README for Version 1 Nighttime VIIRS Day/Night Band Composites Updated 20171218 The Earth Observations Group (EOG) at NOAA/NCEI is producing a version 1 suite of average radiance composite images using nighttime data from the Visible Infrared Imaging Radiometer Suite (VIIRS) Day/Night Band (DNB). The version 1 products span the globe from 75N latitude to 65S. The products are produced in 15 arc-second geographic grids and are made available in geotiff format as a set of 6 tiles. The tiles are cut at the equator and each span 120 degrees of latitude. Each tile is actually a set of images containing average radiance values and numbers of available observations. -- Monthly Composites: In the monthly composites, there are many areas of the globe where it is impossible to get good quality data coverage for that month. This can be due to cloud-cover, especially in the tropical regions, or due to solar illumination, as happens toward the poles in their respective summer months. Therefore, it is imperative that users of these data utilize the cloud-free observations file and not assume a value of zero in the average radiance image means that no lights were observed. The version 1 monthly series is run globally using two different configurations. The first excludes any data impacted by stray light. The second includes these data if the radiance vales have undergone the stray-light correction procedure (Reference). These two configurations are denoted in the filenames as "vcm" and "vcmsl" respectively. The "vcmsl" version, that includes the stray-light corrected data, will have more data coverage toward the poles, but will be of reduced quality. It is up to the users to determine which set is best for their applications. -- Annual Composites: The annual composites are only made with the “vcm” version, which excludes any data impacted by stray light. Further processing is done on the annual products to screen out ephemeral lights and background (non-lights). -- File Delivery and Naming Conventions: The version 1 composite products are delivered as compressed tarballs containing geotiff formatted files. Each geotiff filename has 7 filename fields that are separated by an underscore "_". Internal to each field there can be an additional dash separator "-". These fields are followed by a filename extension. The fields are described below using this example filename: SVDNB_npp_20140501-20140531_global_vcmcfg_v10_c201502061154.avg_rade9h Field 1: VIIRS SDR or Product that made the composite "SVDNB" Field 2: satellite name "npp" Field 3: date range "20140501-20140531" Field 4: ROI "global" Field 5: config shortname "vcmcfg" Field 6: version "v10" is version 1.0 Field 7: creation date/time Extension: avg_rade9h Each monthly tarball contains 2 files, the first one is the average DNB radiance, the second contains the number of cloud-free observations used in the average. They are distinguishable by their filename extensions, avg_rade9h.tif and cf_cvg.tif, respectively. Example: SVDNB_npp_20170101-20170131_00N060E_vcmslcfg_v10_c201702241225.tgz contains: SVDNB_npp_20170101-20170131_00N060E_vcmslcfg_v10_c201702241225.avg_rade9h.tif SVDNB_npp_20170101-20170131_00N060E_vcmslcfg_v10_c201702241225.cf_cvg.tif Each annual tarball contains 6 files, 4 contain average DNB radiance values (extension avg_rade9h.tif), one is the total number of cloud-free observations used in the average (extension cf_cvg.tif), and the final one is the number of total DNB observations regardless of cloud cover (extension cvg.tif). The 4 annual average radiance layers are distinguishable by the config shortname (Field 5). This is shown by example below. Example: SVDNB_npp_20150101-20151231_00N060E_vcm_v10_c201701311200.tgz contains: SVDNB_npp_20150101-20151231_00N060E_vcm_v10_c201701311200.avg_rade9h.tif SVDNB_npp_20150101-20151231_00N060E_vcm-ntl_v10_c201701311200.avg_rade9h.tif SVDNB_npp_20150101-20151231_00N060E_vcm-orm-ntl_v10_c201701311200.avg_rade9h.tif SVDNB_npp_20150101-20151231_00N060E_vcm-orm_v10_c201701311200.avg_rade9h.tif SVDNB_npp_20150101-20151231_00N060E_vcm_v10_c201701311200.cf_cvg.tif SVDNB_npp_20150101-20151231_00N060E_vcm_v10_c201701311200.cvg.tif where: "vcm" (VIIRS Cloud Mask) contains the "vcm" average, identical to the monthly "vcm" average radiance products "vcm-ntl" (VIIRS Cloud Mask - Nighttime Lights) contains the "vcm" average, with background (non-lights) set to zero. "vcm-orm" (VIIRS Cloud Mask - Outlier Removed) contains cloud-free average radiance values that have undergone an outlier removal process to filter out fires and other ephemeral lights. "vcm-orm-ntl" (VIIRS Cloud Mask - Outlier Removed - Nighttime Lights) contains the "vcm-orm" average, with background (non-lights) set to zero. -- Data Types/Formats/Units: To reach the widest community of users, files are delivered in compressed tarballs, each containing a set of geotiffs. Files with extensions "avg_rade9h" contain floating point radiance values with units in nanoWatts/cm2/sr. Note that the original DNB radiance values have been multiplied by 1E9. This was done to alleviate issues some software packages were having with the very small numbers in the original units. Finally, the average radiance files are truncated to the hundredth place, which greatly increased the compression rate of the data. Files with extension "cf_cvg" are integer counts of the number of cloud-free coverages, or observations, that went in to constructing the average radiance image. Files with extension “cvg” are integer counts of the number of coverages or total observations available (regardless of cloud-cover). -- Credit: When using the data please credit the product generation to the Earth Observation Group, NOAA/NCEI. For questions contact chris.elvidge@noaa.gov or kim.baugh@noaa.gov.