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Is altitude important for waypoint?


jchristman

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Posted

How important is setting the proper altitude when setting a waypoint? I had a heck of a time finding something today and I'm thinking it was the altitude of the cache.

 

Thanks.

Posted

It seems that most of the lower end handheld units aren't as accurate with altitude as they are with lat long. I could see it being useful if the cache was on a cliff face but not otherwise.

Posted

It's actually impossible to be as accurate with altitude as with lat/long, because VDOP is always bigger than HDOP (by a factor of about 1.5, most of the time.) That's a consequence of geometry: any satellites you can see are necessarily above the horizon, and thus "clustered" in one hemisphere.

 

That said, recreational GPS units don't tend to pay any attention to the altitude at all when telling you which way to go (have you ever seen the arrow point "up"? Me either icon_smile.gif )

 

warm.gif

Posted

It's actually impossible to be as accurate with altitude as with lat/long, because VDOP is always bigger than HDOP (by a factor of about 1.5, most of the time.) That's a consequence of geometry: any satellites you can see are necessarily above the horizon, and thus "clustered" in one hemisphere.

 

That said, recreational GPS units don't tend to pay any attention to the altitude at all when telling you which way to go (have you ever seen the arrow point "up"? Me either icon_smile.gif )

 

warm.gif

Posted

The only time I'd be concerned about the altitude of a geocache waypoint is if I had to decide whether the cache was at the top or the bottom of a cliff.

 

However, if the altitude indicated by your GPSr is way off because you only have a 2D fix (3 satellites or very poor geometry), then it can affect the horizontal position calculated by the unit. I've observed this when first getting a fix on the ground after using the GPSr at 30 kft on a flight. The initial 2D fix can be a mile off and then gets corrected as additional satellites are received and the unit gets a 3D fix with a more correct altitude.

If reception is poor and the unit is staying in 2D mode you can manually enter an estimate of the right altitude and get a better horizontal position as a result.

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