GPS Altitude Error !?!?!?

**ShawJohn** · 10-04-2004, 01:20 PM

I've noticed that my BU-303 reports that I'm 170-177ft above see level when I'm parked on a slipway with the rear wheels in the sea?!?

Now I know there are various other factors that can affect sea level like moon phase, and air pressure but something isn't right here.

I'm pressuming that the tides will affect this reading but to be 170ft different wouldn't that be a Tsunami tide ;-)

Is there anyway to calibrate the altitude etc...

John

**SAScooby** · 10-04-2004, 01:39 PM

how many satellites does it see and where exactly is it mounted ?

**Danceheaven** · 10-04-2004, 01:41 PM

Good point!!
Ive noticed that the alt is never that accurate, unfortunatly i never got that close to the sea!
Factors i can think off that will affect the height would be the number if sats you have a look on too,
more than 4 definatly, the more the better/accurate!
Prehaps its central sea level and not coast line level? still 170 ft is a big difference to play with?
what where the Weather condictions like?, rain can play a big factor on communication of satellite transmissions in the Ghz band

**Marsupial** · 10-04-2004, 02:00 PM

the sea level is a standard - its the mean sea level of all seas on earth, and yours can be a pretty higher or lower the other ones. Maybe you are at sea level at your place and are above or below actual sea level.

I wouldn't be bothered by this info, unless you are planning something where altitude is a primordial information.

**Danceheaven** · 10-04-2004, 02:03 PM

Originally Posted by Marsupial

the sea level is a standard - its the mean sea level of all seas on earth, and yours can be a pretty higher or lower the other ones. Maybe you are at sea level at your place and are above or below actual sea level.

I wouldn't be bothered by this info, unless you are planning something where altitude is a primordial information.

Ahhh so why doesnt all the water level out to one level like in my bath

Moon?

**carpcnoobie** · 10-04-2004, 02:51 PM

I've noticed this too, and just ignored it. I've noticed on my GPS sometimes it says I'm going down, and I'm going up a hill. Or vice versa.

The only thing I can theorize is that GPS is meant to triangulate your position. This triangulation is along a horizontal plane. There is no way for the sats to triangulate your position vertically. I imagine the alt. information is simply calculated by the time it takes the signal to actually get to the receiver. This can be thrown off by MANY atmospheric factors.

I've also read that the position of the sats that you're using at a given moment can determine how accurate your positioning is. If you have 2 sats that are fairly close together, then it will not be as accurate as say 3 sats that are spaced out well.

Once again, I'm not an expert or anything. So, if someone else wants to explain this better, please feel free.

**ukchris** · 10-04-2004, 02:58 PM

Sea level is not static, it varies around the globe, what is generally talked about as "Sea Level" is actually just an average. The earth isn't round, it's actually quite an odd shape which contributes significantly to all of this. You therefore have to consider where sea level is being measued from, in this case a set of satellites. As the mis-shapen earth rotates the distance from the satellite to various locations will vary.

As the satellites are put up by the US government I'd think they use whatever Americans thing of as sea level, therefore you're more likely to be off if you're overseas than in the US.

Makes you wonder about missile guidance though, hmm, maybe they use altimeters too, or maybe the satellite imaging used for targetting is measured from the same satellites and therefore helps them avoid missile bath-time.

I can live without knowing how hogh I am, as long as I have a vague idea WHERE I am!

**Marsupial** · 10-04-2004, 07:03 PM

Originally Posted by Danceheaven

Ahhh so why doesnt all the water level out to one level like in my bath

Moon?

Moon is a factor, as the level of the sea at a specific location will change in time because of it, but another big factor is stated bu UKCHRIS - the shape. Your bath has water only on one side. Think about a huge round bath that has water all around and in constant motion... do you think all water will be at the same level?
Particularly when there are obstacle to the motion of the water, such as islands or continents...

Nevertheless, I think that unless you have military GPS you won't get good readings on all the axis...

**EBFoxbat** · 10-04-2004, 07:03 PM

MY GPS uses atompshereic pressure to calculate altitude.

**kiltjim** · 10-04-2004, 08:54 PM

I'm not an expert, so I couldn't tell you if the sea level in specific areas differs around the globe. I always thought sea was pretty much at the same level globally except for the tides.

I would think it would be the GPS. Older GPS units had error built in, so that you couldn't build a rocket with pin point accuracy. If you look at the raw stream of GPS data, you will see that it jumps around alot. VisualGPS shows you this. While monitoring mine over a day or so, it jumped upto 500'. Download it, and just have your GPS sit on you desk for a day or so. That should tell you if its the GPS, or the sea, or whatever.