ESA's GOCE Satellite Provides Gravity Map of Earth 119
kaulike writes "The European Space Agency's GOCE satellite, launched in March 2009, has provided a spectacular, highly detailed map of our favorite gravity well. This map shows the normalized surface of the earth as defined by gravity, showing the relative altitude differences from the average for each surveyed point. The article provides the helpful metaphor that a ball resting on this surface would not roll anywhere, even though there would be visual slopes, as gravity is equalized across the globe. There is a fascinating deep area in the Indian ocean (-100M) and a high area near Iceland (+80M), proving conclusively that our world is not homogeneous in terms of density (or practically any other measure). Does anyone know whether these anomalies correspond to known geographic phenomena? Deposits of heavy metals perhaps, or hotspots where the mantle is thinner? I know little about geodetic stuff, but I'm curious about the reasons for wrinkles in the data set."
Re:Where's a 1920x1200pxl Image? (Score:0, Informative)
some people have a taste for pixel accuracy
Re:Map of our favorite gravity well (Score:2, Informative)
Not quite full HD but still is a higher res: 1081 x 541
http://www.usna.edu/Users/oceano/pguth/website/so432web/ww15mgh.jpg [usna.edu]
The U.S. military already has one of these (Score:4, Informative)
The U.S. military already has one of these
It's used in inertial navigation for weapons systems. Interestingly, the inertial navigation software itself is available as source code for download, but the data of the map itself is classified to prevent its use by non-U.S. aggressors. Also, for what its worth, the military data resolution is far better than the 100km between data points, as it is with GOCE, but is the resolution falls off on non-projected weapons trajectory route splines.
See also the geoid from the earlier GRACE observations (animated spinning globe) which were 322km resolution, along with a more technical discussion of GOCE:
http://www.scientificblogging.com/planetbye/grace_goce [scientificblogging.com]
-- Terry
Re:cool (Score:5, Informative)
The map is showing the deviation from an ideal spheroid that would result in the observed gravity variation. So, positive meters basically means that if the Earth were made of stuff of a uniform density, the surface of the earth would be this many meters above the ideal surface (gravity is weaker here than expected). Conversely, negative meters means gravity is stronger here than expected, and so correlates to a "low" (low elevation being closer to the center of mass of the earth, meaning stronger gravity).
The map is essentially showing what the surface of the earth would look like if all variation in gravity (what they observed) was due to variation in the shape of the earth, rather than density. At least, I think that is what they are showing - I don't think the article actually states if this is raw data or if it has been processed (to apply a free-air correction, for example, which would remove variability due to the actual variations in elevation of the earth's surface).
Make sense? It isn't as complicated as my half-assed explanation might make it seem (well, it is complicated, but the concept is simple).
Re:cool (Score:3, Informative)
No, because that image shows one how height above the ellipsoid (not spheroid as someone said above) model of the earth (as used in GPS / etc.) minus the geoid model deviation (what these satellites are, in effect, measuring) equals your orthometric height (what most people call "elevation").
I don't believe this helps the GP's query, because it doesn't explain what the geoid model is.
What most people never think about is that old-school surveying (with a conventional level) always, even before we understood that the earth's gravity varied, compensated for the variation. A surveyor's level, be it an old one where the circular plate is made level with a spirit bubble or a newer one where the recticle floats and self-compensates, is always looking perpendicular to the line between it and the center of gravity. Everyone always assumed, back in the day, that this really meant the center of the Earth, but the positive side effect is that elevation measurements "carried" with a surveyor's level were always compensating for these differences in gravitational potential.
So much so, that all the major geoid models used to this day are heavily based upon high-precision GPS observations (of height above the ellipsoid) on benchmarks possessing elevations leveled in.
Exactly (Score:3, Informative)
Exactly... "the resolution falls off on non-projected weapons trajectory route splines".
You get very good data for the areas in which you want to fly your birds, and lesser data for where you don't expect to do that. This is necessary to, for example, use inertial guidance rather than active TFR in a cruise missile and keep it below the enemy radar.
-- Terry
Some background: The Satellite Itself (Score:5, Informative)
It's worth taking a read [bbc.co.uk] of the satellite itself. Apparently, the accelerometers themselves (3 pairs of them) are mounted to within one picometre (that is micro-micro-metre). Gravity measurements are to within 10^-13 G. All pushed ahead by a cool xenon ion engine :)
That's some serious engineering precision. A bit more than your average accelerometer [sparkfun.com] in your iPhone.
There's a bit more on how it works in this article [bbc.co.uk].
Of course, the raw data looks a lots uglier [bbc.co.uk] than the beautiful image of the final result, but if the research is for climate change, then manipulating raw data is what they do best ;)
high points are high gravity. (Re:cool) (Score:2, Informative)
Think of it another way... The observed 'center' of gravity is always perpendicular to the slope of the geoid.
Thus consider a piece of slope tilted like a forward slash ( / ) The gravity would have to be stronger on the right hand side to hold the ball flat against the slope... Thus the gravity is stronger on the 'high' side, and weaker on the 'low' side.