Elevation For Waypoint

[+The Daisy Seed Co.]

Hello,

I am trying to learn how to use my new GPS. It is a Magellan SporTrak. I was following the instructions for entering a geocache waypoint. It has a field to enter elevation. I don't see elevation as part of the geocache info. Is that field required? I am confused. Any help is appreciated.

[+briansnat]

No need for elevation.

[+smullis]

I have a Magellan Meridian Platinum and have found no use for the elevation field when setting a waypoint other than nice-to-know. I did install the Topo map and use that, particularly when out and about in rural areas...always nice to know when you're heading toward a cliff so you can circumvent it. I have seen some caches post the elevation, but I only think it would really matter if the cache is hidden high enough to require oxygen.

[+GEO*Trailblazer 1]

Yes there is a need,

for the more advanced the Elevation is the Z,

In the Equation X+Y+Z your 3rd dimension(3D)

X= LONGITUDE

Y= LATITUDE

Z= ELEVATION

You will find that it helps you zero in faster.

Also a Long-Lat. with 7889' is at a different position than a Long-Lat with 1000'.

Just think about it for a spell.

Edited September 15, 2004 by GEO*Trailblazer 1

[+Prime Suspect]

It's essentially pointless for geocaching, and won't have any effect on the performance of the GPS. Don't waste your time entering it.

 

Markwelled.

[+GEO*Trailblazer 1]

X-Y-Z CONVERSIONS

We are talking

Vector Analysis

Angular velocity or Vector analysis shortcut.

GPS/RTK Real Time Kinematic

Edited September 15, 2004 by GEO*Trailblazer 1

[+Prime Suspect]

X-Y-Z CONVERSIONS

We are talking

Vector Analysis

Angular velocity or Vector analysis shortcut.

GPS/RTK Real Time Kinematic

Entering the elevation for a waypoint will have zero effect on the unit's ability to calculate its current position. As I said, it's a waste of time.

 

Here's quick test. Go outside and fire up your GPS. Wait until you've got good locks, then set a waypoint. Stay where you are, and edit that waypoint by adding 50,000 feet to the elevation value it stored when you created the waypoint, then save it. Now do a GOTO to that waypoint, and see if it tells you the waypoint it 50,000 feet away. That's right, it won't. The elevation entry is pretty much informational, and is not used in calculating locations.

[+GEO*Trailblazer 1]

OK if you say so.

I know how it works and have been working with the GPS for over 10 years.

Experience in aquisition of a point can be determined faster if the X,Y,Z is known that is why the NGS and or any other high accuracy survey's, goes to such great extremes leveling.

Imagine in just a few years your handheld will get centimeter grade accuracy.

We can discuss till the end of time whether or not it works for you or anybody else but it works for me.

I have been to several geocaching competitions and seem to be the one who can find (0 in on) the points the fastest.99% of the time.

I am also working with the CERT search and rescue mapping here and can get faster aquisition when up and down mountians.

 

I linked to the math and unless you are a scientist and can read and comprehend things in a short time did not read the materials provided,

especially the RTK Real Time Kinematic,which is the type of GPS they are using now for centimeter grade accuracy....the elavation is on it too.

I am not a scientist either and am still after all these years learning new things,elevation is one but unless you use it a lot you do not see the benifit.

 

I hope you do not take this the wrong way but thought I would post my experiences and observations.

 

I could suggest you do an opposite test,go to a Control Point(Benchmark 1st order accuracy/GPS).change the elevation and watch your GPS jump around on getting a fix.

 

Kinda like the cruise control..........my car has one, (but it is irrelavant) to use it, it's just there?

[+Prime Suspect]

OK if you say so.

I know how it works and have been working with the GPS for over 10 years.

Experience in aquisition of a point can be determined faster if the X,Y,Z is known that is why the NGS and or any other high accuracy survey's, goes to such great extremes leveling.

Imagine in just a few years your handheld will get centimeter grade accuracy.

We can discuss till the end of time whether or not it works for you or anybody else but it works for me.

I have been to several geocaching competitions and seem to be the one who can find (0 in on) the points the fastest.99% of the time.

I am also working with the CERT search and rescue mapping here and can get faster aquisition when up and down mountians.

 

I linked to the math and unless you are a scientist and can read and comprehend things in a short time did not read the materials provided,

especially the RTK Real Time Kinematic,which is the type of GPS they are using now for centimeter grade accuracy....the elavation is on it too.

I am not a scientist either and am still after all these years learning new things,elevation is one but unless you use it a lot you do not see the benifit.

 

I hope you do not take this the wrong way but thought I would post my experiences and observations.

 

I could suggest you do an opposite test,go to a Control Point(Benchmark 1st order accuracy/GPS).change the elevation and watch your GPS jump around on getting a fix.

 

Kinda like the cruise control..........my car has one, (but it is irrelavant) to use it, it's just there?

Please explain to me how entering elevation information for a waypoint, has any effect on getting a fix at your current location. And remember, we're not talking about calibrating the unit. Maybe you need to re-read the OP's original post. I think you're off on a tangent that has nothing to do with the question at hand.

[+GEO*Trailblazer 1]

I do not know how to explain the how.

I am not an electronics engineer.

 

The only way I know for sure is how my units act while adding or not adding elevation.

Experience.

When looking for benchmarks/geocaches I have found it gets you right to the mark if elevation is entered and it floats around when it is not entered.

 

I will have to study more to get you the answer for the question you asked.

I know that the satts use the Ocean (Mean sea level) to calculate,I am sure for reasons beyond my feeble comprehension this is part of the electronic signals being used for the calculations.

The curvature of the earth and elevation are also a part of the function of the calculations.

 

But again this is not really an answer to the question asked.

I will work on it.

 

I am sorry if I am off on a tangent.............sometimes I get that way .

 

My understanding of th OP is:

 

Does that field matter(elevation)?

My book says if you have it ,enter it.

but for first aquisition it not manditory but helpful.

Edited September 16, 2004 by GEO*Trailblazer 1

[+Prime Suspect]

Beware of trusting anecdotal evidence.

[+GEO*Trailblazer 1]

For sure thanks.

I am more of the observational type though.

OBSERVATION

And tend towards the experimental aspects.

EXPERIMENT and reply in experimental physics of the matter.

Experimental physics.

I am only giving you my experiences.

EXPERIENCE.

 

So lets keep elevating the subjective reasoning of elevation.

ELEVATION

 

And I hope you know I am just having fun so please have fun too.

[+GEO*Trailblazer 1]

I will entertain more facts of history,of this happening(anecdotal),story.

WAYPOINT/DATA POINT

 

The Datum

see:Vail-curve. hmm!!

 

I found where elevation is mentioned for the GPS.

 

Technical description

The system consists of a "constellation" of at least 24 satellites in 6 orbital planes. The GPS satellites were manufactured by Rockwell; the first was launched in February, 1978 (Block I), and the final (24th), satellite was launched in 1994. Each satellite circles the Earth twice every day at an altitude of 20,200 kilometers (12,600 miles). The satellites carry atomic clocks and constantly broadcast the precise time according to their own clock, along with administrative information including the orbital elements of their own motion, as determined by a set of ground-based observatories.

 

The receiver does not need a precise clock, but does need to have a clock with good short-term stability and received signals from four satellites in order to find its own latitude, longitude, elevation, and the precise time. The receiver computes the distance to each of the four satellites by the difference between local time and the time the satellite signals were sent (this distance is called a pseudorange ). It then decodes the satellites' locations from their radio signals and an internal database. The receiver should now be located at the intersection of four spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals. The receiver does not have a very precise clock and thus cannot know the time delays. However, it can measure with high precision the differences between the times when the various messages were received. This yields 3 hyperboloids of revolution of two sheets, whose intersection point gives the precise location of the receiver. This is why at least four satellites are needed: fewer than 3 satellites yield 2 hyperboloids, whose intersection is a curve; it's impossible to know where the receiver is located along the curve without supplemental information, such as elevation. If elevation information is already known, only signals from three satellites are needed (the point is then defined as the intersection of two hyperboloids and an ellipsoid representing the Earth at this altitude).

 

When there are n > 4 satellites, the n-1 hyperboloids should, assuming a perfect model and measurements, intersect on a single point. In reality, the surfaces rarely intersect, because of various errors. The question of finding the point P can be reformulated into finding its three coordinates as well as n numbers ri such that for all i, PSi-ri is close to zero, and the various ri-rj are close to C.Δij where C is the speed of light and Δij are the time differences between signals i and j. For instance, a least squares method may be used to find an optimal solution. In practice, GPS calculations are more complex (repeat measurements etc...).

 

There are several causes: The initial local time is a guess due to the relatively unprecise clock of the receiver, the radio signals move more slowly as they pass through the ionosphere, and the receiver may be moving. To counteract these variables, the receiver then applies an offset to the local time (and therefore to the spheres' radii) so that the spheres finally do intersect in one point. Once the receiver is roughly localized, most receivers mathematically correct for the ionospheric delay, which is least when the satellite is directly overhead and becomes greater toward the horizon, as more of the ionosphere is traversed by the satellite signal. Since it is common for the receiver to be moving, some receivers attempt to fit the spheres to a directed line segment.

 

The receiver contains a mathematical model to account for these influences, and the satellites also broadcast some related information which helps the receiver in estimating the correct speed of propagation. High-end receiver/antenna systems make use of both L1 and L2 frequencies to aid in the determination of atmospheric delays. Because certain delay sources, such as the ionosphere, affect the speed of radio waves based on their frequencies, dual frequency receivers can actually measure the effects on the signals.

 

In order to measure the time delay between satellite and receiver, the satellite sends a repeating 1,023 bit long pseudo random sequence; the receiver knows the seed of the sequence, constructs an identical sequence and shifts it until the two sequences match.

 

Different satellites use different sequences, which lets them all broadcast on the same frequencies while still allowing receivers to distinguish between satellites. This is an application of Code Division Multiple Access, CDMA.

There are two frequencies in use: 1575.42 MHz (referred to as L1), and 1227.60 MHz (L2). The L1 signal carries a publicly usable coarse-acquisition (C/A) code as well as an encrypted P(Y) code. The L2 signal usually carries only the P(Y) code. The keys required to directly use the P(Y) code are tightly controlled by the U.S. government and are generally provided only for military use. In spite of not having the P(Y) code encryption key, several high-end GPS receiver manufacturers have developed techniques for utilizing this signal to increase accuracy and remove error caused by the ionosphere.

 

A minor detail is that the atomic clocks on the satellites are set to "GPS time", which is the number of seconds since midnight, January 5, 1980. It is ahead of UTC because it doesn't follow leap seconds. Receivers thus apply a clock correction factor, (which is periodically transmitted along with the other data), and optionally adjust for a local time zone in order to display the correct time. The clocks on the satellites are also affected by both special, and general relativity, which causes them to run at a slightly slower rate than do clocks on the Earth's surface. This amounts to a discrepancy of around 38 microseconds per day, which is corrected by electronics on each satellite. This offset is a dramatic proof of the theory of relativity in a real-world system, as it is exactly that predicted by the theory, within the limits of accuracy of measurement.

 

GLOBAL POSITIONING SYSTEM

Edited September 16, 2004 by GEO*Trailblazer 1

[+Prime Suspect]

I will entertain more facts of history,of this happening(anecdotal),story.

WAYPOINT/DATA POINT

 

The Datum

see:Vail-curve. hmm!!

 

I found where elevation is mentioned for the GPS.

 

Technical description

The system consists of a "constellation" of at least 24 satellites in 6 orbital planes. The GPS satellites were manufactured by

[yada yada yada clipped]

Yeah, that's basic GPS 101. But, like all the (seemingly random) prior links you've posted, it has nothing to do with the question at hand.

 

I never questioned whether or not a GPS has the ability to calculate its altitude from SV signals. Of course it does (though not very well), as long at it can get a signal from at least 4 SVs.

 

The question is, is there any need to enter the elevation values for waypoints? Does elevation play any part in calculating bearing and distance when executing a waypoint, in your basic handheld GPS. The answer, as I said before, is "No".

 

The GPS system uses a theoretical ellipsoid model of the shape of the earth. As the earth is far from being a true sphere, this mathematical model attempts to define (roughly) its true shape. When we refer to the WGS-84 datum, that's what we're talking about. When a GPS calculates its elevation using the SVs signals, it's being calculated not from true sea-level, but from the imaginary surface of this ellipsoid.

 

Likewise, when calculating bearing and distance between the GPS receiver and a waypoint, those calculations are done as if both the receiver and the waypoint are on the ellipsoid surface. The calculated elevation of the receiver, and the entered elevation of the waypoint are not involved in the calculation, in any way.

 

But, if you need some more evidence that there's no performance benefit to be gained by entering in elevation values for waypoints (remember, that is the question at hand), then perhaps you'll pay a little attention to those who should know.

 

I asked Garmin and Magellan technicians the same question: "Does the entry of an elevation value for a waypoint have any effect on the performance of the GPS unit? Is the performance in any way degraded if no elevation value is entered?"

 

Thank you for contacting Garmin International,

 

Sir, during a go to the unit does not take into account any elevation

information.  So it will neither hinder, nor, help performance. 

 

If, you have any further questions please feel free to contact us.

 

Thank You,

Jake Dahlstrom

Senior Product Support

GARMIN International

800-800-1020

<http://www.garmin.com/>

Fax 1-913-440-8480

Mr. [...], the elevation does not affect the operation or accuracy of the receiver since the receiver only considers coordinates and not elevation.

 

Thank you and have a great day,

 

Robert Davis

Magellan Technical Support

800) 707-9971 - USA & Canada

909) 394-5000  -Outside USA

Thales Navigation

960 Overland Court

San Dimas, CA 92407

 

So, like I originally said, it's essentially pointless for geocaching, and won't have any effect on the performance of the GPS. Don't waste your time entering it.

 

And I think I've wasted enought time on this thread.

[+Night Stalker]

The answer is a qualified yes. I searched for a cache with RK that was in a parking garage. One of the clues was the elevation. It was the only way we knew what floor to look for the cache on.

 

[+GEO*Trailblazer 1]

The receiver does not need a precise clock, but does need to have a clock with good short-term stability and received signals from four satellites in order to find its own latitude, longitude, elevation, and the precise time.

 

Why would it say this about elevation then.

I have found 700+ Benchmarks.The experience comes from many many observations,

And I am sorry you think you are wasting your time so I will stop ribbing you.

And end it with a sorry.

 

edit speliing

Edited September 18, 2004 by GEO*Trailblazer 1

[+EraSeek]

Scotty, beam me down. W122 47.223 N47 48.001 . But dammit! Get the elevation right this time!!!!!

 

It matters if you live in a 3 dimensional world, but not if you live in Flatland.

[+Prime Suspect]

Scotty, beam me down. W122  47.223  N47 48.001 . But dammit! Get the elevation right this time!!!!!

 

It matters if you live in a 3 dimensional world, but not if you live in Flatland.

It's not exactly Flatland, but Ellipsoid Land, which is pretty close.

 

Go outside with your GPS and get a good lock on at least 4 satellites. Mark a waypoint where you're standing. Now edit that waypoint by adding 50,000ft to the elevation value. Standing in the same spot, do a Go-To for that waypoint. Does it say the distance to the waypoint is 9.46 miles, or a couple of feet?

 

Welcome to Ellipsoid Land, where only latitude and longitude are used to calculate distance and bearing.

 

And if anyone else is having trouble wrapping their mind around this, I would suggest moving the discussion to the GPS Units and Software forum, where it really belongs.

[+EraSeek]

Oh who cares where it is. This whole thing does present some issues I'd like to consider more. Yes I think you are right that for basic Geocaching elevation does not matter. You can input coords without elevations and it will take you to a 2 dimensional point in a GOTO. And yet elevation does play a part. It will be part of the solution when getting a satellite fix. In fact my GPS manual says "NEW ELEVATION- This option can be used to enter a different elevation. If the gps has a 2D GPS location, entering a known elevation can increase the GPS accuracy."

 

Now if you think about it when your unit triangulates (really trilaterization or something like that) the satellite fix, it is not 2 dimensional, but 3 dimensional, so elevation would have to matter in some manner. Perhaps only in elevation display. Worth researching a bit more.

[davidbrit2]

Hmm, this raises an interesting question. Are there actually any high end units currently available that take elevation into account when navigating to a waypoint? I'm sure all our humble off-the-shelf units from Galyans, Circuit City, et. al. don't do it (including my eTrex) but I'd imagine there's equipment out there somewhere with this capability. You know, just in case someone wants to plant a cache on a vertical cliff face. Ha ha.

[+EraSeek]

The interesting thing here is that the GPS sloution and fix are 3D, but the unit treats Lat/Long and Elev seperately in display and GOTO. You would think that they would be treated as one. Thus elevation would be part of the waypoint "it's 500' up there". Perhaps it is treated seperately to increase the perception of accuracy. I would like to see it the other way where elevation is an intergrated part of the position.

[+EraSeek]

"Orthometric Height (MSL)

 

 

A minimum of four GPS NAVSTAR satellites (more can be used in overdetermined PVT solutions) are used to determine three position coordinates and time. Position coordinates are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates. Most receivers compute (and store) positions in geodetic coordinates (datum WGS-84) latitude, longitude and height above the ellipsoid (HAE). Note - height calculations are independent of any user selected horizontal datum.

 

The height displayed on most consumer handheld GPS receivers is orthometric height, the height above mean sea level (MSL). It is straight forward to approximate MSL world wide by interpolation of the GEOID model (table) and making the simple calculation:

 

 

GEOID03 is a refined model of the geoid in the United States, which supersedes the previous models."

 

So some interesting info. I also looked at horizontal and verticle accuracy and found that the accuracy is improving over time probably due to system improvements. Generally now less than 3m horizontally and 5-6 meters vertically under perfect conditiions.

 

Find an image of these models here:

http://www.edu-observatory.org/gps/height.html

OK maybe a bit much for "getting started"

Edited September 19, 2004 by EraSeek