With most sources of satellite remote sensing data, spatial resolution and temporal resolution are inversely related. In other words, as spatial resolution increases, temporal resolution decreases and vice versa. For example, the VIIRS sensor onboard S-NPP and NOAA-20 has a swath width of 3,000 kilometers and each instrument images the entire surface of the Earth at least once daily. The swath width of MODIS onboard Terra and Aqua is narrower at 2,330 km providing slightly less than daily global coverage. However, the finest spatial resolution of active fire detection data that can be gleaned from VIIRS and MODIS at 375 meters and 1 kilometer, respectively, are considered coarse resolution data by most definitions.
In comparison to VIIRS and MODIS, the Operational Land Imager (OLI) onboard both Landsat 8 and Landsat 9 provides active fire detections at relatively finer spatial resolution (30 meters) (please see the related NASA FIRMS blog entry that introduces the Landsat active fire product). Unlike VIIRS and MODIS, OLI has a much narrower swath width of 185 kilometers. The swath of OLI is less than 1/10th the width of a MODIS swath and provides much lower temporal resolution/sampling by comparison (Fig 1).
Figure 1 - Example of a 185 km Landsat 9 OLI swath (red) compared to a 2,330 km Terra MODIS swath (orange). Terra MODIS can easily image the entire continental US daily in three consecutive orbits. However, Landsat 8 and Landsat 9 OLI each require multiple orbits over 16 days to provide the same coverage.
How do these characteristics affect the daily availability of active fire data?
The relatively large swath widths of VIIRS and MODIS sensors provide comprehensive satellite observations and derived active fire detection data for the entire globe on a daily basis. Additionally, both sensors provide multiple observations daily within a relatively short period of time where consecutive swaths may overlap, more so at higher latitudes (Fig 2).
Figure 2 - Example of MODIS and VIIRS orbit tracks showing overlap between tracks, especially at higher latitudes.
Conversely, each Landsat OLI sensor requires 16 days to completely image the surface of the Earth. So, OLI cannot fully image large geographic areas in one day, such as the continental United States, and can only provide partial coverage. However, since the orbits of the two Landsat satellites are 8 days out of phase, the combined temporal resolution of the two OLI sensors enables complete coverage once every 8 days (Fig 3).
Figure 3 – The top graphic provides an example of the swath coverage for the continental US by OLI onboard Landsat 8 (orange) and Landsat 9 (red) on July 13, 2022. Orbits for each satellite move to the east and west daily and complete a full cycle of coverage every 16 days. The bottom graphic displays the HLS imagery product in FIRMS for July 13, 2022, which contains the acquired Landsat 8 and Landsat 9 imagery over land. Areas along each swath that exceed the predetermined cloud cover threshold are excluded from HLS product in FIRMS. The combined daily coverage of OLI on both Landsat satellites can image the entire continental US, and the globe, every 8 days.
When viewing daily active fire detection data in FIRMS, users should be aware of the high temporal resolution characteristics of VIIRS and MODIS versus Landsat. In Figure 4, active fires detected by Terra MODIS and Landsat on September 10, 2022, for the Pacific Northwest are provided. Landsat active fire detection data, displayed as yellow points, is only available within the 185-kilometer OLI swath. However, MODIS active fire detection data, displayed as red points, is available for the entire region within and outside of the OLI swath. Consequently, updates to the finer resolution Landsat active fire detection data for the entire Earth cannot be provided daily.
Figure 4 - Active fires detected in the Pacific Northwest on September 10, 2022. 1 kilometer MODIS active fire detections (red) are available for the entire region while 30-meter Landsat 9 active fire detections (yellow) are limited to the 185-kilometer swath (WRS-2 Path 42) acquired on this date.
The NASA Worldview Snapshots tool enables users to easily create satellite imagery subsets for anywhere in the world. The tool provides access to current and historical imagery captured by Terra and Aqua MODIS and S-NPP and NOAA-20 VIIRS. By simply selecting the desired image product, acquisition date, spatial resolution, output file type as well as entering the latitude/longitude bounding box, users can quickly generate an imagery subset to use for visualization in a GIS or as a graphic in a presentation or document.
For the convenience of users who need to create satellite imagery subsets of geopolitical units, Snapshots has recently been enhanced to include predefined latitude/longitude bounding boxes for countries. After setting the basic parameters for an imagery subset, users can simply select their desired country of interest from the Countries dropdown list in Country/Region Presets box below the map. The red box that subsequently appears on the map indicates the bounding box of the subset to be generated. Users can also increase the default size of the subset boundary extent by 5% to 20% by selecting the desired 5% interval under the Padding dropdown list or manually edit the predefined coordinates as desired in the Bounding Box section of the interface.
Users can also generate imagery subsets for a US state or Canadian province or territory. After selecting either of these two countries, a user can select the State / Province / Region dropdown list to update the subset bounding box extent to the targeted state, province or territory.
A September 9, 2022 Aqua MODIS true color composite image for Oregon generated from NASA Worldview Snapshots. The subset bounding box is defined by the map extent of the state, predefined in Snapshots, and extended by an additional 5% in both dimensions.
A screenshot of the Worldview Snapshots definition for an imagery subset of the state or Oregon. The updated interface enables users to select a country (as well as a US state and Canadian province/territory) and use its associated map extent as the bounding box coordinates for the subset boundary. If desired, users can increase the size of the predefined bounding box in 5% intervals or edit manually edit the coordinates.