How to use OpenAltimetry for ICESat-2 data products

OpenAltimetry (OA) is a freely accessible map-based data visualization and discovery tool for exploring surface elevation profiles. With OA, you can see how surface heights change across the Earth over time. Using OA, you can easily visualize data, and download the data in various formats directly from the website. You do not need any special software to access OA data; all you need is a web browser.

Data available through OA

Note: This information is time-sensitive and subject to change.

OA offers access to seven altimetry data products from the NASA Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) mission. These products cover distinct types of elevation profiles from late 2018 to present:

1. ATL03: Global Geolocated Photon Data
2. ATL06: Land Ice Height
3. ATL07: Sea Ice Height
4. ATL08: Land and Vegetation Height
5. ATL10: Sea Ice Freeboard
6. ATL12: Ocean Surface Height
7. ATL13: Inland Water Surface Data

OA also offers access to GLAH06 Global Elevation Data from the Ice, Cloud, and land Elevation / Geoscience Laser Altimeter System (ICESat) mission. ICESat is the predecessor to ICESat-2. ICESat collected data from 2003 through early 2010.

Time required for data access: important!

Please note that OA processes a tremendous amount of data that is archived in NASA’s Earthdata Cloud. Depending on what you select, you may need to wait several minutes for the data to load into the map interface. As we evolve the tool, one of our goals is to improve processing speed.

Differences between ICESat and ICESat-2 data visualizations

Although the OA interfaces for ICESat and ICESat-2 data are generally similar, they have significant differences.

Data offerings:

• ICESat data include one data set, GLAH06 Global Elevation Data with observations from 2003 to 2010.
• ICESat-2 data include seven data sets with observations spanning 2018 to present.

Instrument capabilities:

• ICESat used one beam.
• ICESat-2 uses six beams.

Temporal displays:

• ICESat data show multiple dates.
• ICESat-2 data show one day at a time.

Tool options:

• When the map interface loads for ICESat, the vertical toolbar on the left side of the screen allows you to zoom in and out of the map and reset your selection.
• When the map interface loads for ICESat-2, the vertical toolbar on the left side of the screen offers additional tools, such as the option to load MODIS cloud cover imagery for the selected date.

ICESat-2 data source

The data products visualized in OA are produced from the Advanced Topographic Laser Altimeter System (ATLAS) instrument aboard the Ice, Cloud and land Elevation Satellite-2 (ICESat-2).

ATLAS has a single laser split into six beams and arranged in three pairs to optimally gauge the slope of Earth's surface. The device measures heights across the planet's surface by comparing the time required for a laser pulse to return to the receiver as the satellite orbits the Earth. ICESat-2 emits 10,000 laser pulses per second compared with only 40 pulses per second from its predecessor, the Ice, Cloud and land Elevation Satellite Geoscience Laser Altimeter System (ICESat). ICESat-2 takes measurements every 28 inches as it propels through space.

The underlying data products are also freely accessible from the NASA National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC) in the ICESat-2 collection.

Overview of tools for ICESat-2 data

The OA opening screen for ICESat-2 data looks something like this:

A vertical toolbar of icons runs down the left side of the screen, and a horizontal toolbar appears in the upper right corner. Hovering over any of the icons displays a description of the tool name. From top to bottom, the left-side map controls include:

1. Zoom in/out buttons
2. Zoom in/out slider bar
3. Search or zoom using specific geocoordinates
4. Filter by beam options (including a color-coding legend for the six beams)
5. Filter by track options
6. Calendar toggle that opens/closes a selector to choose the date (By default, the calendar is open.)
7. Cloud icon that adds MODIS imagery with cloud cover to the map for the selected date
8. A toggle to show/hide all ICESat-2 tracks
9. Reset button to restore the default display

From left to right, the top-right map controls include:

1. A globe icon that allows you to choose the map projection: Geographic (global), Arctic, and Antarctic
2. A tags icon that allows you to annotate the map (This requires you to be logged in to your Earthdata account.)
3. An i(nformation) icon that allows you to select from a variety of map base layers
4. An envelope icon that opens a contact form that you can fill out with questions or comments.
5. Below the icons, a select list displays the short name for each available data product. You can hover over each short name to display a summary description or click the “More info” link to see the full list of data product summaries.

Data-access tips

• The default view of OA displays the entire Earth on the most recent date that data are available. At the bottom of the map, you can see the current date selection.
• When the map first loads it will display ICESat-2 ground reference tracks as green lines.
• You can switch between data products by selecting from the data product list in the top right corner of the screen. If you are not sure what the short name stands for, simply hover your cursor over the name to display a summary description of the product.
• You can load MODIS cloud imagery to determine if the data availability for the selected date is limited by cloud cover.
• As you zoom in, available photon beam data will appear as a series of dots along the tracks. Dots are color coded to the six photon beams available from ICESat-2.
• The beams are always the same distance apart from each other. However, the beams can shift from one side of the reference ground track to the other side over time. OA lets you easily see how the beams drift across the reference tracks to better understand where the data points are geographically on any given date.
• Once a beam or grouping of beams is selected, a popup window will appear with data visualization options for elevation profiles and beam heights.
• Interactive visualizations of data graphs typically load in a new window or tab.

Recommended steps for finding the data

To see the data available for the date selected, you must first zoom into a geographic region of interest.

1. Pick the product you want to visualize from the list of available data products in the upper right. Note: ATL03: Global Geolocated Photon Data cannot be selected through this menu, but the data are offered from every elevation profile.
2. Use the map controls on the left to zoom to an area of interest.
3. Once you have zoomed in enough, ICESat-2 reference tracks and ATLAS data points will appear on the map and the Select a Region button will also appear in the top left corner.
   1. To select one data point, simply click on it.
   2. To select a grouping of data points, click the Select a Region button, then click on the map, and drag your cursor to draw a selection box.
4. Once your selection has been made, a popup window will appear with options.
   1. For one data point, you will see an option to View photon data.
   2. For a selected region, you will see more options:
      1. View elevation profile will load plots visualizing the selected elevation profiles with download options listed at the bottom of each plot. If you click the hamburger menu on the top right of the plot, you will see more download options.
      2. View signal photons will load plots visualizing the selected photo heights. download options listed at the bottom of each plot. If you click the hamburger menu on the top right of the plot, you will see more download options.
      3. Close will close the popup window and remove the region selection.

ICESat-2 data-access note: You can only view data for one day at a time. (ICESat-2 takes 91 days to observe the entire planet.) Clicking on a ground track, even when it is inactive, opens a popup window listing dates with available data. Clicking on any of those dates updates the map and allows you to access data.

Tips for working with ICESat-2 elevation profiles

• Moving the cursor over the graph opens popup windows with detailed statistics such as the specific latitude and longitude, beam, and observed height.
• Along the bottom of any graph, you can toggle on or off the display of specific beams. You can also select or deselect all. OpenAltimetry’s default setting is to select and display all.
• Clicking and dragging over a portion of the graph zooms in to those observations, and you can reset the zoom after doing so.
• The three-line hamburger menu in the upper right of each graph offers multiple options for printing or downloading the graph, or for viewing or downloading the underlying data. Additional download options display across the bottom of the graph.
• In the data graphs for ICESat-2, you can switch between elevation profile or photon height displays by choosing the tabs at the top of the charts.
• You can select other observation dates from the date selection box in the upper left corner.

How to download HDF5 files via Harmony

OpenAltimetry uses the Harmony framework to provide access to ICESat-2 subsetted HDF5 data files. From the elevation profile display, at the bottom the graph, choose the option to download subsetted HDF5 using Harmony. Completing this process requires that you be logged in to NASA Earthdata. Once you are logged in, clicking the Harmony link initiates data file creation for the bounding box you have selected. OpenAltimetry will display a notice showing the status of your order. From that status box, click My Orders Page to access your OpenAltimetry orders.

NASA Earthdata account user capabilities

You can use the OpenAltimetry interface without logging in to the NASA Earthdata system, but logging in makes additional features available, including access to full data sets and annotations, and downloading HDF5 files via Harmony. You can add private annotations for your own use and/or search publicly available annotations. Adding public annotations requires privileges granted by the OpenAltimetry team.

To log in: From the menu in the upper right, choose My OpenAltimetry. From the resulting screen, you can log in with your Earthdata login credentials. NASA Earthdata accounts are free and provide many benefits to NASA data users, learn more about how to register for an account.

Additional resources and tutorials

• How to cite OpenAltimetry
• Known issues & updates
• NASA ICESat-2 technical specifications
• OpenAltimetry Tutorial (from 2019; some features have changed)
• Open Altimetry: Advanced Discovery, Processing & Visualization Services for ICESat & ICESat-2 Data (from 2021)