LRO Image question

[mooeypoo]

So, NASA released this amazing topographical map of the far side of the moon, based on images from the Lunar Reconnaissance Orbiter (LRO):

 

 

 

(Source: http://www.nasa.gov/mission_pages/LRO/news/lro-topo.html)

 

But this got me thinking. On Earth we define 0-elevation as sea-level. Anything below is negative, anything above is positive. But there's no sea on the moon, or liquid water -- where's the 0-elevation defined, then? Why is this map showing negative elevation values?

 

It just peaqued my curiosity, wasn't entirely sure.

 

Thanks,

 

~mooey

[insane_alien]

they use the average radius of the planet/moon/object.

 

some features will be above and others below.

 

this is because it is hard to find the absolute lowest(or highest) point but relatively trivial to find the average.

[imatfaal]

I must admit I didn't realise the massive changes in altitude on the moon. Just north (?) of the mid line and to the right of centre there is a green crater within a cm (on my display ) of a white crest around a larger crater. According to rough guesses this is a change of over 10km in altitude in a horizontal distance of around 250km - that's one hell of a hole

[John Cuthber]

Have you seen Ben Nevis?

OK, it's only about a seventh as high, but it's only 5 miles or so from the coast.

[IM Egdall]

Super picture of Moon's far side! All the craters makes me think how vulnerable we are to an astoriod collisions, One's gonna hit us eventually. We really do need to develop a program to deflect astoroids.

[mooeypoo]

Makes you wonder how many of those asteroids would've hit the Earth if it wasn't for the moon, too..

[Appolinaria]

Ugh, gross.

[mooeypoo]

Ugh, gross.

 

Gross? The picture?? Really!?

[Appolinaria]

Gross? The picture?? Really!?

 

Eh, Clusterphobia.

 

Super picture of Moon's far side! All the craters makes me think how vulnerable we are to an astoriod collisions, One's gonna hit us eventually. We really do need to develop a program to deflect astoroids.

 

 

 

isnt our atmosphere pretty efficient for the vast majority of stardust crap that's flung at us?

[mooeypoo]

 

isnt our atmosphere pretty efficient for the vast majority of stardust crap that's flung at us?

 

Star dust, yes. What produced the big-ass craters on the moon? No.

[Appolinaria]

Star dust, yes. What produced the big-ass craters on the moon? No.

 

Well doesn't our atmosphere turn those same big-ass meteors into star dust? If we had no atmosphere, I'm pretty sure we would be speckled with craters that size.

 

Also, our planet is bubbling, oozing, replacing old crust with new crust... mending the wounds from God's galactic trebuchet. The moon seems to be pretty dead and unchanging, proudly displaying the evidence throughout time.

[insane_alien]

Well doesn't our atmosphere turn those same big-ass meteors into star dust? If we had no atmosphere, I'm pretty sure we would be speckled with craters that size.

 

what makes you think we aren't? erosion and plants do a damn good job of hiding them, things the moon doesn't have.

[John Cuthber]

Surely the Earth shields the Moon more than the Moon shields the Earth?

[Appolinaria]

Surely the Earth shields the Moon more than the Moon shields the Earth?

 

Gravity, no?