Garmin 128 GPS

[JOEMORAN]

I have a Garmin 128 fitted to my sail boat (Yamaha 30). The istrument displays an altitude reading of 72 all the time. Surely this should be around about 0 (sea-level) Do I need to do something about this? If yes, what do I do and how do I do it? Thank you.

 

[Trevor_swfyc]

Joe, I have noticed similar on my Furuno and Garmin handheld. It has not worried me because one thing I can be sure of when I am out in the boat. I am at sea level, unless perhaps if I have run aground.
If you are concerned perhaps you should get an alti-meter, light aircraft would not use GPS for altitude would they?. I think you have discovered why.

All the best
Trevor

 

[Gunfleet]

The GPS places you in a sphere. It makes no assumptions about whether you are getting wet or not. Look on the bright side... if 70ft is as far as you are out in the two dimensional plane too, that's pretty accurate positioning.

 

[Piers]

Some sets have the abilty to fix the altitude for height of aerial above the water. But given GPS doesn't know the state of the tide, this is somewhat irrelevant, and doesn't really help). You may also have the facility to between 2D or 3D fixes.

If you have this choice, I'd either fix the altitude, or choose 2D, so it doesn't become a distraction. Whatever you choose, it won't affect the position reading.

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm

 

[tome]

Joe

The Garmin 128 automatically runs in 3D mode unless there are fewer than 4 satellites, when it runs in 2D mode. It's possible to edit the height, but only whilst running in 2D mode.

The Garmin default datum is WGS-84. This is a whole-world datum which assumes that the earth is a perfect ellipsoid. In reality, it's more like a potato with a lot of undulations. This means that the zero height of the WGS-84 ellipsoid doesn't coincide very accurately with real-world zero height (ie sea level). To improve matters, you should switch to a local datum which gives a much better fit. For UK waters (except the channel Islands) this is normally OSGB36. Channel Island and continental charts tend to be ED50 (european Datum of 1950). You'll see the datum in use below the title of the chart.

To set your Garmin to OSGB36 select "Page" until you get to the Main Menu page. Scroll down to "Navigation" and press "Enter". Scroll down to Map Datum, press "Enter" again and go through the list until you see "Ord Srvy GB". Press "Enter" and then return to your navigation display. You should now see your altitude much closer to sea level.

A word of caution! GPS is less accurate in the vertical axis and you can expect an error of 2-3 times the horizontal error. If your set is accurate to 5 - 10m horizontal, then the vertical error could be as much as 10 -30 m. However, it should now start to average out at sea level (plus the height of your antenna).

Good luck!
TomE

 

[Plum]

Don't worry, this is normal. The altitude field can be changed to show a more usefull attribute, for example, average speed