Elevation

[avslpfan81085]

Not sure if anyone else had an answer for this... I have a Garmin eTrex, and I know in it it uses the coordinates as well as elevation. well for one, everytime I try to enter in a new waypoint, the gps has a completely different elevation, but same coordinates... i'm talking hundreds of feet difference when I'm just sitting on the couch. Anyway, that wasn't the real question (merely a comment/observation)... what I want to know is if the elevation is going to screw anything up... ie. If i'm at the exact coordinates, but if the elevation that is in the GPS doesn't match that of my position, will it still let me know if I'm there, or is the elevation just kind of there?

[+briansnat]

If your eTrex doesn't have a barometric altimeter (only the Vista, Vista CX and Summit do), it uses sats to determine elevation. This is not the most accurate method and is often off by a hundred feet or so. Probably even more if you are in your living room and not getting a good sat lock. Even if it does have a barometric altimeter you will need to calibrate it frequently as it atmospheric conditions are consstantly changing.

 

That said, the elevation has no bearing on the coordinates. Its just there for informational purposes and will not affect your unit's accuracy when navigating to a waypoint.

Edited January 15, 2007 by briansnat

[+Wayfinders]

The horizontal coordinate is not effected by the elevation so you're good to go.

[+Alan2]

He raised a point that's never been clear in mind becauseI cannot get past the algebra and geometry in my mind.

 

The question is how does the GPS know you're at a particular location regardless of your height at that location?

 

Let's you're standing at sea level and you take a reading. The GPS system and sats calculate your location. Then you climb a hundred foot ladder at the same location. The first satellite west of you gives signals to your GPS that you're further west,. How do the other satellites also not place youi in a location different from your actual location not lnowing you're standing on a ladder?

Edited January 15, 2007 by Alan2

[4x4van]

Let's you're standing at sea level and you take a reading. The GPS system and sats calculate your location. Then you climb a hundred foot ladder at the same location. The first satellite west of you gives signals to your GPS that you're further west,. How do the other satellites also not place youi in a location different from your actual location not lnowing you're standing on a ladder?

Because the other satelites are at different angle from you than that first one. By use of triangulation, they can tell where you are, and they really don't care much about your vertical position. It's all about that Algebra & Geometry thing again!

Edited January 15, 2007 by 4x4van

[+tozainamboku]

He raised a point that's never been clear in mind becauseI cannot get past the algebra and geometry in my mind.

 

The question is how does the GPS know you're at a particular location regardless of your height at that location?

 

Let's you're standing at sea level and you take a reading. The GPS system and sats calculate your location. Then you climb a hundred foot ladder at the same location. The first satellite west of you gives signals to your GPS that you're further west,. How do the other satellites also not place youi in a location different from your actual location not lnowing you're standing on a ladder?

 

Your GPS will usually tell you if you have a 2D or 3D fix. For a 3D the GPS needs to get data from at least 4 satellites. It can then compute your location and elevation. If you only have 3 satellites, the GPS will give you a 2D fix. It does this by assuming your altitude. Depending on your GPS and where you were the last time you had it on this may or may not give you an accurate latitude and longitude. Most models will assume the last elevation you had. If you have a unit with a barometric altimeter, it will use that. But even that could be off by quite a bit if the weather has changed since you last had the unit on. Looking for caches in the So. California mountains, have found that I can be hundreds of feet off when I have a 2D fix. On the other hand, if I were always at sea level (or always cached where it was relatively flat) a 2D fix would be just as accurate a 3D fix.

[Donald, Daisy and Kids]

If your eTrex doesn't have a barometric altimeter (only the Vista, Vista CX and Summit do), it uses sats to determine elevation. This is not the most accurate method and is often off by a hundred feet or so. Probably even more if you are in your living room and not getting a good sat lock. Even if it does have a barometric altimeter you will need to calibrate it frequently as it atmospheric conditions are consstantly changing.

 

That said, the elevation has no bearing on the coordinates. Its just there for informational purposes and will not affect your unit's accuracy when navigating to a waypoint.

 

Ive found with my vista CX that the altimeter needs to be calibrated every time I turn it on. The compass allso needs to be re-calibrated everytime the batteries are changed.

Edited January 15, 2007 by Donald, Daisy + kids

[+cache_test_dummies]

Great topic. I've never really thought about all this before. Thanks for asking the question.

 

Oh, and, um, I'm kinda embarrassed to ask this because I've used my Garmin Vista for a couple of years now. But does anybody know the best way to calibrate the altimeter? I've never done it, or even thought to do it, up until now. And all of a sudden, it seems like a good idea.

 

(Sorry for the OT question.)

[Donald, Daisy and Kids]

Great topic. I've never really thought about all this before. Thanks for asking the question.

 

Oh, and, um, I'm kinda embarrassed to ask this because I've used my Garmin Vista for a couple of years now. But does anybody know the best way to calibrate the altimeter? I've never done it, or even thought to do it, up until now. And all of a sudden, it seems like a good idea.

 

(Sorry for the OT question.)

 

Not sure for the Vista but for the Vista cX my manual states:

"because the Vista cX relies on barometric pressure to determine the elevation and the pressure at any given elevation can fluctuate, you can calibrate the altimiter to increase its accuracy. You must know the elevation above sea level or pressure at your current location.

 

To manually calibrate the altimeter:

1: press the menu key to open the altimiter page options

2: Highlight "calibrate altimeter", and press enter to open the calibration page

3:at the message "Do you know the corrrect elevation" highligh YES and press enter, or If you do not know the elevation select NO and then select yes at the pressure option.

 

If you do not know the pressure select NO to use the default GPS elevation.

[+iamjbob151]

[+cache_test_dummies]

If you do not know the pressure select NO to use the default GPS elevation.

Thanks. The procedure seems to be the same for my Vista.

 

But since I'm not sure of my current elevation, and I don't know the correct pressure, I don't have sufficient information to calibrate the altimeter.

 

I'll have to try this again when I'm next to a barometer.

 

edit: spelling

Edited January 16, 2007 by cache_test_dummies

[Donald, Daisy and Kids]

But since I'm not sure of my current elevation, and I don't know the correct pressure, I don't have sufficient information to calibrate the altimeter.

 

I'll have to try this again when I'm next to a barometer.

 

I have previously gone to a known location (usually the airport) and calibrated mine there. If you ask the air services they will usually tell you the air pressure and guide you to one of their known altitude markers so you can calibrate.

[+cache_test_dummies]

I have previously gone to a known location (usually the airport) and calibrated mine there. If you ask the air services they will usually tell you the air pressure and guide you to one of their known altitude markers so you can calibrate.

briansnat is telling me that I need to calibrate my altimeter frequently, and you are telling me that I should go to the airport to do it. I guess I'll either have to move to the airport, or continue to get along with an uncalibrated altimeter.

[+Alan2]

The Vista will calibrate its barometric altitude to the satellites. Make sure you have the latest download for the Vista from Garmin. It can do this automatically. Or you can check the altitude and then hit calibrate to set the barometric altitude. I don't have my Vista handy to check the exact page and functions, but it is there. It could be on the satellite or altitude screens or both.

Edited January 16, 2007 by Alan2

[+StarBrand]

I've always tried to over-simplfy the altitude thing - the world is a remarkably flat smooth object when viewed from 26,000 miles out in space. So then Latitude and Longitude is meant to describe a point on the surface of that object despite any "minor" bumbs along the surface.

[+Prime Suspect]

I have previously gone to a known location (usually the airport) and calibrated mine there. If you ask the air services they will usually tell you the air pressure and guide you to one of their known altitude markers so you can calibrate.

briansnat is telling me that I need to calibrate my altimeter frequently, and you are telling me that I should go to the airport to do it. I guess I'll either have to move to the airport, or continue to get along with an uncalibrated altimeter.

Well, no, not really. Find a spot with a known altitude (got a regional airstrip nearby? maybe a benchmark?). Go there, then calibrate the GPS to that altitude. Now go back home, and take a reading at your front door. Unless you live over an active volcano, you can use that value to calibrate the GPS at home.

 

Or, if you can just turn on the auto-calibrate mode (or I should say, leave it on, since that's the default). When I've had an opportunity to check it, it's never been off more than 10' from the marked altitude.

Edited January 16, 2007 by Prime Suspect

[+dkwolf]

Or you can go to the USGS National Map viewer, look at the USGS 1:24,000 digital topo, or the NED (National Elevation Dataset) and find the elevation of your neighborhood, or a point you can find on the ground relatively easy.

 

Or, for a closer-to-geocaching activity, go to the gc.com frontpage, search for a benchmark and look through the descriptions to find one for vertical control....typically benchmark control discs. Go find that, and use the recorded elevation for that point.

[+cache_test_dummies]

Outstanding suggestions for determining elevation for altitude calibration. Thanks to all who responded to my question.

 

As a result of the recent ice storm, out house lost power a couple of days ago (it still hasn't been restored), so I haven't had a chance to try any of these ideas. But I will.

[Transalpchris]

Hey, i'm new at this forum so greetings to all.

I wonder what software i can use on my pc to get a chart of my elevation over time.

I have a "etrex summit" so the data is stored inside the unit. I think it would be nice to view my elevation difference that i had during travel.

Thanks.

[+Cedar Grove Seekers]

Using our old Garmin etrex (yellow) for our first 500 finds, we often found that when our GPS bounced around a lot, we'd change the elevation on the waypoint to reflect the actual elevation, and the GPS would often settle down and give a more accurate reading. We don't know if this was just coincidental, but conceptually it makes sense to us that elevation would have some impact.

[+Alan2]

Coincidence. The GPS receiver uses the satellite signal to determine its location and provide an altitude readout. Entering altitude data is only for "show".

 

On the other hand, entering data may slow you down so the GPS receiver is not bouncing around as you enter the data. This means getting better and more accurate reception.

 

So continue what you've been doing. It seems to work.

[+Cedar Grove Seekers]

Well I certainly don't want to stand around changing my elevation if it does nothing - I could alwys be looking for the cache while I wait for the GPS to settle down.

 

What I'm struggling with is this. If my waypoint is set to N40 00.000 W080 00.000 elevation 100 feet, and I'm at N40 00.000 W080 00.000 but the actual elevation is 2000 feet, why would the GPS and satellites think I am where I should be? The angles and distances will certainly differ between where I am and where I want to be, merely due to the difference in elevation.

[+tozainamboku]

Well I certainly don't want to stand around changing my elevation if it does nothing - I could alwys be looking for the cache while I wait for the GPS to settle down.

 

What I'm struggling with is this. If my waypoint is set to N40 00.000 W080 00.000 elevation 100 feet, and I'm at N40 00.000 W080 00.000 but the actual elevation is 2000 feet, why would the GPS and satellites think I am where I should be? The angles and distances will certainly differ between where I am and where I want to be, merely due to the difference in elevation.

I though I explained it above 2664724[/snapback].

One reason setting the elevation may cause your GPS not to jump around so much is if it is going between a 2D fix and a 3D as it sees more satellites. I've notice my GPS it will drop some satellites and try others until it gets a 3D solution that works for more than 4 satellites. I suspect that telling it the right elevation to start with will allow it to get a solution quicker.

 

Most units will ignore elevation when calculating the distance to a waypoint. If you are at N40 00.000 W080 00.000 elevation 2000 feet and have a waypoint set to N40 00.000 W080 00.000 elev. sea level, the GPS will say you are zero feet away, not 2000 ft. So you don't need to set the elevation of the cache, as that won't make any difference.

[+Cedar Grove Seekers]

Thanks Tozainamboku. Your 2D / 3D fix explanation makes sense (sorry I missed it earlier in the thread).