- [Cory] This isn't going to end well.
Today we're on a mission
that's a little esoteric.
We're measuring the height of trees
as accurately as we can.
We've got one team out and about in Central Park
and another in Northern California,
home to some of the tallest trees in the world.
This is awful.
We're so uncomfortable.
- [William] Cory is going to be jealous.
- [Cory] It's all for a good cause.
NASA asked us to.
Actually, they asked everyone to.
- [William] It's a really big tree.
- [Cory] They're trying to mobilize an army
of citizen scientists to help double-check
the work of one of their newest satellites.
- [Announcer] We have good indication of
separation of the ICESat-2 spacecraft.
- [Cory] And we're not going to let them down.
That's a good tree.
Forty-two degrees at 50 feet.
The satellite that we're assisting today
is called ICESat-2.
It was launched in September of 2018
with a pretty remarkable goal.
It's measuring the height of everything on Earth.
Land, ice, water, and vegetation like trees.
Every 90 minutes, ICESat-2 completes
a lap around the planet, and along the way,
shoots pulses of lasers down to the surface.
Using the speed of light as a constant,
NASA can calculate the distance
between the ground and the satellite.
This gives them a line of height measurements
wherever ICESat-2 flies, so orbit by orbit,
NASA receives increasingly dense
lines of data that span the globe.
They're just not sure how much
they can actually trust their readings.
Turned into a pretty crappy day.
- I have no idea how we're going to get
far enough away from the trees
to get a good measurement of them, but
we'll cross that bridge when we come to it.
- We're getting involved because NASA
really wants to find out the height of forests.
We know that about 30 percent of the Earth
is covered in trees, but that's a
two-dimensional measurement.
It doesn't give you a sense of
the sheer tonnage of trees out there.
For that, we need volume.
But trees are really hard to measure from space.
When a laser hits a tree,
photons bounce off the canopy,
off leaves somewhere in the middle,
and off the ground.
Scientists have to suss out
which photons bounced off what,
and sometimes the satellite
only gets a single photon back.
The point is, there's a lot of calculation involved
and NASA would love for us to check their work.
We can try this one.
The app NASA built to get us involved
is called GLOBE Observer.
We installed it on our phones
and called NASA up for a walk-through.
- So my name is Holli Kohl.
I am the project coordinator for GLOBE Observer.
- [Cory] NASA's app is a part of
a broad program to engage the public
with environmental sciences.
With the app, anyone can take measurements
of land cover, clouds, mosquitoes, and tree height.
All those measurements have different uses
for different researchers, but for trees,
it's about matching the data we're collecting
with what ICESat-2 sees
when it flies over that same area.
- The satellite does have global coverage,
so at some point in the future,
we'll fly over and then fly over again
and again and again.
As we collect more and more data,
we're providing a wider database
for those ICESat-2 scientists to draw from
as they start to validate
what their satellite's telling them.
- [Cory] The app is pretty user-friendly.
First, you tell NASA how tall you are.
That'll come in handy later.
Then, pick a tree.
- Okay, we're going to try to record
the height of this very tall tree.
- [Cory] To measure a tree, you hold the phone
at eye height, point the camera at
the base of the tree, and take a picture.
Then, pivot the phone back without changing
its position in front of you,
and snap a photo of the treetop.
Once you have both photos,
the app will ask you to walk from
where you're standing to the base of the tree
and record your number of strides.
- It's going to be a bit of a hike.
One, two, three.
- [Cory] Four, five, six, seven, eight.
- [William] 52, 53, 54, 55.
- [Cory] 15, 16, 17, 18.
- Okay, I call it 73.
- [Cory] This gives the app everything
it needs to estimate the tree's height.
49.3.
Now how did the GLOBE app turn a couple of
photos and steps into a height measurement?
Well, our younger viewers
might have figured this out already.
- If you think back to high school,
you'll remember that if you know
the length of one side of the triangle
and then the two angles, the right angle
and then one other angle,
you can calculate the length of the other side,
and that's what we're doing.
- [Cory] Basically, the app is doing trigonometry.
Picture a right triangle where side A is the tree.
The app guesses side B, the ground,
based on how many steps you take.
And it uses your phone's gyroscope
to measure the angle opposite the tree.
That's enough to calculate the tree's height,
which is side A of the triangle plus your height.
Just for fun, we checked the app's work
with a clinometer, pen and paper,
tape measure, and calculator.
A little bit of shaky math later
and we had an answer that exactly matched
the average of our three readings.
It's 50.3.
Will's going to get a redwood that's like 300 feet tall.
- So the calculated tree height
is 292 feet and 4 inches.
The actual height is 286 feet.
That is pretty darn close.
- [Cory] Back in the office, we saw that
our measurements were already published
to the GLOBE website for anyone in the world
to see and use, but we wanted to know what
scientists might do with our data,
so we called one up.
Paul Montesano, a research scientist at NASA.
Paul studies forest height and density
in places like Alaska and Siberia.
- And I study these forests
primarily using remote sensing data,
data collected from satellites or airborne platforms.
- Paul says that GLOBE app data
can check more than just ICESat-2.
He relies on airborne flyovers of forests,
which use a similar laser measurement technique
to paint incredibly detailed pictures of tree heights.
And he measures trees on the ground himself,
and uses satellite data, too.
If it sounds like a lot of overlap, that's the point.
- If you only have one measurement,
you don't know how wrong you are.
All measurements are off, right.
A measurement is just an estimate.
- We're not under any delusions.
Our own measurements are tiny, tiny drops
in a huge bucket of data
that scientists like Paul use.
But by mashing up all the available data,
scientists can build more and more accurate pictures
of forests around the globe.
- We'll have this multitier system of
scaling up our most accurate and precise measurements
from the ground to the satellite platform.
- Which brings us to the final question: why?
Why catalog all the trees
and all the forests in the world?
There are a lot of reasons, but many
boil down to one word: carbon.
- One of the overarching goals, I guess,
of Earth scientists today
is to understand the carbon budget.
- Human activity produces a lot of carbon dioxide,
which alters the overall carbon budget,
the net flow of carbon through
different areas of the planet.
There's a big pool of carbon in the atmosphere,
another dissolved in the oceans, and on land,
gigatons of carbon are found in forests.
Maybe half of a tree's dry weight is carbon.
If scientists can figure out how much tree matter
there is on Earth, they can guess at
how forests will change the amount of carbon
in the air, as humans pump more in.
- Are these forests serving as
a source of carbon to the atmosphere,
or are they serving as a sink of carbon,
sucking up more carbon than they're giving off?
We're interested in the net exchange of carbon
from the terrestrial pool.
- Those are big questions,
but finding answers means zooming way in,
from satellites sweeping the forests of the world,
all the way down to individual trees,
and measuring them, sometimes one by one
just like we did.
- I think I've roughly measured
either the diameters or the heights
of maybe about 15,000 trees in my life.
It's a lot of work.
- Hey guys, if you download
the GLOBE Observer app yourself,
Holli and Paul have a couple of requests.
First, measure your stride length,
and second, don't forget to measure
the same tree a couple of times.
Those things will help improve your accuracy.
Thanks.