Whoopee!

[Team T&S]

dadgum I love this thing. Last night I had my first geocahing experience, walked up to within 6' of the cache at around midnight.

 

It's got the same software version it came with which is 4.52. The only bug I can see is the inability to calibrate vertical. That does bug me. I should be able to walk up to any benchmark and adjust the vertical readout to fit.

[peter]

The only bug I can see is the inability to calibrate vertical. That does bug me. I should be able to walk up to any benchmark and adjust the vertical readout to fit.

That's not a bug but an inherent characteristic of GPS measurements. Unlike pressure-based altimeters where calibration is needed to compensate for gradual changes in atmospheric pressure, the errors associated with GPS measurements tend to be more random. Adjusting to a known height at one place wouldn't necessarily make the reading more accurate at the next place where you may well be using a slightly different set of satellites.

 

So the only calibration that makes sense is one that's done based on real-time measurements of each separate satellite signal and this is the principle behind WAAS and other forms of DGPS. (The 12 doesn't do WAAS, but it would accept an input from a DGPS beacon receiver.)

[Team T&S]

That's not a bug but an inherent characteristic of GPS measurements.

Huh? Who told you that?

 

lol of course it's a bug! Or would be...The booklet tells how to enter an alt. But the unit won't let you do it...or will it?

 

Aha! After owning it and using it for six years, I finally just figured out how to enter an altitude. You have to do so while the unit is still acquiring.

 

I geuss even though it's always bugged me, it never bugged me enough to read the directions this closely...

 

I've had this g12 since 'I think '99, use it extensively for ocean fishing and at work.

 

Not only would I recommend it to anyone, I wouldn't recommend any other unless it costs less, and not many units cost less than the 12. I was surprised to find they still offer it!

 

Mind you I wouldn't recommend against any other unit, hey it's your money. Feel free to make yourself comfortable. It's all about fun.

 

I wouldn't give an extra 2¢ for WAAS unless I was trying to land an airplane by remote control. My only use for the altitude readout is as an additonal tool to help my place myself on a topo map, so I'd like it to be at least as tight as the contour interval of whatever map I'm using, thus my desire to calibrate on local BM whenever I can find one. The book specificlly describes the procedure:

 

Altitude Field

When the GPS 12 is acquiring satellites or navigating in the 2d mode, the last known altitude is used to determine your position. You may also manually enter your altitude for greater accuracy. In cases where the GPS 12 has 2D coverage, entering your approximate altitude will enable the reciever to determine a more accurate position fix.

 

To enter an altitude:

1. Highlight the "ALT" field, and press enter.

2. Enter a value, and press enter.

 

Unfortunately step 1 is a bit misleading- if the unit is still acquiring and if you highlight the field below the "alt." field, (which could also be called the "alt." field, I suppose,) then you get a chance to enter a BM alt. I'll be interested to see if after fully acquiring, the thing holds the given alt. or not. I'll find out tomorrow and report back.

[peter]

Once the unit gets enough satellites for a 3D solution (usually 4, but could be more if the geometry is very poor) it'll determine the altitude on its own pretty quickly and ignore the value you previously entered.

 

Entering the value when it doesn't have a 3D solution isn't 'calibrating' the altitude like it would be with a pressure-based altimeter. Rather it's giving the unit enough information to give you a better lat/long fix. Normally GPS receivers need 4 satellites to solve for 4 unknowns (lat, long, alt., and time). With only 3 satellites they need to assume a value for the altitude so they can calculate the remaining 3 unknowns. That's why the unit lets you enter a value under that circumstance since a more accurate entered altitude will let it give you a more accurate lat/long position.

 

But the way I read your first post was that you were expecting that entering a known altitude at one place (say at a benchmark) would then give you more accurate altitude values at other places. That's not the way GPS receivers work except for the few models that incorporate pressure sensors.

[Team T&S]

I see. Good info.

 

I called it a "bug" because though the book says you can enter your own alt, in fact you can't- or so I thought:p

 

Your description of things helps explain why the unit acts as it does. The manual is poorly worded; I know others who have independently had the same gripe about the "bug" keeping them from tightening up alt. accuracy. So we carry altimeters. That won't change soon!

 

Using a base station and rover, we regularly obtain gps vertical accuracies of a few hundreds of a foot. I'm certain that correction data makes this possible.

 

I'm not surprised to find it doesn't work that way with a single hh rover.

 

But when I'm 50 mi offshore and it says I'm at alt. 200'...

 

I'll be experimenting tomorrow with multiple bm's on the same datum: Go to several, spread out over a couple miles, see what the unit says vs. record; Then reboot and enter a bm value at one, then go back through them to see if anything changes. Not that I'm really all that worried about it, but I'm extremely curious.

[Team T&S]

well, acq. alts very close to record alts. Entered value changed as predicted. Thanks again for the insight. Wish vert. was so tight all the time.

[Prof. Y. Lupardi]

There is something else going on with the 'alt' the GPS12 gives.

In my country we have a local 0-level (called NAP) that extends (within a few dm.) over mainland Europe.

The top of my head (hat realy with GPS antenna on it!) is 2 m above NAP.

The difference between my height and the theoretical surface of the WGS84 ellipsoid is 44 m.

What alt does my GPS12 give me? Not something of 44 m but about 2 m (+/- 20 m accuracy). So somewhere there is a table of geoidheight in use.

[Team T&S]

Peter?

[peter]

What alt does my GPS12 give me? Not something of 44 m but about 2 m (+/- 20 m accuracy). So somewhere there is a table of geoidheight in use.

The 12 (and other Garmin and Magellan GPS receivers) reports the estimated height above the geoid, i.e. the model of where mean sea level would be. This is done using a lookup table to approximate the difference between the geoid and the WGS-84 ellipsoid. The lookup table value used by the unit for this difference is specified in the NMEA $GPGGA message.

[Team T&S]

Thanks Peter. Man you sure are knowledgeable about this stuff. Do you work with it? Or are you one of those guys who spontaneously sponges up large amounts of data? Either way I sure appreciate what you bring to the table here.