# Accurately measuring level over long distances? **URL:** \ **Category:** Factual Questions\ **Created:** [December 13, 2023, 3:07pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555 "2023-12-13T15:07:26Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [December 14, 2023, 3:07pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/21 "2023-12-14T15:07:28Z")
Back in the 19th century, believers in Cyrus Reed Teed’s Hollow Earth theory performed just such an expereiment. Since they believed they believed that we live inside a “bubble” in an infinitely thick uni=verse of rock and dirt, their idea was that if you projected a level line a few feet above the earth long enough, it would eventually run into the earth, as any chord will intersect its circle. They found that it did, which suggests some error in their experimental methods. Teed was calling himself “Koresh” (from the Persian original of “Cyrus”) long before David down in Waco, Texas. > **[We Live Inside \[Dr. Cyrus Reed Teed and the Koreshan Unity\]](https://order-of-the-jackalope.com/we-live-inside/)** > > Dr. Cyrus Reed Teed may have been a madman, or the Messiah, but only he was bold enough to tell the truth: that we live on the inside of a hollow Earth. ---
**Author:** ![DocCathode](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doccathode/32/18773_2.png) [@DocCathode](https://boards.straightdope.com/u/DocCathode)\ **Post date:** [December 14, 2023, 3:23pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/22 "2023-12-14T15:23:44Z")
What am I, invisible? ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [December 14, 2023, 3:43pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/23 "2023-12-14T15:43:30Z")
Sorry – I missed your brief comment in a quick scan. Although, to be fair, you didn’t give explanation or context. Even on the Dope Board, you can’t expect everyopne to be familiar with Teed, or the fact that his followers actually tried the experiment. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 14, 2023, 3:52pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/24 "2023-12-14T15:52:47Z")
Yeah. I figured @DocCathode’s “Teed” was an oblique cite to some SF book I’d never read or silly movie I’d never seen. Yet another of the vast panoply of allusions I don’t get. IOW: No Sir you’re not invisible. Just far more subtle than your Unwashed audience (or at least this Unwashed part of it) can appreciate. 😁 Today I learned … And thank you both for that. ---
**Author:** ![DocCathode](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doccathode/32/18773_2.png) [@DocCathode](https://boards.straightdope.com/u/DocCathode)\ **Post date:** [December 14, 2023, 3:58pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/25 "2023-12-14T15:58:05Z")
I first learned of Teed in Eccentric Lives And Peculiar Notions. He also has an entry in Paradox Press Big Book Of Weirdos. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [December 14, 2023, 4:20pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/26 "2023-12-14T16:20:45Z")
> [@DocCathode](#): > > He also has an entry in Paradox Press Big Book Of Weirdos. Yeah – they even include an illustrated story of their doing the “horizontal line” test. But I fitst learned about Teed from Martin Gardner’s **Fads and Fallacies [in the Name of Science]** ---
**Author:** ![DocCathode](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doccathode/32/18773_2.png) [@DocCathode](https://boards.straightdope.com/u/DocCathode)\ **Post date:** [December 14, 2023, 4:45pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/27 "2023-12-14T16:45:51Z")
Behold! [The Cellular Cosmogony: Table of Contents](https://sacred-texts.com/earth/cc/cc01.htm) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 14, 2023, 4:48pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/28 "2023-12-14T16:48:34Z")
One is awestruck. Or dumbstruck, since I’m certainly dumber for having read a few pages. This Teed fellow was wacky as the late Mr. Timecube, but far, far more coherent. Quite a _Magnum Opus_ of delusion. Proof that insanity need not be chaotic. Thank you for that. ---
**Author:** ![DocCathode](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doccathode/32/18773_2.png) [@DocCathode](https://boards.straightdope.com/u/DocCathode)\ **Post date:** [December 14, 2023, 4:52pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/29 "2023-12-14T16:52:12Z")
I’d post some pics from Eccentric Lives but it’s stil packed away from the move. Teed’s cult center is still preserved in Florida and I’d very much like to visit one day. ---
**Author:** ![Timz](https://avatars.discourse-cdn.com/v4/letter/t/6a8cbe/32.png) [@Timz](https://boards.straightdope.com/u/Timz)\ **Post date:** [July 29, 2025, 11:57pm UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/30 "2025-07-29T23:57:46Z")
> [@Machine\_Elf](#): > > But how do you measure level over much larger distances If you want the best accuracy you can get, you do it about the same way people did it 100+ years ago. On any US topo map, you’ll see many small crosses, labeled “BM \_\_\_\_” with the elevation above sea level, in feet. That elevation is usually the elevation of a convex brass disc set into rock or a concrete mass, and you can look it up at the NGS website to get the details about it. How was that elevation determined? By carefully measuring how much higher it was than another point maybe 100 meters away, the elevation of which you already knew, because you’ve already measured how much higher that point was than another point, 100 meters further back toward where you started. In other words, you go point to point to point, 100 meters at a step, for hundreds of miles. How to assess your accuracy? Nothing to do but do it again and compare. If you’re leveling between benchmarks a mile apart, and you want to claim the best accuracy, the two elevation differences are supposed to agree within 2 millimeters. Main difference between now and 100 years ago: then, the instrument had a bubble level to get the telescope line of sight horizontal. Instruments nowadays have a pendulum device inside them and mirrors/prisms to automatically level the line of sight, once you’ve roughly leveled the instrument well enough to get into the pendulum’s final-correction range. One complication: if you start at sea level at Seattle and go down the coast, benchmark to benchmark, all the way to San Diego, you’ll find that sea level at San Diego isn’t the same as sea level at Seattle. Maybe half a meter difference, as I recall. So “sea level” is hard to define. Under the current NAVD88 system, benchmarks in San Francisco are considered to be about 0.8 meters higher than they were in the NGVD29 system, while benchmarks in NY are 0.3 meters lower than they were in NGVD29. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 30, 2025, 2:15am UTC](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555/31 "2025-07-30T02:15:28Z")
As you know but others might not “sea level” is a very complicated idea once you point your magnifying glass at it. > **[Sea level](https://en.wikipedia.org/wiki/Sea_level)** > > Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datum – a standardised geodetic datum – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A co... [Previous page](https://boards.straightdope.com/t/accurately-measuring-level-over-long-distances/994555.md?page=1)