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GPS negates Elevation and Depth

Old 05-20-2008, 12:22 AM
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Default GPS negates Elevation and Depth

My backup GPS is a Garmin handheld.

It has an "elevation" and a "depth" field.

I also notice that regardless of whatever data I put into these two fields. That distance is not included in the overall distance calcaluation.

For sake of a simple example. I will use the pythagorean theorem. a2 + b2 = c2 or 3 + 4 = 5.
Point A to Point B is exactly 3 feet. Point C is 4 feet directly above Point B. Mathematic says that Point A to Point C should be 5 feet.

My handheld is showing that from Point A to Point B is 3 feet. It is showing that from Point A to Point C is also 3 feet totally negating the evelation.


Can anyone else verify this?
Old 05-20-2008, 12:33 AM
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Default Re: GPS negates Elevation and Depth

Huh? If point B is three feet above point A then your elevation at point B is three feet.

And what are your points??? Like this?


B*
*
*
*
***************
A C

Also, the elevation calculation from a hand held GPS is not exactly accurate. It is accurate enough I suppose. But I would not trust it. My hand held Garmin GPS says I am 20 - 200 feet above sea level when I am standing on the beach with my feet in the Gulf Of Mexico. I am not that tall.

The GPS is much more accurate when it comes to my position on the surface of the planet.
Old 05-20-2008, 12:41 AM
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Default Re: GPS negates Elevation and Depth

Distance between 2 points as shown on your GPS is basically a horizontal distance between the XY and does not take into account the elevation
Old 05-20-2008, 12:44 AM
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Default Re: GPS negates Elevation and Depth

Imagine I live at A at sea level. My buddy live at C on a steep mountain.

Hopefully this diagram will further clarify my question.

C
/ *
/ *
/ *
/ *
* * * *
A B

A to B = 3ft
B to C = 4ft

A to C = According to the pythagorean theorem this should be 5ft.

According to my GPS. A to C is still 3ft away. Total disregard for any elvation/depth.

GPS only measuring the linear distances.


Old 05-20-2008, 03:57 AM
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Default Re: GPS negates Elevation and Depth

According to my GPS. A to C is still 3ft away. Total disregard for any elvation/depth.

GPS only measuring the linear distances.
That's exactly right, that's what it measures, that's what it is supposed to measure, A to B, this is the horz dist from A to C
Old 05-20-2008, 05:34 AM
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Default RE: GPS negates Elevation and Depth

Kerry is correct.

Your handheld GPS is not going to provide an accurate elevation measurement; it simply does not have what it needs to do so. The uncertainty in the vertical plane is on the order of approximately 4 times that of the horizontal uncertainty. You can hold your unit an inch above sea level and reset the elevation and within 5 minutes you will be well above that. The purpose of handhelds is not to provide a reasonably accurate elevation near sea level; again, they can't. If you are climbing mountains, then an error a couple of hundred feet is likely not an issue for you. If you are landing your plane using your handheld as a substitute for a true altimeter, well, yer gonna crash!

Why do they have/advertise the elevation function?

Marketing.


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Old 05-20-2008, 07:14 AM
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Default Re: GPS negates Elevation and Depth

Altitude can be derived from the the GPS signals using the same math that gives us lat and long, but the angles are so close that accuracy is not that good. The advantage of the elevation calculation is lost on boaters but provides some inexact data for climbers, pilots, and bikers who would be using topographic charts. Perhaps more useful, but still inexact height information is available from Garmin's handhelds with an S in the model number. These have sensors built in for barometric and magnetic heading information. I'm interested in the barometric data to track changes in pressure as a piece of the weather puzzle, and have fould the compass feature useful in the dinghy, which doesn't have a real one.

Hey, you don't use every blade in that Leatherman, either.
Old 05-20-2008, 12:34 PM
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Default Re: GPS negates Elevation and Depth

I have taken this hand held on a flight before and it registered 22,500 feet so I know for sure it does read elevation. As for accuracy - who knows.

So for those that do know the answer to my question. In the distance formula. Shortest straight line between two ponits. Rather both points be on sea level or being one point on sea level and the other point on a mountain top. Is elevation/depth even calcaluated at all?

I know my GPS is reading elevation/depth however it is NOT calcaluating elevation/depth. Just want some further verification if other GPS units are the same.
Old 05-20-2008, 03:32 PM
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Default Re: GPS negates Elevation and Depth

Is elevation/depth even calcaluated at all?
Elevation is computed as part of the 3D position solution BUT is NOT used in distance calculations in your type of receiver. There is no other way to say this as the distance calc is HORIZONTAL and has nothing to do with the vertical component.

I know my GPS is reading elevation/depth however it is NOT calcaluating elevation/depth. Just want some further verification if other GPS units are the same.
Your GPS is computing height/elevation as part of the 3D position solution. Basically all recreational GPS receivers do this so as to determine XY values but they can not determine height with any degreee of accuracy and not as accurate as horizontal. Higher order GPS receivers in survey applications will/can have the option to provide/display slope distance but slope distance even in survey applications is not all that important as it is the horizontal distance that is mostly required in survey applications.

Old 05-20-2008, 03:38 PM
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Default Re: GPS negates Elevation and Depth

Also the "depth" field is more than likely only available if connected to a depth sounder (via NMEA or part of a combo etc) and "depth" is not part odf any computed position solution.

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