Garmin 2720 and elevation info

The 2720 is my first Garmin GPS. Is it typical for Garmin not to include
elevation info?
Are there any inexpensive electronic gadgets out there that would provide
elevations?
Is there any software updates that will do it?
Tim fm Ct

A pocket altimeter, usually had for under $50. This is what was used prior
to GPS.

I have a garmin E-trex that displays elevations, but it is so old (tiny
screen, com port data cable, too much time lag when changing positions in
the field (i turn east, it is still reading north)) I am thinking of
upgrading to the new Garmin mapping GPS. I have placed the lat, lon and
elevation (from my E-trex) on a digital map before, and it is usually pretty
close to the map's contour elevation.

tim fm ct wrote:


The Garmins that will record tracks include the GPS elevation. It saves
the lat/long position, the date and time, and the GPS elevation. Those
three elements of data will let you use MapSource (or other GPS
utilities) to calculate statistics such as the speeds, average speed,
elapsed times and distances, and also to draw elevation profiles from
the track point elevations.

Your 2720 will record 2,000 track points and you have a setting to
choose whether it does that continuously (overwriting the oldest point)
or stop collecting when the capacity is reached. You can download the
track to MapSource or with any of a number of GPS utilities. If you
wanted to drive longer distances and save tracks, you would have to
pause and download the track occasionally.

The bad news about GPS elevations is that they are not based on sea
level elevations, are pretty inaccurate, and can vary widely from one to
the next.

GPS elevations are relative to a geoid not to sea level or terrain
elevations and the accuracy of the elevations is much less than the
horizontal accuracy.

The only models that do a good job on determining and recording
elevations are the handhelds that have barometers in them (60CSx, 76CSx,
etc.).

Jack

I believe your 2720 will provide elevation, but you may have to search
through the various different screens to find it. E.g., my eMap shows
elevation only on the 'GPS Info' page which isn't the most
convenient. But it is there.

It is inherently a little less accurate (about a factor of 1.5) than
horizontal due to the basic geometry, but is still quite good. And
the geoid is a model of sea-level, so the elevation data provided is
relative to mean sea level. One indication that the elevation data is
actually pretty good is that using my eMap (which does not have the
barometric feature) I noticed that the elevations were consistently
about 15' lower when I was at places of known altitude. I checked
Garmin's geoid vs. ellipsoid table compared to the USGS values and
found that they differed by almost 5 meters - thus accounting for the
15' lower reported altitudes.

One caveat about the altitude data is that it requires a minimum of 4
satellites and more are preferred, so it can be way off in situations
where much of the sky is blocked by terrain, buildings, or especially
dense foliage.

peter wrote:

When I record tracks on boat deliveries and look at them from MapSource
they look to be almost wildly erratic in comparison to the actual
elevation changes that that occurred.

I see total variations of 50 feet or so. The way MapSource presents the
profile makes it appear to be of little or no use. The variation in
elevations from one track point to the next, if you just read the
numbers, is reasonable enough and seems to reflect the accuracy trends
you mention.

One thing I did notice was that as I go 200 NM from Cape Cod to Mount
Desert Island, it is "downhill". I start out seeing reported elevations
that are up around 40-70 feet or so and finish up down around elevations
of 10 or 20 feet.

And that is with a 76Cx propped on the dash of a boat moving in a
straight line at 20 knots for seven or eight hours at a stretch. The
76Cx has a WAAS fix with 10 to 12 channels of data, the wave heights are
two feet or less, and the track shows a total variation of 50 feet or so
but it just look wildly erratic as MapSource presents it.

Mainers always talk about "going up to Boston" and many people think it
is because they went north or uphill to get to the steamer landings in
the old days. It is confusing enough, as someone that is "from away",
to communicate with these folks. But if you try to tell them that the
reality is that they actually go "down" to Boston, they think you're nuts.

I'll be "from away" for the rest of my life there is no cure for being
from away. In fact, I have my epitaph chosen already. It will say

Jack





--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)