Tag Archives: Terra

Cloud-Free Heard Island

Composite, cloud-free satellite imagery of Heard Island, being produced in QGIS.  Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8, EO-1) data (public domain).
Composite, cloud-free satellite imagery of Heard Island, being produced in QGIS. Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8, EO-1) data (public domain).

Heard Island is a pretty cloudy place most of the time. However, there are occasional times when the weather clears, particularly on the southeastern (leeward) side of the island. On rare occasions, the northwest and southwest sides of the island come out from the clouds as a satellite passes over.

For the past two years, I have been watching Heard Island using true-color imagery from four satellites: Terra, Aqua, Landsat 8, and EO-1. I have posted previously about satellite imagery from these instruments. Although every image of the Island I have seen has clouds in it covering a portion of the island, I was curious whether or not I had accumulated clear imagery of the entirety of Heard Island.

In part, this question was spurred by a follower on Twitter asking about eruptive activity at Heard. I had to admit I didn’t really know whether the activity was low-level and continuous (like Kilauea) or more intermittent. Given that our knowledge of its eruptive activity is primarily from satellite observations, do the satellite “thermal anomalies” correspond to short eruptive events, or simply a cloud-free view of the volcano?

For high-resolution imagery of Heard Island, the datasets of interest are from EO-1 ALI, and Landsat 8 OLI. The two MODIS instruments (one on Aqua, one on Terra) are moderate-resolution, and while 250-m resolution is sufficient for some purposes, this one needs more. Looking through the archives, I was able to find EO-1 ALI data primarily for Mawson Peak and points southeast, and Landsat 8 OLI covered much of the island, particularly the northwest.

Not only is having cloud-free, high-resolution data important for me, but I want the data to be recent. There has been a retreat of up to 5.5 km for some of the glaciers since 1947, and the Google Maps imagery of that area (Stephenson Lagoon) is horribly outdated. Fortunately, I found most of the island covered in large swaths with images from 2014 onward, and mostly 2016. There was even good imagery from when I was on Heard Island! Our ship, the Braveheart, is visible as a few white pixels in Atlas Roads (just north of Atlas Cove), slightly closer to the Azorella Peninsula than to the Laurens Peninsula. The tents and campsite are too small and darkly colored to be visible on this image.

Braveheart in Atlas Roads and the campsite (non-contrasting) at Atlas Cove, Heard Island.  Satellite image pixels are 15 m across, and the Azorella Peninsula isthmus (along Campsite label) is 1 km wide.  Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8 OLI) data (public domain).
Braveheart in Atlas Roads and the campsite (non-contrasting) at Atlas Cove, Heard Island. Satellite image pixels are 15 m across, and the Azorella Peninsula isthmus (along Campsite label) is 1 km wide. Image credit: Bill Mitchell (CC-BY), using USGS (Landsat 8 OLI) data (public domain).

A small portion of the island between Atlas Cove and Mawson Peak was the most difficult to find. With the topography of the island, the steady stream of wind, and the humid air, the 2.5 km by 2.5 km region was cloudy pretty much all the time. Eventually, using the EO-1 ALI instrument and going back to early 2010, I found a reasonably clear image of it.

Once I had the images (after combining true-color and panchromatic brightness data in QGIS), I needed to stitch them together. Thanks to the wonderful QGIS training manual, I was able to create vector (polygon) layers which corresponded to the clear region of each image (plus some surrounding ocean). At this point the troublesome mostly-cloudy spot became evident, and the search was on for imagery to fill the void.

Creating polygons for clipping the satellite imagery using QGIS.  Four polygons are shown here, including the small polygon of much cloudiness.  A fifth dataset was subsequently incorporated.
Creating polygons for clipping the satellite imagery using QGIS. Four polygons are shown here, including the small polygon of much cloudiness. A fifth dataset was subsequently incorporated.

Finally, I tried to put them together. This turned out to be more trouble than it was worth for my purposes, having only five images. Several of the images had differing resolutions (10 m/pixel for EO-1 ALI, 15 m/pixel for Landsat 8 OLI). Additionally, since I was handling these in their raw format, color balances/exposures were not consistent across images. I decided it best, then, to leave them separate, and sent them around to the Heard Island Expedition team.

Soon I had an email from the expedition leader: he was very interested in the imagery, but it wasn’t opening in Google Earth. Some searching later, I found that Google Earth works best with a certain map projection (EPSG:4326), and when exporting the GeoTIFF, I needed to select “rendered image” rather than “raw data”. I re-exported the images, zipped them up, and tested it out on another computer: success! This Google Earth friendly imagery is now available here (17 MB zip).

One continuation of this project would be to keep looking through the documentation on GeoTiffs to find out how to make the rendered images use a transparent, not white, border where there is no data. That would likely let me create a virtual raster catalog to load all of them in one go, rather than having to load them separately.

Geoscientist’s Toolkit: Terra MODIS

Terra satellite being prepared for placement in the payload fairing.  Image credit: NASA (public domain).
Terra satellite being prepared for placement in the payload fairing. Image credit: NASA (public domain).

In my previous post on satellite communications, I discussed two types of satellites: geostationary and low-Earth-orbit. One of NASA’s low-Earth-orbit satellites, orbiting at an altitude of 705 km (438 mi), is Terra.

Launched in December of 1999, Terra is in a polar orbit, and is sun-synchronous—it makes its north-to-south pass on the daylight side of Earth, crossing the equator around 10:30 AM in the local time zone. As 10:30 AM moves around the Earth, so too does Terra, with each orbit taking 99 minutes.

Aboard Terra is one of my favorite instruments: the MODerate resolution Imaging Spectroradiometer, or MODIS. Unpacking the name, we find that MODIS has moderate resolution: its best resolution is about 250 m/pixel. It is an imaging instrument (i.e. it sends back pretty pictures), and it is a spectroradiometer, meaning that it measures the amount of light (radiometer) across a spectrum of wavelengths (visible and infrared, in this case). Most of my use of the instrument is for its true-color imagery, or “Bands 1-4-3” (corresponding to red, green and blue). An example image is shown below.

Minneapolis area seen by NASA's Terra satellite Sept. 30, 2015.  The Minneapolis and St. Paul airport is the concrete-colored smudge just left of center; St. Cloud is in the upper left, Winona toward the bottom right, and at furthest bottom right is La Crosse, WI.  Image credit: NASA (public domain).
Minneapolis area seen by NASA’s Terra satellite Sept. 30, 2015. The Minneapolis and St. Paul airport is the concrete-colored smudge just left of center; St. Cloud is in the upper left, Winona toward the bottom right, and at furthest bottom right is La Crosse, WI. Image credit: excerpt from NASA imagery (public domain).

MODIS is a push-broom type imager. It takes one very wide “picture” (2,330 km East-West), and splits that into 36 spectral bands. As the spacecraft flies (North-to-South), those wide “pictures” are put together along the track of the satellite to create a swath image. The instrument’s resolution is highest at the center of the image.

One great thing about MODIS is that it has pretty good spatial coverage (that’s the advantage of the moderate resolution). In 24 hours, it will get images of most of the Earth, but with a few gaps between swaths at the equator. Orbits are offset day-to-day (with a 16-day cycle), so it takes two days to get full global coverage. Global maps are produced daily (give or take) by NASA Earth Observations, and tend to have a day or two of lag behind real-time.

Terra MODIS image of Earth, Oct. 7, 2015.  The tan-grey streaks in the center of the swath over some equatorial regions is caused by glare from the sun reflecting off the ocean surface.  Image credit: NASA Earth Observations (public domain).
Terra MODIS image of Earth, Oct. 7, 2015. The tan-grey streaks in the center of the swath over some equatorial regions is caused by glare from the sun reflecting off the ocean surface. Image credit: NASA Earth Observations (public domain).

You may notice that in the picture of the whole world, Antarctica is nicely lit up, but the data for the North pole is missing? What’s up with that? Is NASA taking part in a conspiracy with Santa to hide his gift-production and distribution facilities?

In a word: no.

In more words, having recently passed the September equinox (autumnal equinox to folks in the northern hemisphere), the North pole is now in darkness at 10:30 AM “local time”. It doesn’t really matter what you choose as local time, because it’s dark regardless. With it being dark, the instrument is off.

Beyond pretty pictures, Terra MODIS is used for scientific purposes. Its images can detect wildfires,[1] be used to estimate area burned by fires, monitor drought severity and snow cover, study aerosols and atmospheric pollutants, and even chlorophyll (phytoplankton) concentrations in the ocean.

Using the images to understand the productivity of plants can in turn influence the estimates for how much carbon is being removed from the atmosphere, and can serve as a gauge of ecosystem health in remote areas. Volcanic eruptions, major wildfire events, and even thick pollution from human sources can be seen in these images. By analyzing MODIS data, scientists can gauge how much of various types of atmospheric gases are being emitted by wildfires.[2, 3]

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[1] Near-real-time swath data are available from the Rapid Response website.

[2] Mebust, A. K., Russell, A. R., Hudman, R. C., Valin, L. C., and Cohen, R. C.: Characterization of wildfire NOx emissions using MODIS fire radiative power and OMI tropospheric NO2 columns, Atmos. Chem. Phys., 11, 5839-5851, doi:10.5194/acp-11-5839-2011, 2011. [Open access]

[3] Mebust, A. K. and Cohen, R. C.: Space-based observations of fire NOx emission coefficients: a global biome-scale comparison, Atmos. Chem. Phys., 14, 2509-2524, doi:10.5194/acp-14-2509-2014, 2014. [Open access]

Compton Glacier Calving Seen from Space

Heard Island on a clear morning, seen by the MODIS instrument on NASA's Terra satellite.  July 31, 2015.  Image credit: NASA GSFC (Terra/MODIS).
Heard Island on a clear morning, seen by the MODIS instrument on NASA’s Terra satellite. July 31, 2015. Image credit: NASA GSFC (Terra/MODIS).

July 31st was a remarkable day on Heard Island, for several reasons. First, the weather was clear—a rare event in itself. Second, both NASA’s Terra and Aqua satellites had Heard Island reasonably near the center of their swath images. That’s not super-rare, but it’s probably <25%. Third, not only was the weather clear, but it was clear for both satellite overpasses, so both Terra and Aqua had good views of the island.

Many days, as I check the satellite images to see if Heard Island is visible, I end up playing “where in this image is Heard Island”. Imagine my surprise when I saw the Terra MODIS preview image from the morning pass, and there was a nice, bright white spot with some swirling grey vortices pointing toward it. The full-resolution image is shown above (cropped). It’s exactly the charismatic image I watch for, even though the resolution is moderate.*

I scrolled down the page to the Aqua MODIS images, which come from the early afternoon. Although Heard Island was a little off to the side of the image, leading to some artifacts, it was still free of the usual obscuring clouds. What a day! Two great images from when the island was within the usable part of the MODIS swaths.

Heard Island, standing in stark contrast to the dark blue waters of the Indian Ocean, July 31, 2015.  Image credit: NASA GSFC (Aqua/MODIS).
Heard Island, standing in stark contrast to the dark blue waters of the Indian Ocean, on the afternoon of July 31, 2015 as seen by NASA’s Aqua satellite. Image credit: NASA GSFC (Aqua/MODIS).

As I looked more closely, I noticed something odd about the afternoon image: Compton Lagoon, in the northeast corner of the island, had a very odd shape. Usually it looks rather like it does in this map from the Australian Antarctic Division:

Topographic map of Heard Island, published July, 1999.  Compton Lagoon is prominent in the northeast.  Image Credit: Australian Antarctic Division.
Topographic map of Heard Island, published July, 1999. Compton Lagoon is prominent in the northeast. Image Credit: Australian Antarctic Division.

Let’s look more closely at the satellite images.

Heard Island, morning of July 31, 2015. (Terra MODIS, as above; annotations mine).
Heard Island, morning of July 31, 2015. (Terra MODIS, as above; annotations mine).
Heard Island, afternoon of July 31, 2015.  (Image from Aqua MODIS, as above; annotations mine).
Heard Island, afternoon of July 31, 2015. (Image from Aqua MODIS, as above; annotations mine).

Some of the difference between images comes from the North Barrier ridge, which runs from high up the volcano down to the west of Compton Lagoon, bounding the Compton Glacier to the northwest. With the sun in the northeast in the morning and northwest in the afternoon, the ridge stands out much more in the afternoon when it casts a shadow on the light glacier.

The lagoon, however, is quite different. Much of what was blue lagoon in the morning is grey in the afternoon, and the glacier seems to be a bit darker grey near its toe. I interpret that as evidence for a significant calving event, where ice, snow, and rocks from the glacier break off and slide/fall into the lagoon. A wind from the northeast (evidenced by the clouds) helps to keep the floating ice toward the west end of the lagoon.

Of course, it would be nice to have a second image showing the ice floating around in the lagoon, or a higher resolution image of the glacier. Unfortunately, since these images were taken, the images have been cloudy and/or off to the side of the field where distortion and artifacts are at their worst. I was hoping that the EO-1 satellite or Landsat 8 would get a good image with their 30 m resolution, but that doesn’t seem to be the case. That just goes to highlight how incredible these images are!

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* That’s the MOD in MODIS, the MODerate resolution Imaging Spectroradiometer; at its best (directly beneath the satellite) the resolution is 250 m/pixel.