I have long known that GPS receivers provide altitude information, but I had a poor understanding of how they do it. It’s complicated. By the way, the Earth’s shape is more irregular than you may think.
January 12, 2010 at 1:18 am
my dad says:
It is widely accepted that a level terrestrial surface is everywhere perpendicular to the local gravity vector. The Earth's 'average' shape is thus a surrogate for the Earth's gravity field. Average sea level only approximates this surface more or less well depending on your area and time span of interest and your need for precision, though in many texts sea level on a continent-less Earth is evoked to 'clarify' the concept of elevation. This partially explains the need for geodesists to continually revise the reference Geoid (and have alternate definitions for different purposes). The geoid is a mathematical construct to which elevations near the Earth's surface are referenced.
One aspect of Adrian's tjopic that his link does NOT try to cover well is the many ways (causes and time scales) of change in the Earth's shape. Nothing is constant. Thinking in terms of the gravity field may make changes easier to embrace. The Earth's shape continually remoulds on a full range of spatial and temporal scales that I find fascinating in conjunction with man's need and ability to think of it as fixed.
A separate but related question: Computers and the internet have revolutionized knowledge availability and advancement. Everyone uses the latest Geoids and does so appropriately. Very few people, maybe no one outside of a few mathematicians and top surveyors, did so when I was in school. Sputnik changed that and more. I wonder, if the understanding of a geoid (or shape of the Earth ('s gravity field)) is more 'sophisticated' now or whether that concept (like others) has been made more obscure by 'electronics' that lessened the need to understanding even for those who did need to use the details. Sophisticated use does not only not require sophisticated understanding; it does not even require any concept of the fundamentals. The basics are usually in the old books, not the high tech manuals. True at least in surveying, statistics, and magic.
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