Results 1 to 4 of 4

Thread: Altitude

  1. #1
    Newbie SUNAJ's Avatar
    Join Date
    May 2006


    I wonder if any one has been looking on the altitude in Destinator/CF

    I donít know if itís a bug. But when I am at the beach itís showing 40 meters with 7 to 9 locked satellites, and that is a bit high.

  2. #2
    Fusion Brain Creator 2k1Toaster's Avatar
    Join Date
    Mar 2006
    Colorado, but Canadian!
    40m, or 40ft?
    Fusion Brain Version 6 Released!
    1.9in x 2.9in -- 47mm x 73mm
    30 Digital Outputs -- Directly drive a relay
    15 Analogue Inputs -- Read sensors like temperature, light, distance, acceleration, and more
    Buy now in the Store

  3. #3
    Newbie SUNAJ's Avatar
    Join Date
    May 2006
    40m approximately while I am using metric system. (Have not tested feet)

  4. #4
    It ain't easy being a green moderator meddler's Avatar
    Join Date
    Aug 2002
    Steps out the front of Henson's workshop or Sydney
    The error is built into the GPS system. I had a look and found the garmin website had a good explanation.

    So why do GPS's measure elevation as the least accurate measurement
    It's generally agreed that for GPS use, you're most interested in where you are (x-y-axis) and not how high up you are (z-axis). When you're driving or walking, you want to know where you're at; you already know you're on the ground. There are precision instruments for measuring altitude. So the Earth-approximate-shape calculation is optimized for X-Y accuracy.

    Per Garmin, consumer uncorrected GPS's accuracy along the X-Y axis varies from 16 horizontal feet, to a usual average of 33, and a general max of 50 feet (though they can be off by more). Most full-function GPS's report their degree of precision. Remember--if you're off by 16 feet that's in any direction; a circle with a 16 foot radius has an area of pi*r^2, or Area = 805 square feet. 33 ft radius= 3,421 sqft, and a 50 foot radius = 7,854 sqft.

    Again, vertical precision even less accurate--so think of yourself standing dead center inside an imaginary football standing vertically. You're in the center of a circle, with the walls 33 feet away in every walkable direction, and that top-bottom of accuracy is even farther.

    There are a couple forms of correction which makes it more accurate if your GPS can receive it
    • DGPS--Differential GPS. Requires an external beacon receiver to be plugged into your GPS.
    • WAAS --Wide Area Augmentation System, if your GPS can receive it.

    It's worth noting that some newer GPS's (ex: Garmin 76CS, some others) come with a barometer and benchmark correction for altitude.
    • Correction Method 1: If you find a USGS altitude benchmark, enter in the altitude.
    • Correction Method 2: If you know your correct barometric pressure, enter it in.
    • Correction Method 3: Enter in both!
    The unit use that data to correct it's own GPS estimate as you move. Of course, if you have a thunderstorm heading your way fast, the barometer altitude adjustment actually makes it worse. You just can't win.

    The main reason GPS altitude is less accurate is because of the Earth's shape. If the Earth were a perfect circle, it would be easier. It's actually more like a flattened orange. Several mathematical models have come about and can be read on GPS FAQ/Fact/Info pages. To make it more complicated, sea level varies widely! That's why we use "above Mean Sea Level" or MSL as the altitude measurement. Sea level varies with the tide, obviously. (Avg tide: 3 feet. Biggest tide: 52 feet! (click)) It also varies with density of the rock below it, and the force of ocean currents--which is why you can't just cut a path through Panama, but instead have to go through locks that raise or lower you, depending on your direction of travel.
    For a great and easy to read article on how they constantly re-calculate Mean Sea Level and all the why's, Click Here

    As a result, your GPS approximates/triangulates your location not on a perfect circle, but on a approximation of the earth's shape called a "Geoid" and then guesses how high you are compared to the world Mean Sea Level.
    Never let the truth get in the way of a good story

Similar Threads

  1. Replies: 0
    Last Post: 01-16-2006, 03:23 PM
  2. How to add Altitude information in FreeDrive
    By rdsoft in forum FreeDrive
    Replies: 1
    Last Post: 12-19-2005, 04:14 PM
  3. Altitude indicator
    By Mr.Fabio in forum Map Monkey
    Replies: 5
    Last Post: 04-28-2005, 04:27 PM
  4. Altitude wrong
    By badass101 in forum MM Bugs
    Replies: 0
    Last Post: 04-15-2005, 04:54 AM
  5. GPS Altitude Error !?!?!?
    By ShawJohn in forum GPS
    Replies: 20
    Last Post: 10-14-2004, 10:17 AM


Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts