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Issue with elevation readings


The other day, I figured out how to get my 1350 to show the elevation instead of the arrival time. That's cool, I think, and so I try it. Overall, it works well, but sometimes it has some strange readings (like below sea level readings), and sometimes the readings go down when the road is going up, (and visa versa), and is sometimes inconsistent (I pull into my garage, and it says 72 feet, when I come out o the same garage, it says 90 feet.)

It seems that when this is the at its worst, the conditions outside are not good - cloudy, foggy, etc. I also noticed at the same time the car icon on the screen "fishtales" (trust me - the real car isn't doing that).

I am assuming that the behavior I'm seeing is because I'm not getting a strong signal - what do others think?


  • AndreyT 105 Points
    The principles employed by GPS system are sufficiently good to determine your current longitude and latitude. However, they provide only very poor precision when it comes to determining your elevation. The "elevation" feature in your GPS is there only for very loose reference purposes. It is basically a free side-effect of GPS algorithms, with little practical value because of its inherent lack of precision.

    Devices that need more precise altitude measurement (like sport GPS devices) are often equipped with built-in barometric altimeters, which can provide very precise altitude measurements (relative and absolute). Car GPS devices generally do not.

    The "fishtailing" that you sometimes observe with the car icon on the GPS screen is also a consequence of the typical GPS operation. The GPS does not really know which way your car is pointing. It has no way of knowing that. The only way it can orient your car icon properly against the map is to constantly poll your car's location. As your car's location changes, the GPS device can determine the current travel trajectory and the current orientation of the car against the map. However, if the consecutive GPS location measurements are not precise, the trajectory seen by the device becomes zig-zagged, which results in car icon's "fishtailing". The slower your car moves, the more prominent that fishtailing can get.

    When you car is stationary, the GPS device cannot possibly determine which way your car is pointing. In such cases it won't be able to orient the car icon properly. If you drive in reverse, the GPS will show the car icon turned around 180 degrees (since it has no way to know that your are driving in reverse). These are just extreme examples of the very same "fishtailing" you observe.
  • Boyd 1999 Points
    Here is Garmin's FAQ{66f1b0a0-4cd6-11dc-4733-000000000000}

    Frequently Asked Questions


    GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, they are two different systems, although they have a relationship that has been modeled.

    The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating.
  • tbdbitl 31 Points
    Thanks to all of you who replied. Very interesting responses
  • Emmadw 10 Points
    Thanks! Was wondering why I frequently appear to be below sea level (I live just about it, but not below!)
  • alanb 556 Points
    Emmadw said:

    Thanks! Was wondering why I frequently appear to be below sea level (I live just about it, but not below!)

    The calculation method is called GWPM (Global Warming Predictive Model) which means you may want to trade your nuvi for one of the marine models
  • rocket scientist 196 Points
    edited October 2015
    Keep in mind that "sea level" is a commonly used reference datum here on mother earth, but it's completely arbitrary to a GPS satellite. Considering that they are orbiting something like 10,000 miles above the earth, I don't think that +/- 400 feet or so is all that bad.
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