On Wednesday, May 30, 2012 3:52:41 PM UTC-7, Martin Gregorie wrote:
On Wed, 30 May 2012 07:03:43 -0700, Darryl Ramm wrote:

I'm hoping you know those settings used in the devices from whoever you
purchase them. I think its is important that the approving agencies are
provided with that data as well, and that they understand it. And I
think it would help with transparency if that data was in future made
public as a part of any approval process. With many GPS chipsets telling
us the model chipset used without that extra data is next to useless.

Daryl,

In my quite limited experience the only GPS I know for certain smoothed
the trace was the Garmin GPS II+ and I know why as well: it only stores a
few points (1024 or 2048 IIRC) and so to hold all of a long trail it has
to condense runs of fixes into a straight line with just the ends
retained. Of course this would have made it utterly useless as a COTS
logger. However, AFAIK it has never messed about with the fixes it
outputs. At least I've never seen any sign of this in the traces dumped
from my EW Model D logger, which is invariably connected to one of my
Garmin GPS II+ units.

On a slightly different topic: GPS altitude. I've always known that all
GPS altitudes are relative to the WG-84 geoid but have never known how
precisely that corresponds sea level, so I finally did some research and
it turns out that its within +/- 1 metre of AMSL.

That is less than the error in a standard GPS receiver's height
measurement. I don't believe I've ever seen an EPE of less than 3 metres.
Just now one of my GPS II+ units said EPE=5m when I took it outside. So,
my guess is that for almost all our purposes its reasonable to take a
valid GPS height as being equivalent to altitude AMSL provided an error
of +/- 20-25 feet is acceptable for the task in hand.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Martin that is all nice, but for particular devices here people are observing altitude errors of 1,000' or so (presumably caused by 2D fixes being marked incorrectly as valid 3D fixes). To me that's the issue, not whether GPS altitude in principle is usable. It is, but as I think this is showing the devil is in the details of how devices work/are usable in detail in the field (and consequently what the specifications they are required to meet are and how they are approved for use). Hopefully there are some quick product fixes possible here and some look at improving the specs and/or approval process.

Darryl