Tag Archives: ALI

Show Me the Data: Satellite Observations

Heard Island, March 27, 2013.  Elephant Spit imagery (at right) is from March 3, 2013.  Image has been adjusted to increase bring out detail in exposed land.  Image credit: processed by Bill Mitchell (CC-BY) using data from NASA/EO-1 (public domain).
Heard Island, March 27, 2013. Elephant Spit imagery (at right) is from March 3, 2013. Image has been adjusted to increase bring out detail in exposed land. Image credit: processed by Bill Mitchell (CC-BY) using data from NASA/USGS/EO-1 ALI (public domain).

With Heard Island being remote and uninhabited, studying it can be a bit difficult. However, as readers of this blog (and my Twitter followers) are aware, one of the ways I have been preparing for the expedition is by keeping an eye on it using various satellites and their remote sensing capabilities. Sure, there is often cloudcover at Heard, but some days it’s clearer and on a few of those days, the satellites pass over.

Most of my information comes from NASA’s MODIS instruments, aboard the Terra and Aqua satellites. These have at least every-other-day coverage of everywhere on Earth, although with a moderate resolution of 250 m/pixel. In the morning when I’m catching up on email and comics, I’ll check the near-real-time MODIS image page to see whether there are clear images of Heard Island from either instrument. Finding Heard Island can be difficult: I still usually find the Kerguelen Islands first, then look to the south-southeast. Many times there are indications such as vortices or gravity waves (not gravitational waves, those are different).

A related page is MODVOLC, which uses MODIS for volcano monitoring. In addition to visible light, MODIS can detect several wavelengths of infrared, and the signature from those wavelengths can be used to determine whether there is a likely volcanic eruption occurring at a given place.

MODIS is a great instrument in that it has daily or every-other-day coverage. However, the 250 m/pixel resolution can be quite limiting. For higher-resolution imagery, I look to the ALI instrument on NASA’s EO-1 satellite. These images are available (free registration required) from EarthExplorer, a data search portal from the USGS. ALI has a 30 m/pixel resolution on its color imagery, and 10 m/pixel resolution on the panchromatic image (total light intensity). These can be combined using QGIS into 10 m/pixel color images. By exploring the EO-1 page I found that members of the public can make requests for image targets! Imaging requests are subject to a bunch of conditions (availability of satellite, >30-day lead time, recommended >3 month window for imaging), but the request and any data generated from fulfillment of the request are free.

How did I come to know about these great resources? It takes time, searching, and some attention to detail. MODIS I learned about as a graduate student, from friends who used data products (not the true-color imagery) in their doctoral research on atmospheric chemistry. I came across EO-1 ALI from searching for images of Heard Island: I found some higher-than-MODIS resolution images from NASA which were good about indicating the source satellite/instrument. Citing image sources is incredibly useful, and I’m always disappointed when images (at least, non-screenshot images) are given without any sort of source information.

MODVOLC I learned about from the Smithsonian’s Global Volcanism Program, which cites the sources of their eruption reports. Information about the source plus a little searching yielded an interesting and useful data source.

Satellite Image Processing Revisited

Heard Island on Nov. 20, 2015, with image processing underway in QGIS.  Image credit: Bill Mitchell (CC-BY) with satellite imagery from USGS (EO-1 satellite, ALI instrument).
Heard Island on Nov. 20, 2015, with image processing underway in QGIS. Image credit: Bill Mitchell (CC-BY) with satellite imagery from USGS (EO-1 satellite, ALI instrument).

Following up on my earlier post about satellite image processing, I am happy to report that I have made progress in being able to process images myself! Through a fortunate combination of search terms, timing, and luck, I managed to come across two key pieces of information that I needed.

First, I found out how to make RGB images from raster data layers, such as different spectral bands on a satellite, fairly easily with QGIS. That was a big step forward from how I had been doing it previously, which was inelegant, inefficient, and only mostly worked. Stacking three layers (one each for red, green, and blue) into a virtual raster catalog was just a few clicks away (Raster | Miscellaneous | Build Virtual Raster (Catalog)).

Encouraged by the success with that project, I continued clicking around and stumbled across some mention of pan-sharpening (also pan sharpening), where a panchromatic (all-color) detector at high resolution is used to enhance the resolution of a colored image (sharpen). Alternately, you can think of it in the complementary way, where lower-resolution color data is added to a high-resolution greyscale image. So thanks to this blog post, I was able to find out what I needed to do to make that happen in QGIS (and Orfeo Toolbox).

Of course, it would be too easy for that to work. I didn’t have the Orfeo Toolbox installed which that needed, and ended up having to compile that from source code.* When the compiler finished and the program was installed, I went to tell QGIS where it was—but a bug in QGIS prevented me from entering the folder location. First, having just installed and compiled stuff, I attempted to get the latest version of QGIS and many of the tools on which QGIS relies. Being unsuccessful in making all of those and some of the compiler configuration software play nicely with each other, I eventually remembered I could get updated packages through apt-get, which gets pre-compiled binary files put out by the maintainers of Debian Linux. That solution worked, I added the folder location, and now I can have my pan-sharpened images.

Here for your viewing pleasure is my first properly pan-sharpened image: Heard Island on Nov. 20, 2015, seen in “true color” by the Advanced Land Imager (ALI) on the EO-1 satellite.** I’m not convinced it’s right, and I think the contrast needs to be brought down a bit, but I think it’s close.

Heard Island in true color on Nov. 20, 2015.  Image processing: Bill Mitchell (CC-BY) using data from USGS/EO-1.
Heard Island in true color on Nov. 20, 2015. Image processing: Bill Mitchell (CC-BY) using data from USGS (EO-1/ALI).

* Knowing how to compile software from source code is a rather handy skill.
** Emily Lakdawalla has written a great explanation of what “true color” means.