On the Weather Channel this morning was a brief explanation of how the Grace Satellites were used to measure drought. The driest areas have less gravity because the soil is less dense. I'm assuming they compare these areas, current to past, to calculate any changes.
If you get a chance, watch it. I'm sure it will show several times.
I thought it was interesting but it also brings up another question. Does a drought change gravity enough to affect the geoid? Do we need to start comparing the date of a geoid with the drought index for that year in specific areas?
Curious minds want to know.
James
> I thought it was interesting but it also brings up another question. Does a drought change gravity enough to affect the geoid? Do we need to start comparing the date of a geoid with the drought index for that year in specific areas?
Watched that too this morning.
Apparently the answer to your question is probably "yes". Although to what extent has yet to be determined.
Gravity Anomaly Maps & The Geoid
were they talking about specific gravity cause that would make sense to me.
They didn't go into much detail. It was sort of a Junior High version of what's going on. They did say that dry ground is lighter than wet ground.
James
The measurements made by the Grace satellite are able to detect large-scale variations in the geoid of 5 to 10mm, from one 30 day estimation of the global field to the next. (cf. http://earthobservatory.nasa.gov/Features/GRACE/page3.php)
For most general purpose surveying, it is doubtful that temporal geoid anomalies make a difference any more than other temporal phenomena do, such as solid earth tides, ocean loading tides and pole tides, among others. It all depends on the nature of one's survey (I.e., the intent of the survey and type of measurement being made) which determines whether geophysical variations need to be accounted for. Few surveyors are engaged in work requiring corrections at this level.
The bottom line is that, typically, a static geoid is used as a reference surface. Defining a time-variant geoid as a reference surface would make things pretty ugly!
The trailing ) got included in the link above and causes it to fail. Try this: http://earthobservatory.nasa.gov/Features/GRACE/page3.php