Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from R. Greg Vaughan, research scientist with the U.S. Geological Survey.
There are many satellites that collect data of Earth’s surface and atmosphere, which help us advance our knowledge and understanding of Earth’s interconnected systems. Many of the images acquired by Earth-observing satellites — including those launched and operated by NASA, NOAA, ESA, and the USGS — are freely available to the public. Here, we provide a “how-to” guide for searching, downloading, and viewing satellite data from Yellowstone, using Landsat 8 as an example.
Landsat 8, launched in 2013, is a collaborative mission between NASA and the USGS that acquires visible and thermal infrared images of Earth’s surface. These data are used to measure surface temperatures across the broad expanse of Yellowstone National Park and were used to identify a new thermal area in the east part of the caldera.
One way to search for, view, and download satellite images from Yellowstone (or anywhere else) is by using the USGS Earth Explorer web tool (https://earthexplorer.usgs.gov/). To download data, you have to create an account (which is free), but no login is required just to search for data.
Let’s start the search by defining the search criteria. First, go to USGS Earth Explorer; notice the interactive map that allows you to move around anywhere in the world (by clicking and dragging your mouse or using your finger if on a touch screen). You can also zoom in and out (using the roller on your mouse, clicking the + and – icons in the upper right corner of the map, or using two fingers if you have a touch screen). For this example, move the map to the northwest corner of Wyoming and zoom in a little bit.
There are two primary search criteria: geographic area and date range. To define a search area you can create a point anywhere on the map just by clicking with a mouse or a finger tap on a touch screen. You can create a polygon of any shape or size by defining several points and can edit the location of individual points by clicking and dragging them. You can delete them one at a time by clicking the red X icon, or delete them all by clicking the Clear Coordinates button.
To define the date range, go to the Date Range tab and enter the “from:” and “to:” dates using the calendar icons or by typing them in. Dates must be entered using the format: MM/DD/YYYY.
Next, click the Data Sets button to see the list of all the data sets than can be viewed. There are a lot of them. Just focus on Landsat by clicking the + icon next to Landsat to expand all the options. Again, there are a lot of options. What you probably want are the Landsat Collection 2 Level-1 data. (To understand what the other options are, see the appendix below).
Click on the + icon next to the Landsat Collection 2 Level-1 data, then click the Landsat 8 OLI/TIRS C2 L1 box. OLI stands for Operational Land Imager — this is the instrument that measures surface reflected radiance in visible, near infrared, and shortwave infrared wavelengths. TIRS stands for Thermal Infrared Sensor — this is the instrument that measures surface emitted radiance in thermal infrared wavelengths. C2 = Collection 2; and L1 = Level 1. Next, click the Results button to see what images are available for the data collection, time frame, and area on the ground that you specified.
Search results are listed in the panel to the left of the map. For each scene, there is a thumbnail image, which you can click on to see a larger version of the low-resolution browse image, along with its metadata (detailed information about the image). Why are some of the thumbnails black? Because these are dates when Landsat 8 acquired data at night — the default browse image is a color visible image, which shows no data at night. But if you click on the thumbnail you can scroll to see the thermal infrared browse image. Landsat 8 data are acquired at night over selected volcanic or active geothermal targets, so if you are searching for Landsat 8 scenes over another area, you may only get daytime scenes.
To download data, you can create a free account by clicking the Login link in the upper right of the main page. On the Login page, click Create New Account and follow the instructions to create a username and password. Once you have your account and are logged in, and are viewing your search results, the download icon will be available.
To download one of the scenes, just click on the download icon. A new window will open with lots of options. There are three different types of images, plus the full-resolution scientific product bundle. The Quality image provides a visual description of the quality of the pixels within a scene that may help the user determine its utility. For example, it tags pixels that are cloudy. The Natural Color image is the one that uses Landsat 8’s visible channels to simulate what you would see with your eyes. The Thermal image is a greyscale image that shows the intensity of radiance measured in the thermal infrared channels (bright areas are warmer and dark areas are cooler). The full-resolution scientific product bundle (usually about 1Gb in size) has the full-resolution GeoTIFF images for each Landsat 8 channel, as well as metadata and quality assessment data.
For the Quality, Natural Color, and Thermal images, there are two different file formats available: JPG and GeoTIFF. JPGs are just images. GeoTIFFs are images that contain geographic location information, which can be opened into various geographic information system (GIS) applications or geographic browsers. Some of these data can also be viewed in the online Geology of Yellowstone map.
That’s all there is to it. It might seem complicated, but once you’ve tried it a few times you’ll quickly get the hang of it, and can download a variety of satellite data from all over the world.
With the recent launch of Landsat 9 on Sept. 27, 2021, we will soon have even more data to use for studying and monitoring Yellowstone’s thermal areas.
Appendix: Landsat data types listed in USGS Earth Explorer: Landsat Legacy data are images and image mosaics that were acquired from previous Landsat missions, going back to 1972. Collection 1 data are data that were calibrated in the early stages of the Landsat 8 mission. These data have all been reprocessed using improved calibration techniques and have been replaced by Collection 2 data. Collection 1 data will no longer be available after Dec. 31, 2021. Collection 2 data are the data with the most up-to-date advancements in data processing, algorithm development, improved radiometric calibration and geolocation accuracy, and data access and distribution capabilities. Level-2 data are specific high-level science data products, such as calibrated surface reflectance and surface temperature. Analysis Ready Data are data that have been calibrated and processed to a very high level for scientific analysis in monitoring and assessing landscape change.