded as outliers. It is less important to use a closely matched anniversary scene. • Some drawbacks to the technique are that it depends on proprietary software and is sensitive to image registration. IGARSS 2004 Symposium Anchorage, Alaska

  22. Preliminary assessments of new products IGARSS 2004 Symposium Anchorage, Alaska

  23. Preliminary Assessments of Level-1G SLC-off Products • The standard Level-1G SLC-off products, the radiometrically and geometrically corrected products with data gaps were assessed by NASA, USGS and select operational users as reported previously • The Level-1G SLC-off interpolated product is available to the public, some have been purchased but the product’s utility has not been formally assessed. IGARSS 2004 Symposium Anchorage, Alaska

  24. Preliminary Assessments of Level-1G SLC-off to SLC-on Gap-filled Products CARPE Landsat Users • For local scale studies within the CARPE landscapes the data will be of very limited value (outside of the central unaffected swath) • gap filled products will not be useful for deriving maps in an automated or semi-automated fashion and should not be used for extensive change detection • For regional scale and photo interpretation studies the data will still be useful, as there is enough good data to interpret and map land cover and land cover changes between the data gaps. • A number of enhancements are being investigated which may increase data utility, including side lap compositing, but corrections using adjacent or previous scenes will be impacted by the limited availability of cloud free acquisitions for the Congo region IGARSS 2004 Symposium Anchorage, Alaska

  25. SLC-off acquired 3/21/2004 WRS2 path 182 Row 59 RGB bands 4,5,7 IGARSS 2004 Symposium Anchorage, Alaska 100 km

  26. SLC-on acquired 2/15/2003 WRS2 path 182 Row 59 RGB bands 4,5,7 IGARSS 2004 Symposium Anchorage, Alaska 100 km

  27. Subset D Subset F Subset E Subset A Subset B Subset C Level-1G SLC-off to SLC-on Gap-filled Primary scene - SLC-off acquired 3/21/2004 Fill scene – SLC-on acquired 2/15/2003 WRS2 path 182 Row 59 RGB bands 4,5,7 IGARSS 2004 Symposium Anchorage, Alaska 100 km

  28. Congo Basin Phase I Gap filled product SLC-off data (March 21, 2004) Merged with SLC-on data (February 15, 2003) Data acquired prior to SLC anomaly February 15, 2003 SLC-on 5 km IGARSS 2004 Symposium Anchorage, Alaska

  29. Congo Basin Phase I Gap filled product March 21, 2004 SLC-off data merged with February 15, 2003 SLC-on data Data acquired prior to SLC anomaly February 15, 2003 SLC-on 5 km Areas of gap filled data are outlined. IGARSS 2004 Symposium Anchorage, Alaska

  30. Preliminary Assessments of Level-1G SLC-off to SLC-off Gap-filled Products • Geologic Mapping • Fire Assessments • Emergency burn severity maps • Extended burn assessments • Impervious Surface and Canopy Mapping • Crop Type Mapping • Monitoring Land Use/Land Cover Trends in West Africa IGARSS 2004 Symposium Anchorage, Alaska

  31. SLC-off to SLC-off Data for Geologic Mapping • Study focused on enhanced image interpretability • Most distinct artifacts occurred in thermal infrared band (band 6) • Often difficult to distinguish between SLC-on and SLC-off to SLC-off gap-filled products used in this study • Study suggests that utility of SLC-off gap-filled data is no less than that of SLC-on data acquired prior to 31 May 2003 • Suggested further evaluation with additional data SLC-On Band 7 Histogram SLC-Off Band 7 Histogram IGARSS 2004 Symposium Anchorage, Alaska

  32. SLC-off to SLC-off Data for Emergency Burn Severity Maps • For emergency application, a satellite image must be acquired and analyzed as soon as possible after the fire is contained or declared out. Within this time constraint (1-2 weeks), waiting for a subsequent acquisition for filling the gaps is not feasible. • Level-1G SLC-off to SLC-off products are not an option for this application. IGARSS 2004 Symposium Anchorage, Alaska

  33. SLC-off to SLC-off Data for Extended Burn Severity Maps • Extended assessment burn severity maps are usually compiled using post-fire imagery acquired during the following growing season. • First, the Normalized Burn Ratio (NBR) is computed for each date: • NBR = (Band 4 – Band 7) / (Band 4 + Band 7) • NBR values range from –1.0 to +1.0 and are scaled to fit the range of –1000 to +1000. • Then, the DNBR is computed: DNBR = (Pre-fire NBR) – (Post-Fire NBR) IGARSS 2004 Symposium Anchorage, Alaska

  34. (a) (b) SLC-off to SLC-off Data for Extended Burn Severity Maps Enlargement of the 5-class burn severity map derived from the gap filled (a) and the original imagery (b). • This analysis demonstrated that the gap-filled product does not significantly alter the results of a non-emergency burn severity assessment. IGARSS 2004 Symposium Anchorage, Alaska

  35. SLC-off to SLC-off Data for Impervious Surface and Canopy Mapping • In this study, three SLC-off to SLC-off gap-filled images were used to create impervious surface and percent canopy estimates • The impervious surface prediction fared reasonably well with the gap filled product, as most impervious areas are pseudo-invariant, therefore impervious cover estimation should be relatively consistent even if filled with poorly matching dates. • The gap filling process evidently produced enough variability in the localized training area and subsequent rules that different results when extrapolating to the larger scale were seen. This could be minimized if training data from non-gap filled areas was used, but this is not a practical option. We conclude that consistent canopy estimation is unlikely with gap filled data. IGARSS 2004 Symposium Anchorage, Alaska

  36. SLC-off to SLC-off Data for Impervious Surface and Canopy Mapping Level-1G SLC-off to SLC-off gap-filled Level-1G SLC-off IGARSS 2004 Symposium Anchorage, Alaska

  37. SLC-off to SLC-off Data for Impervious Surface and Canopy Mapping Canopy Estimate for Level-1G SLC-off to SLC-off gap-filled Canopy Estimate for Level-1G SLC-off IGARSS 2004 Symposium Anchorage, Alaska

  38. SLC-off to SLC-off Data for Crop Type Mapping Above – Crop type areas Upper right – SLC-on mapping Lower right – Gap-filled mapping IGARSS 2004 Symposium Anchorage, Alaska

  39. SLC-off to SLC-off Data for Crop Type Mapping • Summary of comparison of the classified map based on the original Landsat image to the classified map based on the SLC-off gap-filled image: • 15.2% were misclassified using the histogram match method • average differences between the total area for all cover types 2.1-9.8% using the histogram matched method • The investigator is studying the potential a segmentation model based approach to gap-filling and has obtained better results than using the SLC-off to SLC-off data used in this study. IGARSS 2004 Symposium Anchorage, Alaska

  40. SLC-off to SLC-off Data for Monitoring Land Use/Land Cover Trends in West Africa • Landsat 7 imagery is one of the pillars of an ongoing program to monitor and map land use and land cover trends across West Africa. • The test image selected in southern Senegal represents a fair degree of land use / land cover complexity, further complicated by patterns of burn scars. The five specific study sites within this image fell near the left and right margins of the image, in areas where the data gaps are wide. The gap filled imagery resulted in quite visible perturbations of the landscape features within a given land use / land cover class, but produced only slight differences in the area of the various classes, amounting to no more than 1.5 percent change in area within 100 km² study sites. For most classes, the area differences were negligible. Thus, we remain optimistic that gap-filled Landsat images will be useful for approaches that use photo-interpretation to quantify and map land use / land cover in West Africa. IGARSS 2004 Symposium Anchorage, Alaska

  41. SLC-off to SLC-off Data for Monitoring Land Use/Land Cover Trends in West Africa IGARSS 2004 Symposium Anchorage, Alaska

  42. Ordering ETM+ Level-1G SLC-off Products • Scene selection • Clouds • Snow • Water level • Season • Gap phase location • Gap placement is random in WRS scenes • Estimated gap location is calculated using available ephemeris • Relative gap placement is identified by the “gap phase statistic” • Gap phase statistics from multiple SLC-off images used to predict total ground coverage for merged scenes IGARSS 2004 Symposium Anchorage, Alaska

  43. User feedback*(We are looking for it and appreciate all we get) IGARSS 2004 Symposium Anchorage, Alaska

  44. Landsat 7 User Questionnaire • Was the SLC-off data you purchased successful for your application(s)? • For what specific application(s) was the SLC-off data purchased? • What specific challenges (if any) did you experience with your data analysis and/or applications tools? • Have you considered or purchased alternative data sources? (If yes, please identify if possible) • What would be the primary limitation(s) of your alternative data sources? • Please list any ideas you may have that would enhance the future utility and/or usability of SLC-off products for your applications. IGARSS 2004 Symposium Anchorage, Alaska

  45. Additional Information IGARSS 2004 Symposium Anchorage, Alaska

  46. Anomaly Resolution Attempts • There are no redundancies for the SLC device itself. The timing and control for the SLC are driven by electronics and there are backup systems for these. • Possible failure modes postulated • 112 mechanical faults within the SLC postulated by the team • Only 5 significantly matched the symptoms • 53 electrical faults within the SLC electronics postulated by the team • None matched all 7 symptoms • After the initial identification of possible failure modes for the SLC, a switch over to redundant electronics was made. This switch over resulted in no change to the collected imagery and the control electronics were returned to their original state for continued operation with the SLC turned off. IGARSS 2004 Symposium Anchorage, Alaska

  47. Potential Image Compensation Techniques • Other options were examined to compensate for the failed SLC. Since it has not been possible to restart the SLC some other method of obtaining the missing ground coverage is necessary. Methods that were considered at the high level fall into two broad categories: flight operations modification and image modification. • Flight operations modification included things like yawing or pitching the spacecraft or raising the orbit, with the goal of either changing the locations of gaps or reducing the size of the gaps. These methods were rejected for near term implementation as they require a great deal of analysis and if they are feasible they either impact the long term science goals of the Landsat program or they pose a risk to mission operations. (cartoon of yaw effects) • Image modification includes methods ranging from single imagery interpolation to multi-image/multi-instrument image merging. These methods provided a number of options, some of which could be implemented very quickly. The majority of these methods do not provide near-instantaneous full scene coverage. The exceptions are image merges from instruments collecting wide areas (MODIS) or flying in close formation (ALI). IGARSS 2004 Symposium Anchorage, Alaska

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