Does GPS offer tidal correction?

[studiot]

I've been following several discussion between GPS experts(?) and I wondered what GPS does for tidal corrections when the position is reduced to a chart?

Edited July 28, 2014 by studiot

[xyzt]

I've been following several discussion between GPS experts(?) and I wondered what GPS does for tidal corrections when the position is reduced to a chart?

I am not sure I understand what you mean by "tidal correction" but all the corrections are listed here.

[studiot]

 

I am not sure I understand what you mean by "tidal correction"

 

GPS is a 3D space system, but terrestrial positioning is referred to a chart on some projection or other. That projection is onto some reference surface, but craft a rearely positioned directly on that durface due to marine tides. In the old days we used to have to make a manual correction for this, I wondered if the modern systems have a way of doing this automatically and what this would be.

I am not referring to earth tides, which are too slow.

[xyzt]

 

GPS is a 3D space system, but terrestrial positioning is referred to a chart on some projection or other.

The chart used by GPS is not flat, it is a fairly exact approximation of the geoid.

[studiot]

Which geoid?

 

Tides go up and down.

[John Cuthber]

Last time I used GPS it gave altitude WRT mean sea level (though not very accurately).

[studiot]

 

Last time I used GPS it gave altitude WRT mean sea level (though not very accurately).

 

 

Thank you John.

 

So that means that to determine the depth of water you would have to know the time and tide tables and perform calculation.

 

So it would not be automatic.

Edited July 28, 2014 by studiot

[John Cuthber]

"So that means that to determine the depth of water you would have to know the time and tide tables and perform calculation."

Or swing the lead.

 

If this

http://gpsinformation.net/main/altitude.htm

is right then the specification for how well it can judge altitude (15M) is bigger than the tidal range practically anywhere.

[studiot]

 

In any case, it is extremely unwise to overly depend on the altitude readout of a GPS. Those who use GPS altitude to aid in landing their small plane should have their insurance policies paid up at all times.

 

I like this last line from your link, very drole.

 

It's a long time since I moved oil rigs about, but the message seems still to be, don't rely on GPS.

Edited July 28, 2014 by studiot

[xyzt]

 

 

It's a long time since I moved oil rigs about, but the message seems still to be, don't rely on GPS.

...for altitude determination.

[studiot]

But unless you know the altitude, you don't know the plan coordinates.

[xyzt]

But unless you know the altitude, you don't know the plan coordinates.

You know them with the given error. caveat emptor.

[studiot]

When I used to move rigs, we used Decca, Trisponder and Syledis.

 

Occasionally they failed and we had to get the sextant out.

 

Nowadays there is (hopefully) enough redundancy in the GPS system to avoid the need for this.

 

But this is (an interesting) digression from the OP, which was about tides.

 

The need for tidal correction really comes in for marine geodetic surveying work and I was interested to learn if modern technology has managed to remove the need for significant post-fix calculation work.

[John Cuthber]

I'm just airing my ignorance here, but what accuracy can you get with a sextant (in good conditions)?

 

For example, let's assume you can put the rig down on the sea bed so it stops moving and you have good visibility of the sun (and the stars if you want to wait till night time) but you can't see land to use that as a guide.

[imatfaal]

Max would be about a quarter of a nautical mile (one quarter of one arcminute) - in reality you would have to be very good to get about 1 nautical mile

[studiot]

The point is that rigs are so heavy, that once they are in motion, you cannot just stop them and anchor (you probably can't anchor anyway for fear of hitting a pipeline).

 

No one would start off without better position control than a sextant, but once in motion the aim would be to bring the rig to a safe halt if EPS was lost.

 

This would mean monitoring the closing angle between other objects within view. There are always plenty of objects in an offshore oilfield.

Edited July 28, 2014 by studiot

[md65536]

So that means that to determine the depth of water you would have to know the time and tide tables and perform calculation.

 

So it would not be automatic.

I'm trying to follow this conversation but I don't get it. Is there an example of what you'd want it to do? I can't see why anyone would expect a GPS by default to give altitude relative to the tide level. If it did, would that mean that the readings on land would also fluctuate with the tides? Or that your altitude coordinate would be different depending on if you were on sea or land?

 

I can see nautical GPS applications for that. A lot of GPS devices run custom software, and I wouldn't doubt that someone has incorporated tide data into some software (even something general, might have some kind of "sea mode" setting). If not, it should be possible to automate that. But that should be handled after getting the position from the satellites, it shouldn't be built in to the satellite system.

[xyzt]

I'm trying to follow this conversation but I don't get it. Is there an example of what you'd want it to do? I can't see why anyone would expect a GPS by default to give altitude relative to the tide level. If it did, would that mean that the readings on land would also fluctuate with the tides? Or that your altitude coordinate would be different depending on if you were on sea or land?

 

I can see nautical GPS applications for that. A lot of GPS devices run custom software, and I wouldn't doubt that someone has incorporated tide data into some software (even something general, might have some kind of "sea mode" setting). If not, it should be possible to automate that. But that should be handled after getting the position from the satellites, it shouldn't be built in to the satellite system.

He wants to get the "altitude" adjusted to the tides. The answer is that it is being done but the errors are rather large. See here.