# Measuring the flatness of the earth

I have a nagging question from my research into the flat earth society. They are actually a fairly scientific bunch, if you simply ignore certain science. For example, there’s a YouTube where a guy who gets in his truck in Australia and drives a flat road. He starts out explaining how to look up the curve of the earth and if you look at it as an actual straight line extending from where you are, the earth “drops” away from that line. So, he theorizes, as he drives, that drop should register on his GPS. I know he just has the wrong instrument, that if he flew in a jet in that actual straight line, with the gyroscopic device that maintains zero altitude from where it left the ground, he would see that “drop”.

But, here’s the question, how does GPS take into account the curvature? Some sort of triangulation I guess, but what basis is it using for sea level? Is each satellite given an altitude, based on its geosynchronous location? I’m not even sure what I would put in google for this one.

I have no idea how they get the earth is flat. They’ve obviously haven’t been to the Ozarks or the Blue Ridge Mountains or even the Rocky Mountains, only to name just a few in the U.S. alone.

They are actually a fairly scientific bunch, if you simply ignore certain science.
Isn't that an oxymoron?

Isn’t fidelity to honesty a cornerstone of science?

Part 1: The “Ellipsoidal Model” — Your GPS receiver measures elevation with respect to “the ellipsoid”

What should I remember about ellipsoidal data?
All elevation data is based on a defined “vertical datum”
A vertical datum is a representation of the earth’s surface (considered to be 0 meters)
GPS receivers measure elevation with reference to an ellipsoid
Ellipsoidal data is accurate, but not a common elevation reference

Part 2: Mean Sea Level — What is the MSL my GPS receiver outputs? (And could I use that?)
The vertical reference often used to represent the earth’s surface called the “mean sea level,” or MSL. MSL is a local tidal datum that can be used as a reference for elevation when close to the shoreline. However, once you get more than a few kilometers inland, MSL becomes impractical. Your GPS / GNSS receiver already outputs global MSL, because MSL is a standard for position output.

What should I remember about MSL?
Mean sea level “MSL” is an elevation reference output by GPS receivers
MSL elevations don’t take into account local factors

Part 3: The Geoid — A model of the Earth’s gravity field which best fits a global MSL
The geoid
What is the geoid?

https://eos-gnss.com/elevation-for-beginners

Now I know.

ps

Why don't the elevations on your maps agree with those provided by my GPS system? Which ones are correct?

They both might be correct. The reason that the elevations do not agree might be because they are set on different datums. Most maps use NAD27 and the elevations are based on mean sea level. Your GPS receiver uses WGS84 and the elevations are based on the NAD83 ellipsoid. These datum shifts can result in difference of tens of meters horizontal and hundreds of meters vertically.

www . usgs . gov

Isn’t that an oxymoron? -- CC
When put briefly like that yes. I knew it sounded weird when I wrote it, but didn't want to elaborate. There's a book on the history witchcraft that came out a few years ago that describes this. Once they decided witches are something to investigate, the rest of the tests and connections they make are mostly logical and involve collecting data.

With flat earth, look at the history of how we figured it out. There’s very little direct experience of living on a ball hanging in space, let alone one that is moving very fast. To “see” the curvature, using the shadows cast by the sun, you need large enough measuring sticks, and cooperation by someone willing to do the measuring, and communication across a distance beyond yelling to each other. None of this had any practical application at the time.

I think we have it easy now because “round world” is in our language. We hear it as get all the rest of our culture. As we mature, we start to doubt that culture, this is healthy for the survival of the species, but, for some, the doubt mechanism overtakes the reasoning mechanism.