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The Importance of Calibration EarthCache

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Geocache Description:

Geocache Description:

For this Earthcache, you will go to a roadcut on Santa Lucia Canyon Road and examine an excellent exposure of diatomite, also known as diatomaceous earth or (DE).

This part of the California coastline is composed of many different kinds of rocks that have been pushed together and uplifted by plate tectonics over millions of years. The extensive outcrop of light-colored rock before you is composed almost entirely of the fossilized remains of diatoms, a single-celled alga that has a cell wall of silica. Researchers working in this area have concluded that this layer, formed at the bottom of an ocean, is between 600 and 800 feet thick and was formed in the middle to lower Miocene age, about 14 to 15 million years ago. For reference, the apes first evolved, arose, and diversified during the early Miocene (Aquitanian and Burdigalian stages), becoming widespread in the Old World.

Diatomite is formed almost entirely of the shells of small aquatic creatures that lived when this area was covered in water. These algae are the only organism on Earth with cell walls composed of transparent silica. They can be found today in almost every aquatic system on earth and produce about half of the oxygen in our atmosphere.

Diatomite is used to filter water, to absorb hazardous materials, even as an agent in kitty litter. It has so many uses that many consumers are only a few feet away from products created with diatomaceous earth. The Imerys company has a mine and processing plant about one mile south of Lompoc which, according to their webpage, is “one of the planet’s largest worked diatomite mines and only marine deposit in Lompoc, California.”

Diatomite—along with other materials like gypsum, silica, and salts— are found in certain regions on the Earth that provide spatially uniform sources of reflected sunlight. These are useful for calibrating satellite instruments including those on Landsat. Examples include:

  • the Sahara and Libyan deserts with diatomite and White Sands in New Mexico covered with gypsum. 
  • Dry lakebeds such as Railroad Valley Playa in Nevada and Roach Lake Playa near the California–Nevada border are also good test sites because, among other reasons, their geologic composition offers a uniform reflectance target. 

Calibrating Sensors in Space

Scientific studies rely on science-grade instruments—instruments that record information reliably and accurately. This reliability is what makes comparisons of Landsat data day-to-day, year-to-year, and sensor-to-sensor possible. Data integrity is maintained by a team of engineers and scientists that constantly monitor and re-calibrate the instrument data throughout its mission lifetime. 

For engineers to calibrate a sensor on a satellite, they need a consistent source of light that is spatially uniform and stable across all spectral bands over time. Examples include sources “on-board” the instrument, such as tungsten halogen lamps, and external light sources such as the Moon or areas on the Earth’s surface such as those mentioned above. 

CAPTION: Scientists collect measurements of light reflecting off the White Sands calibration test site at nearly the same time the Landsat satellite is overhead. These spectral reflectance measurements—along with measurements of atmospheric conditions and sun angle—using a well-calibrated handheld device are compared to the measurements of the Landsat-8 Operational Land Imager (OLI) instrument passing overhead at nearly the same time to determine if a calibration adjustment is required. 

The Moon is also a perfect consistent source of light to use as a calibration source–like a gray card for calibrating a camera’s exposure.  Watch this video about Landsat 8’s Lunar Calibration to learn more about how the Operational Land Imager 2 (OLI-2) on Landsat 9, and its predecessor OLI on Landsat 8, use the surface of the moon for calibration.

The Geologic Point:

This highway road outcrop is diatomaceous earth (DE) and consists of alternating light-colored and rusty, reddish thin layers.  Park at the stated parking coordinates and then proceed carefully across the highway. Pick up a hand sample and look closely at it. Try to crumble a small piece between your fingers.

Please use the messenger service to answer these questions. You may log your find after you have answered the question; you do not need to wait for a reply.

Question 1:
What kind of ecosystem created this thick rock exposure?

Question 2: 
What is a more common name for diatomite?

Question 3:
When you look at the road cut, what characteristics (shape, patterns, texture) is evidence of tectonic activity? 


Question 4:
Pick up a hand sample and look closely at it.  Is the texture fine, or coarse? Try to crumble a small piece between your fingers. What does it remind you of from school? (This word is CODEWORD #9)

Yermoloff, N., & Woodward, A. (1920). The Diatomaceous Earth of Lompoc, Santa Barbara Co., California. Geological Magazine, 57(6), 271-277. doi:10.1017/S0016756800101724

Diatoms of North America Project

Imerys Company,

K.J Thome, (2001), Absolute radiometric calibration of Landsat 7 ETM+ using the reflectance-based method, Remote Sensing of Environment, 78(1–2), 27-38. 

Additional Hints (Decrypt)

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Decryption Key


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