Production Methods for the F162009 Stable Lights Product Kimberly Baugh, CIRES University of Colorado Boulder, Colorado, USA Email: kim.baugh@noaa.gov 1 ‐ 303 ‐ 497 ‐ 4452 Chris Elvidge, NOAA National Geophysical Data Center (NGDC) Boulder, Colorado, USA Email: chris.elvidge@noaa.gov 1 ‐ 303 ‐ 497 ‐ 4452
F162009 Global Stable Lights Product • NGDC’s “Stable Lights” products have values which depict the relative intensity of lit areas. • Lighting deemed ephemeral has been removed and non ‐ lit areas (background) have been set to zero.
DMSP ‐ OLS Data VIS TIR DMSP ‐ OLS sensor has a unique capability to collect low ‐ light imagery. • Polar orbiting • 3000 km swath • 2.7 km ground sample distance • Daily global coverage • Two spectral bands: • VIS (0.4 ‐ 1.1 um) • TIR (10.5 ‐ 12.6 um) • Visible band data values range from 0 ‐ 63 (6 ‐ bit data) Nighttime portion of orbit F16200901281215
Generation of v4b F162009 Stable Lights Product 1) OLS data for satellite F16, year 2009, is gridded to 30 ‐ arc second grid. The highest quality data is kept for inclusion into the product. Highest quality means: mid ‐ swath, cloud ‐ free, zero lunar illuminance, dark nightttime (solar elevation < ‐ 15). 2) An analyst looks at each suborbit to manually mark aurora and gain changes for exclusion. 3) 30 ‐ arc second grids are composited, creating a suite of files including an average visible band image and histograms of input visible band data for each grid cell. 4) Outlier removal process is performed to remove ephemeral lighting events such as fires. 5) Background removal is done which uses no ‐ light areas to generate local background statistics. A “lights mask” is output from this process. 6) Create stable lights image by applying the lights mask to original average visible band data. 7) Align stable lights image to Landscan population grid.
Processing DMSP ‐ OLS Orbits: Flag Bands For each nighttime suborbit, This entire suborbit VIS FLAG a companion “flag” band was flagged as is generated with bit ‐ having zero lunar codes designating: illuminance. • daytime (solar elevation > ‐ 6) • nighttime marginal Red: daytime ( ‐ 15 < solarelevation < ‐ 6) Green: nighttime • zero lunar illuminance marginal (< 0.0005 lux) Black: This area is considered high Solar elevation angles are computed quality nighttime based on lat, lon, and time of data and will be each OLS pixel. processed further. Lunar illuminance is a function of lunar phase, azimuth, and elevation, which are also based on lat, lon, and time of each OLS pixel. Nighttime portion of orbit F16200901281215
Processing DMSP ‐ OLS Orbits: Linescreening • Suborbits containing Red: daytime VIS FLAG Green: nighttime high quality nighttime marginal data are screened by an Yellow: discarded by analyst for aurora and linescreening abrupt gain changes. process • Analyst chooses a start Blue: edge ‐ of ‐ scan and end line of data to data include for compositing. Black: This area is considered high • Data at edges of swath quality nighttime are discarded due to data and will be increased noise and processed further. poorer geolocation (scan angle > 40.91) . Nighttime portion of orbit F16200901281215
Processing DMSP ‐ OLS Orbits: Reprojection OLS vis, tir and corresponding flag bands are gridded to 30 arc ‐ second grids, constrained to latitudes 65S ‐ 75N. For clarity, only mid ‐ swath, linescreened data are shown. Green: nighttime marginal VIS TIR FLAG Nighttime portion of orbit F16200901281215
Processing DMSP ‐ OLS Orbits: Cloud Mask NCEP grids for 2009/01/28 • A cloud mask is generated by comparing the reprojected OLS thermal band to a surface temperature grid. 00:00 UT • National Center for Environmental Prediction (NCEP) creates global surface temperature grids at 0.5, 06:00 UT 1.0, and 2.5 degree resolution at 6 hourly intervals. • The 1.0 degree surface temperature grids were used for 12:00 UT the F162009 stable lights product. 18:00 UT
Processing DMSP ‐ OLS Orbits: Cloud Mask • Difference images are made as Diff = Surface Temp ‐ TIR. • Due to the increased variability in land temperature values, land and ocean regions are processed separately. NCEP Surface Temp. TIR Diff (Land) Diff (Sea)
Processing DMSP ‐ OLS Orbits: Cloud Mask • Thresholds are computed from the difference images in latitudinal tiles as mean+N*stdev. Values greater than this threshold are flagged as clouds. • At high latitudes the land/cloud temperatures converge, so the value of N varies linearly with latitude from N=4 at +/ ‐ 15 to N=1 at +/ ‐ 60. Cloud TIR Diff (Land) Diff (Sea) Mask
Stable Lights Processing: Compositing • The 30 ‐ arc second visible band grids are masked to areas of cloud ‐ free, high ‐ quality nighttime data using the flag N Cloud ‐ free Observations band. • The masked grids are then composited, creating a suite of files including an average visible band image, number of cloud ‐ free observations used, and Average Vis histograms of input visible band data for each grid cell. Histograms are made for each grid cell
Stable Lights Processing: Outlier Removal • Histograms for each output grid cell are analyzed. • Observations are iteratively removed from the top end of the histogram until the standard deviation of the observations has stabilized or >50% of the observations are removed (no convergence) Cell with fires. Process removed highest 10 observations. Cell with town. Process removed highest observation.
Stable Lights Processing: Outlier Removal Average of Visible Band Observations Average After Outliers Removed
Stable Lights Processing: Outlier Removal Avg Vis Before Outlier Removal (F162009) Avg Vis After Outlier Removal (F162009)
Stable Lights Processing: Background Removal Background values vary significantly in the global outlier ‐ removed average image, therefore local background threshold values are computed. 1. Areas known to be light ‐ free are chosen 400 pixels by an analyst. 2. Using the outlier ‐ removed average visible image: a) For each kernel of size 25X25, the 256 400X400 tiles containing this kernel are 25 pixels examined. b) Areas in the kernel with values greater than the maximum light ‐ free values from each tile are tallied as “greater than background”. 3. Stable lights mask is generated as areas considered “greater than background” at least 40% of the time. Tile 003 of 256 Tile 002 of 256 Tile 001 of 256 Tile 016 of 256 Tile 119 of 256 Tile 256 of 256
Stable Lights Processing: Background Removal Light ‐ free areas chosen by an analyst in red Resulting Stable Lights mask
Stable Lights Processing: Background Removal Average After Outliers Removed Average w/ Background Removed
Stable Lights Processing: Align to Landscan Population Grid A cross ‐ correlation technique is used to generate a “best ‐ fit” linear translation between a the Stable Lights grid and the Landscan Population grid. Florida Keys: Before Alignment Florida Keys: After Alignment Florida: Before Alignment Florida: After Alignment Red ‐ >F162009 Stable Lights, Cyan ‐ >Landscan Population Grid
F162009 Global Stable Lights Product • NGDC’s “Stable Lights” products have values which depict the relative intensity of lit areas. • Lighting deemed ephemeral has been removed and non ‐ lit areas (background) have been set to zero.
F162009 Global Stable Lights: Caveats • The Stable Lights Outlier Removal process effectively removes most noise and ephemeral lights such as fires and fishing boats. • Lights from boats and fires which are present in over 50% of the cloud ‐ free observations remain in the Stable Lights product. Avg Vis Avg Vis after Outlier Removal Stable Lights Many lights from These boats are part of Lights from boats boats remain the Stable Lights product
F162009 Global Stable Lights: Caveats • The Stable Lights Outlier Removal process will remove ANY ephemeral light. • Some gas flares, towns with unstable power supplies, and seasonal recreational areas (e.g. ski resorts) will not show up in the Stable Lights product. Gas Flare Removed as ephemeral Absent from Stable Lights Avg Vis Avg Vis after Outlier Removal Stable Lights
F162009 Global Stable Lights: Caveats • Coarse spatial resolution of the OLS 2.7 km GSD 5+ km GIFOV • OLS lights are larger than sources on the ground • “Overglow” surrounds bright sources • No visible band calibration • 6 bit quantization • Urban centers saturate in operational data • No 3 ‐ 5 um band for fires Contrast enhanced to show dim lighting
DMSP Local Times at the Ascending Equatorial Crossing 22:30 21:18 F10 F11 F12 20:06 F13 F14 Time F15 F16 18:54 F17 F18 17:42 16:30 1/1/1992 27/9/1994 23/6/1997 19/3/2000 14/12/2002 9/9/2005 5/6/2008 Date
Version 4b Stable Lights Products • Generated as calendar year products from 1992 ‐ 2009: F10: 1992*, 1993, 1994 F12: 1994*, 1995, 1996, 1997, 1998, 1999, 2000 F14: 1997*, 1998, 1999, 2000, 2001, 2002 F15: 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008* F16: 2004, 2005, 2006, 2007, 2008, 2009 *indicates partial year due to start/end of usable data • Data available online at: http://www.ngdc.noaa.gov/dmsp/downloadV4composites.html
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