Flutterby™! : gravity model 2006-10-24 17:18:36.978483+00

gravity model

2006-10-24 17:18:36.978483+00 by Dan Lyke 2 comments

One of the things I'd never thought about before I did some playing with mapping, projections and geographic datums, was how hard it is to pinpoint a geographic position. I mean, you have "down", and that points to the center of the earth, right? Lattitude is a simple matter of measuring that angle to something else that's relatively consistent, and longitude requires a clock, but how hard can it be?

Well, to begin with, you don't really have "down", as this illustration of the lumps in earth's gravity shows. Worse, as glaciers and ice sheets melt, "down" is changing. (via JWZ)

This is also why (and I think I've mentioned this before) you use a licensed surveyor to provide legal delineation of your property (i.e., you can't just use GoogleEarth or GPS). Surveyors use techniques that haven't changed in over a century to describe your property. That's because the actual technique used to mark the property lines doesn't matter! What really matters is that the surveyor is licensed and insured. That establishes a form of legal recourse should a boundary dispute arise. Rather, it provides funding for lawyers to sort out a comprimise should your neighbor decide to use a different surveyor who will get different results!

So how do you precisely determine the location of something? Well, you really don't have to. In Geography (with a big-G), we deal with different scales. Long/Lat only works to a certain scale. It's really good at small scales (like the continental level - compared to the oceans, Chattanooga is around 35N 85W Boulder is around 40N 105W, San Fran is around 38N 122W). As you get to large scales, you start seeing all sorts of sources of error: rounding error in decimals, variations in gravity, movement of the surface of the earth, etc., etc. So for local stuff, you really shouldn't measure based a global point of reference. It should be local - like surveyor's control monuments or RTK DGPS beacons for guiding helicopters to the top of hospitals.

#Comment Re: made: 2006-10-24 20:55:31.291539+00 by: mvandewettering

Interestingly, the GRACE satellites observed a change in the earth's gravitational field in the wake of the 2004 tsunami, suggesting that the density of the sea floor had measurably changed over an area several hundred miles.