Why space matters: Imaging satellite operations, part 16—“Blue are the streets, And all the trees are too”

Obscure picture reference...
Obscure picture reference…

Who could forget 1999’s most famous dance single?  But does anyone remember the song’s group??

Enough dancing—time to talk about satellite operations until we’re blue in the face (sorry, couldn’t resist).  As mentioned in the previous lesson, this lesson is about DigitalGlobe’s WorldView-2 satellite and the blue spectrum of its imagery payload.  Mainly, what is blue used for, aside from 80’s pop album titles, weird lyrics, and terrible puns?  You’d be surprised–in many regards, Eiffel 65 was right.

According to DigitalGlobe’s literature, blue is “designed for water body penetration, making it useful for coastal water mapping. Also, useful for soil/vegetation discrimination, forest type mapping and cultural feature identification.”  So, maybe the streets are blue—and the trees are blue, too (at least when you’re looking only with the satellites’ blue color band)!

Have you noticed the amount of convoluted jibber-jabber in DigitalGlobe’s descriptions?  I’m confident a few folks working for DigitalGlobe used to work in the government.  Let’s try to translate that jibber-jabber.

While their literature confusingly states blue is “designed,” what DigitalGlobe really means is the satellite image sensor on WorldView-2 is designed to take advantage of the nature of blue light wavelength.  They didn’t do any tampering with blue itself. There is no reinventing light (unless you start talking lasers).   To reiterate the point from my previous lesson “Imaging satellite operations, part 14—making it intelligible,” these image sensors work very much like the digital camera sensors you’ve used.

DigitalGlobe uses descriptors such as “water body penetration” and “coastal water mapping” too.  Remember, different colors of light are absorbed and reflected differently by almost everything.  In this case, the language is meant to describe the absorption (so, that word’s red because of a mistake–blue is reflected by water, which is one of the reasons why you see blue as a main water color–red and infrared are heavily absorbed by water.  Please swap “absorption” with “reflection” –tmsb) of blue light by water and the reflection of blue light by vegetation (trees, grass, etc.) and the ground (soil).  Those two subjects, vegetation and soil, will also reflect blue light differently, which is why you see phrases like “soil/vegetation discrimination” popping up in DigitalGlobe’s literature.

This is significant only because that means the blue part of the camera’s sensor is going to show water as nearly black (again, this is not correct–it shows up as black when red is used) .  But at the same time, blue light will reflect back from the ground and trees right next to the water, so depending on the resolution of the image sensor, it can get a pretty accurate picture of the coastline next to water.

In a real world case, the most recent flooding in Colorado probably was using blue to show where water wasn’t supposed to be.  And it probably worked really well, since there was a lot of farmland, too.  So WorldView-2’s image sensor imaged bits of high reflectance using blue (crops and trees), with other parts showing little reflectance (the flooded areas).  You can see bits of DigitalGlobe’s work on the floods here.

Then contrast their work on the floods to their work on the fires Colorado dealt with in the two years prior to the floods.  You can see the evidence of their work here and this PopSci article for satellite imagery uses against fires.  Blue comes in handy there, too, because it can help show the areas burned/remaining after the fire (the differences in wavelength between the trees and the soil).

There is, however, one thing you should remember about DigitalGlobe’s use of colors to view the Earth:  they are not used normally one color at a time, but in a complimentary fashion.  Blue will almost always be used in concert with one or more of the 7 other color bands on WorldView-2’s sensor.  Because the other colors, used in particular combinations based on DigitalGlobe’s customer needs, will help determine what the image is showing.  And that is why we will get into green and perhaps red, next week.

“Yo, listen up here’s a story

About a little satellite

That lives in a blue world…

…I’m blue da ba dee da ba di…”

4 thoughts on “Why space matters: Imaging satellite operations, part 16—“Blue are the streets, And all the trees are too”

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