Satellite Image Processing

Artist's rendering of NASA's EO-1 spacecraft, which holds the Advanced Land Imager (ALI) instrument.  Image credit: NASA.
Artist’s rendering of NASA’s EO-1 spacecraft, which holds the Advanced Land Imager (ALI) instrument. Image credit: NASA.

I have a long-standing fascination with metrology, the science of measuring things.* One of my favorite classes as an undergraduate was on chemical instrumentation and computers, where a major topic of discussion was how you take some chemical or physical property and change that into a voltage. That voltage is then converted to a digital format with an analog-to-digital converter.

Think for a moment about a digital camera: how does that work? Light goes in, and when you press the button it saves an image file, but what is happening in between those steps?

Inside the camera, light hits a photosensitive semiconductor, charges are separated, accumulated, and through electronics and circuitry too complicated for this post, converted into voltages. There’s another little tricky thing, though: your camera is usually has four types of photosensitive semiconductors for each pixel. One is sensitive to red light, another green, another blue, and the fourth is panchromatic—sensitive to all colors of visible light.

Satellite imagery can be fascinating, and is often freely available if you can figure out where to find it (free registration may be required). However, unlike getting pictures from your digital camera, by going directly to the source some additional work may be required to turn the images into what you’re looking for. What needs to be done is determined by the instrument and the type of imagery you want.**

For my purposes, I’m generally interested in true-color imagery (or something reasonably close to true-color) of scenes on Earth. Terra MODIS takes images of most areas every day, but the resolution is only 250 m/px at its best. Other satellites, such as NASA’s EO-1, have instruments with better resolution, but they cover much less area—it may be a few days or weeks between images of a given spot.

Today I am interested in images from the Advanced Land Imager (ALI) on EO-1, which can be found through Earth Explorer. I’ve posted images from ALI in the past, which is how I know the images I want are ones I should be able to make.

Lava flow on Heard Island, April 20, 2013. Image credit: NASA Earth Observatory image by Jesse Allen and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team.
Lava flow on Heard Island, April 20, 2013. Image credit: NASA Earth Observatory image by Jesse Allen and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team.

Earth Explorer has an option to download the data as a GeoTIFF, which imports easily into QGIS. Using the layering features in QGIS, the 630–690 nm band (Band 5, red) can be made to grade from black to red, the 525–605 nm (Band 4, green) to grade from black to green additively on top of the red layer, and the 433–453 nm (Band 2, blue) to grade from black to blue on top of the other two layers. Now we have a composite RGB image.

There’s a problem (sort of) with this RGB image, and you’ll see it quickly if you do your processing on the Heard Island imagery from April 20, 2013 shown above: the resolution of your image isn’t as high.

In this case, the three layers used for the composite image have a resolution of 30 m/px. The NASA image, though, has a resolution of 10 m/pixel. Where does this higher resolution come from?

Remember how I mentioned that digital cameras have a fourth sensor, sensitive to panchromatic light? Well, ALI also has a panchromatic band (Band 1), with resolution of 10 m/pixel.

In order to merge the color layers and the panchromatic layer, the color image needs to be scaled up by a factor of 3 in each dimension, making 3×3 pixel areas of the same color. Then some not-that-complicated steps (which I have yet to fully figure out) are needed to adjust the lightness of those pixels—but not the hue—to match the higher-resolution panchromatic image.

*****
* Also meteorology, the science of weather.
** The same holds true for imagery from other instruments or spacecraft, be it New Horizons, the Curiosity rover on Mars, or the Solar Dynamics Observatory.

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Strict Nature Reserve

Tsingy de Bemaraha Strict Nature Reserve (and UNESCO World Heritage Site).  Image credit: Oliver Lejade (CC-BY-SA).
Tsingy de Bemaraha Strict Nature Reserve (and UNESCO World Heritage Site), featuring awesome karst geology as well as lemurs. Image credit: Oliver Lejade (CC-BY-SA).

A recent email from the Australian Antarctic Division about the Heard Island Expedition permit application and plans reminded me that I haven’t spent much time discussing the protections in place for the island. As you might expect for an IUCN class 1a strict nature reserve and a UNESCO World Heritage Site, there are protections, and they are detailed.

The goal of IUCN strict nature reserves is to preserve a natural landscape or ecosystem which would “be degraded or destroyed when subjected to all but very light human impact,” and secondarily, “to preserve ecosystems, species and geodiversity features in a state as undisturbed by recent human activity as possible.” Heard Island definitely fits this category, because apart from whaling and sealing in the late 1800s, there has been very little human activity there. It is home to the Heard Island cormorant, and provides breeding habitat for millions of birds and many marine mammals. Unlike the other sub-antarctic islands, Heard Island has no known introduced species.

In order to protect Heard Island, and as is required by various classifications (IUCN trict nature reserve, UNESCO World Heritage Site) and laws, there is a comprehensive management plan which lays out policies, procedures, and best practices for preserving the integrity of the site.

Here are some illustrative excerpts which demonstrate that a trip to Heard Island is not undertaken lightly.

5.3.8: Visitors to the Reserve must minimise their use of packaging and wrapping material.

5.3.9: Only detergents which are fully biodegradable and low in phosphates may be used in the Reserve.

5.3.10: Polystyrene beads and similar particulate material must not be taken into the Reserve.

5.3.16: Washing water may be disposed of below the high water mark provided reasonable efforts have been made to remove food matter prior to disposal. Such food matter must be handled in accordance with prescriptions 5.3.13 or 5.3.17.

5.4.7b: Prior to departure for the Reserve, all items travelling in the vessel’s cargo spaces or on deck (such as equipment, stores, field accommodation, vehicles, personal gear shipped as cargo) to be taken ashore in the Territory must be hot-washed, disinfected, fumigated, or otherwise treated, and inspected for contaminants which if found must be removed and destroyed.

5.4.10: All outer clothing to be taken ashore in the Territory must be new or thoroughly cleaned and appropriately treated to kill all organisms (including reproductive material) (e.g. with a biocide or similar).

5.4.29b: Footwear to be taken or worn ashore must be thoroughly scrubbed to remove all organisms, soil, and other contaminants (which if found must be removed and destroyed) and must be treated with a biocide [e.g. bleach].

7.1.9: Intending visitors will be provided information that explains the Reserve’s values, the difficulties and dangers of visitation to the Reserve, and the need to apply for permits.

There is a whole lot more there, and it’s interesting to me at least to see what is prescribed to what level of detail. Maintaining a rodent-free ship is HUGE:

5.4.9: The Director [of the Reserve] must be promptly informed of the detection of any rodent on a vessel that is underway to the Territory. The Director will prohibit the entry of that vessel into the Territory unless the Director can be satisfied that the vessel’s entry into the Territory will not result in the escape of rodents into the Territory.

Reading through documents like this, although a little dry at times, helps set a tone for the expedition, reinforces the primary mission of the reserve: conservation. Not every strict nature reserve has such stringent requirements for entry. There’s a strict nature reserve in Madison, WI, (University of Wisconsin–Madison Lakeshore Nature Preserve) which is open to the public without any permitting process, and without thorough cleaning/disinfecting/bleaching being required of any items and clothing being taken into the park.

I’m continuing to get excited about the opportunity to visit Heard Island, which brings me to chapter 7.4 of the management plan:

Our Aims:

  • The enhancement of public awareness and appreciation of the Reserve’s values.
  • The effective use of off-site measures to present the Reserve to national and international audiences

7.4.1d-7.4.1e: A reserve website will be maintained to provide information about the Reserve. It will include maps of the Reserve [d, and e, ] images of the Reserve.

7.4.2: Where practicable, opportunities will be taken to present the Reserve in appropriate public forums.

I hope I’m already doing well on the aims and on 7.4.2, and you can bet that a lot of photography (and some mapping) on the island will be published with Creative Commons licensing.

Where on Google Earth #512

For WOGE 511, we went to the north end of the Piqiang fault in Xinjiang, China, and saw some spectacular satellite-visible structural geology.

Here for WOGE 512, there is more vegetation:
woge_512

To play, find the location on Google Earth, then leave a comment describing a little about the geology/geography/hydrology etc. of the area and why it is interesting. The winner will choose the next location and host WOGE 513 on their blog. Full rules here, tips/tricks, and a KMZ file of past locations are available.

Good luck!