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May 10 2011

Magellan’s Nightmare

geodesy: a branch of applied mathematics concerned with the determination of the size and shape of the earth and the exact positions of points on its surface and with the description of variations of its gravity field. – Merriam Webster’s Dictionary

Only 18 men out of 237 survived Magellan’s circumnavigation of the globe. Magellan was not one of them. In 1520, when they rounded South America, he thought it would be a short distance to the “Spice Islands” (modern Indonesia) but “no one realized that the greatest expanse of water on the planet lay ahead.” Magellan and his men found out that there was more to this earth than they presumed. They harshly learned that their measurements were grossly wrong and that the earth is not a nice simple sphere but actually a spheroid.

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Geodesy is not simple

When you hear “size and shape of the earth,” it sounds simple enough but then you start hearing that height doesn’t necessarily mean height. You hear terms such as “geopotential” in relation to the variation of gravity. You read about the “geoid of 2003” and “ellipsoid” heights. At this point your brain starts to melt and you turn catatonic. Just tell me the GPS coordinates and elevation!

Why do I need to know about geodesy?

Understanding a little geodesy is important to understanding datums and coordinate systems. Understanding datums and coordinate systems is important because, as engineers, GIS analysts, CADD technicians, and surveyors, nearly everything you do is built within the framework of a datum & coordinate system. Your imagery, topography, and infrastructure data are all information, measurements, and locations that are dependent and relative to a system. Just the other day, some coworkers noticed a 2ft discrepancy between a drainage master plan and the corresponding HEC-RAS model it was based on. Sadly, these mistakes are not uncommon.

In future posts, I’ll dig more into what I think we need to know within geodesy and then move into datums and coordinate systems.

Other decent resources: ”What is Geodesy” (NOAA), ”Geodesy” (Wikipedia)

1 ping

  1. » Where am I? Hydraulically Inclined

    [...] talked a little about geodesy and reference ellipsoids but what is a datum? It’s pretty simple, actually. A datum is way of [...]

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