Section 6 - Data Search and Access
The USGS archive holds data collected by the Landsat suite of satellites, beginning 1972 with Landsat 1. In 2017, an average of 440 Landsat 7 scenes are added to the USGS archive each day, and become available to all users for download at no charge using the interfaces described in this section. Current Landsat 7 products available include:
- LandsatLook Natural Color Image - a full-resolution, 3-band JPG image (approximate size: 8 MB)
- LandsatLook Thermal Image - a full-a resolution, thermal (Band 6) JPG image (approximate size: 4 MB)
- LandsatLook Quality Image - a JPG image containing unsigned integers that represent bit-packed combinations of surface, atmospheric, and sensor conditions that can affect the overall usefulness of a given pixel (approximate size: 500 KB)
- LandsatLook Images with Geographic Reference - a bundle including the Natural, Thermal, and Quality images in GeoTIFF format (Approximate size: 12 MB)
Level-1 Data Product - a compressed file including all individual multispectral and/or thermal band and metadata files (Approximate size: 400 MB)
6.2 Online Aids
Landsat archive searching and downloading is available through three data visualization viewers, all developed at the USGS Earth Resources Observation and Science (EROS) Center: EarthExplorer, the Global Visualization Viewer (GloVis), and the LandsatLook Viewer. All three viewers allow searches from Macs, Personal Computers, and Unix-based systems.
6.2.1 Earth Explorer
Earth Explorer is the primary viewer for accessing aerial, mapping, elevation, and satellite data held in the USGS EROS archives, which includes Landsat data products. An EROS Registration System (ERS) username and password are required to download data products. Some functions of the website work only after a successful login.
Figure 6-1. EarthExplorer Interface
The Search Criteria tab options allow users to select the geographic area of interest by entering a place name, latitude/longitude coordinates, path/row, shape file, or Keyhold Markup Language (KML) file. On this same tab, the Date Range and Number of Results can be specified (see Figure 6-1).
The Data Set tab lists all categories of data held in the USGS EROS Archives. The Landsat section of this tab lists all Landsat datasets, which are organized by collection and processing level. Next to each dataset are options to link to additional information about the data and to toggle the coverage map on or off in the main viewer window.
The Additional Search Criteria tab provides the opportunity to search by specific scene ID, Path/Row, and set cloud cover limits. Users interested in searching nighttime imagery can select that option.
The Results tab allows users to view a browse image of the scene, open a subset of the metadata file, or verify the footprint of a scene on the map (Figure 6-2). The available data products can then be downloaded by clicking the Download Options button (green arrow with a grey disk). Scenes can also be added to a bulk order by selecting the box icon with a yellow arrow. The shopping cart icon is used to order Web Mapping Service (WMS) links.
Figure 6-2. EarthExplorer Results - Browse Image Display
The Result Controls section, located above the results listing, allows users to view the footprints, browse all results, or add the results to a bulk download order (Figure 6-3).
Figure 6-3. EarthExplorer Results Controls
6.2.2 Global Visualization Viewer (GloVis)
GloVis is a browse-based tool that displays all available Landsat scenes held in the USGS archives.
Figure 6-4. Global Visualization Viewer (GloVis) Interface
Upon opening, the viewer displays the most recent, least cloudy image over the vicinity of the USGS EROS Center. To navigate to a specific area, pan and zoom to the area of interest or use the “Jump to…” feature in the upper right to select a WRS path/row, or enter latitude and longitude coordinates. The user can change the cloud cover limits and other filters by using the faceted search options on the left side of the screen.
Scenes can be selected by using the “Select” option in the lower right corner of the viewer. Once all desired scenes have been selected, they can be viewed in the “Selected Scenes (#)” tab.
From this tab, the user can view scene metadata, order or download scenes, show footprints, choose to bulk download, and remove scenes.
Figure 6-5. LandsatLook Viewer "View Selected Scenes" Display
6.2.3 LandsatLook Viewer
The LandsatLook Viewer allows users to enter a place name or pan and zoom around the globe to a specific area of interest.
Figure 6-6. LandsatLook Viewer
Once the desired geographic location is found, criteria (Years, Days of Year, Cloud Cover, and Landsat Sensors) can be modified to narrow the parameters of the search. This interface displays adjacent scenes more realistically mosaicked, giving a more seamless appearance to the view.
A file of the screen display can be exported as imagery or as a PDF with annotations. Full-Resolution Browse (FRB) image files (LandsatLook images) or Level-1 data products can be selected and downloaded by opening the Show Metadata tab on the bottom of the screen. The ERS login screen appears when the Level-1 data products are selected. Once a user logs in, the item basket appears with options to select the WMS link (order) or add the scene to a bulk download order.
Figure 6-7. LandsatLook Viewer Landsat Imagery Display
Figure 6-8. LandsatLook Viewer Show Metadata tab display
6.3 Landsat ETM+ Scene-Based Browse Image
The land area seen in an ETM+ browse image is not common to all bands (see Figure 6-9). The image planes have been staggered according to the following band order arrangement: 8, 1, 2, 3, 4, 7, 5, and 6. For an image collected on a descending pass, Band 8 covers more land area to the west than Band 1, while Band 1 covers more land area to the east. This band staggering effect can be seen in the browse image above. The browse image Bands, 5, 4, 3 are placed in Red, Green, Blue (RGB) color space. Band 5 reaches furthest to the east as evident by the red margin on the right side of the image. Though not as clear, a blue margin exists on the left side of the browse image due to Band 3 and its more western position (see Figure 6-9).
Figure 6-9. Landsat 7 ETM+ SLC-on Browse Image
When viewing a browse image for geographic coverage it's important to keep the band staggering effect in mind. For example, the panchromatic band starts approximately 500 meters before Band 5 but also ends 500 meters earlier. Knowledge of band offsets will prevent coverage surprises when examining a product for the first time.
Following the Scan Line Corrector (SLC) failure, changes were made to the browse scenes available from EarthExplorer, GloVis, and LandsatLook Viewer. Images acquired after July 2003 display as RGB composites and show Bands 5, 4, and 3. Landsat 7 ETM+ scenes acquired from 5/31/2003 to7/14/2003 and 9/3/2003 to 9/17/2003 are not available due to efforts to resurrect the SLC. Browse images are resampled to a pixel size of 180 meters from the original 30-meter data. The browse image shown in Figure 6-10 displays a Landsat 7 ETM+ image with the effects of the SLC-off gaps. For more information on SLC-Off data, visit the Landsat 7 SLC-off webpage.
Each Landsat 7 scene is color-stretched based on individual scene content. This may result in an apparent mismatch of colors between scenes. The browse image previews are uncorrected images in satellite orientation, and can be viewed in a separate window.
Figure 6-10. Landsat 7 ETM+ SLC-off Browse Image
6.4 International Ground Station Landsat Data Archives
The International Ground Stations (IGS) represent an evolving worldwide network of Landsat 7 data collection and product generation centers (see Figure 6 12). IGS data policies and product types may differ from those in the US archive. While foreign landmasses are imaged for onboard storage and eventually downlinked to US-controlled ground stations, the depth of ETM+ coverage at participating IGS was much deeper prior to the changes in the Landsat Project in 2008. Most, if not all, IGS captured every scene imaged by Landsat 7 within their respective acquisition circles.
The Landsat Global Archive Consolidation (LGAC) effort started in 2010, in order to reduce the need for direct contact with any ongoing IGS. This effort from the USGS has the goal of consolidating the Landsat archives from all stations worldwide to make all Landsat scenes, from all missions, available to users.
Consolidating the USGS Landsat record with IGS data includes several challenges:
- Various data formats and processing methods
- Unknown data formats and processing methods (stations no longer active)
- Media storage age and conditions
Various or obsolete technologies used to ingest data
While every technological asset available will be explored, some data may be irrecoverable.
Figure 6-11. Active Landsat 7 International Ground Stations, as of 2018
This effort to systematically acquire, reconcile, and ingest all recoverable foreign data is estimated to last a number of years. As data are successfully ingested, the Landsat scenes will become available for download at no charge from EarthExplorer, GloVis, and LandsatLook Viewer. More information, including maps of acquired LGAC imagery are available.