# Setting Altitude on Legend

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The Altitude seemed off on my Legend by about 30 feet. I have easy access to sea level and wanted to set it to zero. In order to do this, I must turn off my GPS first. How does this help? When I set it to zero and turned it back on I was 80 ft below water! Is there something I'm missing in the instructions, or is it something you just don't try to fix?

I want to die like my grandfather did, peacefully in his sleep. Not screaming and yelling like the passengers in his car.

Most altimeters are based on air pressure and are adjustable so that you can compensate for atmospheric changes due to weather and temperature variations.

However, the altitude determined by the Legend (and most GPS rcvrs. other than the few that incorporate barometric sensors) is based on determining a 3D position in space from time delays of signals received from at least 4 satellites. Therefore there is no adjustment possible or needed with altitudes determined using GPS. That doesn't mean it is always accurate - it isn't and will fluctuate, but there isn't any way to 'adjust out' that fluctuation. OTOH, weather changes are gradual and therefore adjusting a barometric altimeter for them is feasible.

My experience is that with a good sky view you can expect the GPS-derived altitude to be correct within about 50' 95% of the time.

Most consumer GPSr provide the Orthometric Height which is the height above the geoid (the height above mean sea level. MSL).

The Geoid Undulation or Geoidal Separation, is the difference between the Earth's Geoid and the GRS80 ellipsoid.

Your GPSr actually calculates the height above the GRS80 ellipsoid (HAE) and by use of a lookup table calculates... Orthometric Height (MSL) = HAE - Geoid Undulation.

The Orthometric height displayed will always have 1.5 to 2 times the error of that of the horizontal position for the same point.

then what is the point of allowing me to adjust the altitude inside of the GPS? Does it just reset itself when I turn it on?

I want to die like my grandfather did, peacefully in his sleep. Not screaming and yelling like the passengers in his car.

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