Tag Archives: GEDI

A script to find and download GEDI passes

The Global Ecosystem Dynamics Investigation (GEDI) is a spaceborne lidar instrument mounted on the International Space Station (ISS). The GEDI instrument is a geodetic-class lidar with 3 lasers that produce 8 parallel tracks. Each laser illuminates a 25 m footprint on the ground and fires at a rate of 242 times per second. Each footprint is separated by 60 m in the along track direction, at an across-track distance of 600 m between each of the 8 tracks. GEDI’s precise measurements of forest canopy height, canopy vertical structure, and surface elevation will be critical to characterizing and understanding carbon and water cycling processes to further our knowledge of the world we live in: http://www.gedi.umd.edu.

GEDI is mounted on the ISS and therefore its orbit is dictated by that of the ISS. This prevents repeat acquisitions in the same way that Landsat or Sentinel satellite data is collected. While this enables wider coverage, its data acquisition is not consistent. Further to this, GEDI has the ability to point its lasers so that the area on the ground imaged is not necessarily that at nadir beneath the sensor.

To date, GEDI data is available via two means:

Earthdata (https://search.earthdata.nasa.gov/search) provides the data but does not currently provide sufficient visualization so that users can see whether the data intersects their ROI.

Alternatively, the NASA LP DAAC provide the data in list/ftp format (https://e4ftl01.cr.usgs.gov/GEDI/). This allows easy access to all the data but requires that the user knows the orbit that they require. Since February 2020, this has been supplemented by a web tool called GEDI Finder (https://lpdaacsvc.cr.usgs.gov/services/gedifinder). Users pass in bounding box dimensions alongside the GEDI product and version they require and it returns a subset of the list of data that intersects their bounding box. While this is a simple method for accessing data, it requires that a user either manually downloads each one or passes each name to software (wget, curl) to pull the data.

SearchPullGEDI.py was written to make the best of these existing tools and enable the automation of the whole search and download process. It relies heavily on the GEDI Finder tool to search for the data that intersects a bounding box and is, in its simplest form, a wrapper for this tool. It also allows an optional date range to be specified if users are only interested in data collected during a specific time period. SearchPullGEDI.py requires a number of command line options that include GEDI product, version, bounding box, output path and Earthdata login credentials. An overview and example of SearchPullGEDI.py is:

python SearchPullGedi.py -p GEDI02_B -v 001 \
-bb 0.3714633 9.277247 -0.08164874 10.00922 \
-d 2019-01-01 2019-04-30 \
-o /Users/Me/Data/Gedi \
-u MyEarthDataUsername -pw MyEarthDataPassword

whereby:

-p is the GEDI product (e.g., GEDI02_B)

-v is the GEDI version (e.g., 001)

-bb is the bounding box given in UpperLeftLon (maxY), UpperLeftLat (minx), LowerRightLon (minY) and LowerRightLat (maxX). These should be passed to the terminal separated by spaces.

-d is the date range specified in the format YYYY-MM-DD with the start and end data separated with a space

-o is the local path where you want to download the data

-u is your EarthData login username (Note an EarthData account is required)

-pw is your EarthData login password

Running:

python SearchPullData.py -h

will also provide this information.

SearchPullGedi.py contains 3 functions.

  • The first constructs the command line options and passes it to the existing GEDI Finder tool and pulls a list of the GEDI files that are on the html webpage.
  • The second function parses the search results pulled from the GEDI Finder URL and constructs a list where each element is a separate GEDI H5 file.
  • The final function iterates over the list of H5 files and pulls each one using wget software.

SearchPullGEDI.py is available here: https://bitbucket.org/nathanmthomas/bucket-of-rs-and-gis-scripts/src/master/SearchPullGEDI.py

SearchPullGEDI.py requires wget is installed.  On Linux this can be installed through the package manager if not already installed, on macOS you can install this from conda-forge using:

conda create -n gedi -c conda-forge python wget

The command will print the number of files found then start downloading them. Each file is approximatly 1 GB.

Author: Nathan Thomas (@DrNASApants)

Nathan is a UMD Earth System Science Interdisciplinary Center (ESSIC) PostDoc positioned at the NASA Goddard Space Flight Center. Nathan’s research is focused primarily around land cover mapping and characterizing the above ground structure of vegetation, particularly mangrove forests. Through this he uses python to pull, preprocess, calibrate, analyze and display remote sensing info. Some of this code is distributed through his bitbucket: https://bitbucket.org/nathanmthomas/bucket-of-rs-and-gis-scripts/src/master/