Hmmm… more like 107 - Macenstein

Hmmm… more like 107

These readings from my iPhone were taken while standing in the ocean in about 8 inches of water. Assuming the iPhone uses “sea level” as the basis for altitude, I would say the accuracy is closer to 107 feet than 56.

Although, in the iPhone’s defense, I AM just under 50 feet tall…

14 Responses to “Hmmm… more like 107”
  1. Ryan Powell says:

    Not to burst your bubble, or anything, but you DO know that the accuracy applies to horizontal accuracy, right? GPS has a much harder time with vertical measurements, as there is far less differentiation between the satellites on the vertical plane. Although I must admit the MotionX GPS interface certainly doesn’t make that very clear by displaying the accuracy right next to the altitude…

  2. Ryan,
    I think you in fact DID intend to burst my bubble.
    Mission accomplished.
    – The Doc

  3. Jonro says:

    So, did you use your iPhone to help you find your way back to the shore? 😉

  4. Bob says:

    I’m far from an expert but is ‘sea level’ based off one ocean and there are variances? Doesn’t the tide change the level of the water? So if you’re standing in the ocean at high tide, could it be that’s ~50ft higher than low?

  5. dlowe402 says:

    See, that’s what you get Doc. You eat an enormous ice cream cone with a ton of “Sprinkles” and you find yourself waking up lost at sea thinking you’re 50 feet tall. Good thing you had a GPS enabled iPhone to help you get back. You might wanna get an Otter Box for it if you plan to continue on this path of self destructive behavior though.

  6. aga says:

    Bob, you have just solved the worlds energy problems, with Tide changes happening multiple times a day and ~50ft changes in water levels, all we need to do is capture a bit of that change and we have solved the world energy problems.

    Bob, to answer the question though, Tide changes vary in height depending on location, body of water, moon phase, pressure systems, etc.

    on the equator, an ocean tide moves about 3 feet
    in my the bath tub there is no noticeable change
    some lakes have small tide changes, others have none
    you can potentially see ~50ft+ changes if you go to nova scotia

    hope that helps <|:^)

  7. Matt BK says:

    Ryan, I’m glad you pointed that out. It’s very confusing to some people that horizontal accuracy is given but vertical is not (and is often off by 400-some feet).

    Bob, sea level is more of a calculated value than an actual one. Sure, you and Dr. Macenstein can go to two different places at the same time and both say “I’m standing at sea level,” but you have to take into account all the things that aga mentioned. The question is, what mathematical model of Earth do you want to use to describe your elevation? This is why it’s actually quite difficult to determine sea level rise/fall accurately, especially in relation to global warming.

  8. John says:

    Even taking your incorrect assumption into consideration, the results you received from MotionX GPS are apparently still more accurate than any of the GPS units sold to Connecticut drivers who obviously never know exactly where they are going or at which speed they should be traveling.

  9. Steve says:

    Altitude readings from GPS are notoriously inaccurate. It has lots to do with how many satellites are locked on. This actually isn’t a unique iphone issue 🙂

  10. The iPhone as a flotation device?

  11. thisisjohnny says:

    woah! how do you walk with balls that big? taking your iphone for a walk in the water?… you’re far braver than i

    or at least richer, and can afford to replace nice things.

  12. LouRob says:

    Maybe the GPS satellite is in orbit 107 feet higher than they thought.

  13. Doug says:

    Actually, the REAL reason GPS altitude information is less accurate than horizontal position information is due to the WGS84 coordinate system. Coordinate systems have to approximate the Earth as an ovoid since it is NOT a sphere. The ovoid model is less than perfect, hence the higher inaccuracies in vertical position. It’s just that simple.


  14. Kaleberg says:

    The GPS elevation is based on a mathematical model of the earth’s shape called the geoid. It tries to account for the equatorial bulge and other deviations of the earth from a perfect sphere to give you a 3D location for mean sea level. There are at least a dozen different geoids in use today depending on what you want to do and what system you are using. Yeah, that and GPS elevation basically sucks because all the satellites are about the same altitude. If there were another constellation or two of them at higher elevations they might do altitude better.

    P.S. Did you know that if you go down the Mississippi you start out closer to the center of the earth than when you get dumped out in the Gulf?

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