Electronic Barometric Pressure Altimeter

[SwissTeam]

When considering buying either the eXplorist 500 or 600 I decided to go with the 500. Here are the reasons why, I just had the chance to try out one of those fancy watches, a Tissot T-Touch who also does measure the barometric pressure and also the altimeter based on the pressure. So, one morning in my office, which is just a hundred yards from the sea, I set the altimeter to sea level = 0 feet. 9 hours later I checked the altimeter and lo and behold, without moving it, it said 200 feet! Why, because the barometric pressure changed, and therefore also the altimeter ... what a pain. The manual said, that the alitmeter had to be calibrated frequently in order to be accurate, BUT in order to do that, one always has to know the actual elevation.

 

The eXplorist 600, if I'm not mistaken works after the same principle.

 

What do others think about that?

Edited April 5, 2005 by SwissTeam

[+jacques0]

my office, which is just a hundred yards from the sea

What do others think about that?

 

I think I'd be more concerned about global warming than my gps! LOL

[CenTexDodger]

The difference is a GPS with barometric altimeter uses the GPS elevation to "auto-calibrate" the altimeter (at least that is how the 60cs works). It is not as accurate as manually calibrating the unit, but it is good enough.

[+JohnnyVegas]

I have a Suunto watch with the compass, Barometer, Altimeter and time function. I have never seen of a need in the altimeter/barometer for geocaching, Besides when I have compared the altitide reading on my Meridian gold or sport track map running topo software to the reading on my Suunto I have not seen much of a differance.

[peter]

The difference is a GPS with barometric altimeter uses the GPS elevation to "auto-calibrate" the altimeter (at least that is how the 60cs works).

Yes, that is how all of the Garmin receivers with pressure sensors (60/76cs, 76s, Vistas, Summit, Geko301) work and it takes advantage of the strengths of both GPS and pressure-based measurements of altitude. The auto-calibration works on about a 30 minute time scale so short-term GPS fluctuations are averaged out.

 

However, according to the eXplorist 300 manual it does not provide for this type of auto-calibration and therefore offers no advantage over a regular barometric altimeter like the one in the OP's watch. Not clear yet if the 600 will have that as an enhancement over the 300.

[rickertk]

I've been using electronic barometric altimeters for about 10 yrs or so, mostly in a bike computer, but I now have a VistaC which also has one. How useful they are does somewhat depend on what information you are looking to have. 200 ft of change in 9 hours isn't that unusual if weather conditions are changing somewhat. If you need to know absolute elevations with a high degree of accuracy, with a barometric system, you'll definitely need to calibrate it pretty regularly. If you are more interested in relative elevations - how much did you climb on this ride/hike, or over a particular section of road or trail, you don't have to worry about it as much. I rode the same set of hills week in and week out; my starting elevation varied by as much as 3-400 feet, but the climb to the peak of the hill only varied by <100 ft. (this is over a 1600 ft climb). Barometrics will vary more over time in a fixed location than a GPS altitude, but should be able to generate a more accurate track profile over a short period of time, especially if GPS reception is less than ideal.

 

Keith

[+IVxIV]

I would think having a barometric sensor is more beneficial in units that sample that pressure over time, and provides an approximate weather forcast, so you might be warned in advance that a storm is approaching. The Casio Pathfinder wristwatches can do that, can the GPSr's so equipped do that? (ie. if the barometric pressure has been stealily dropping the past few hours, a storm is probably approaching)

 

Tracking altitude is just a bonus.

[+GeoFamof4]

I believe the Meriplat does that. I don't use that feature too often but soon after I purchased it I was in an area under a tornado warning. With nothing else to do but watch the storm out the windows, I turned on my Meriplat to watch the pressure drop. If I remember correctly, it dropped to around 26 in a pretty short time. It does show the current trend too, such as rising or falling.

[SwissTeam]

The difference is a GPS with barometric altimeter uses the GPS elevation to "auto-calibrate" the altimeter (at least that is how the 60cs works). It is not as accurate as manually calibrating the unit, but it is good enough.

So, how much more accurate is a autocalibrated altimeter which is averaged out vs. a pure gps altimeter signal?

[+EScout]

In an earlier post, I explained that I tested my eXplorist 500 and Meridian this weekend at a super accurate benchmark. The elevation at this mark was 317 feet. Both GPSrs were within 15 feet of this, and at one point the 500 was within 5 feet of this.

At other marks in the past, I have found my GPSr often within 20 feet and almost always within 50 feet. In addition, topo software will be another source of elevation. How accurate do you need.

Regarding a compass, I always carry one and I think they are easier to site than a compass screen on GPSr. And you do not have to calibrate it. And mine has a declination scale so I can read true or magnetic. And it is easier to use with a paper map. And if you are hiking any distance you need to bring a compass anyway.

[peter]

So, how much more accurate is a autocalibrated altimeter which is averaged out vs. a pure gps altimeter signal?

Under good reception conditions the pure GPS altimeter should be within about 40' 95% of the time whereas the time-averaged value should be within about 10'. But the real benefit comes when you're travelling under conditions where the GPS reception is frequently marginal. The autocalibration can still work by only considering the periods with good GPS reception, but at any given moment you may have only a 2D position lock so the GPS altitude reading would be based on the last time when it had a 3D lock which might be considerably different from your actual current elevation.

[+Alan2]

Losing lock has an interesting effect on the altitude tracking. When I first got my Vista, I tracked a trip from Queens into Manhattan through the Midtown Tunnel that travels about 50 feet under the East River in NYC. WHen I checked the track on the Mapsource display on my PC, the toll booth was 50 feet under water - the spot was still reading the accurate elevation from when it was under the East River but of course had no satellite lock at the time. It wasn't until I drove a little further and got the first lock after the tolls that the waypoint and altitude updated to the correct altitude in the track storage.

 

That's something to remember when your skiing also. If you lose lock under trees, you might be downhill before the altitude catches up the actual spot on the track.

[+Deneye]

Heading off on a slight tangent here, does anyone know if wrong altitude readings affect the hunt?

 

Cache coordinates are 2 dimensional (lat. & long.), and if the GPSr uses the third dimension (altitude/elevation), it will set the waypoint elevation to the unit's current elevation at the time of inputting the waypoint coords.

 

if there is a difference of say 100 feet or more in elevation/altitude between the recording of the waypoint and the location of the cache, wouldn't this (shouldn't this?) give readings of something like "80 feet to go" even if you happen to be standing next to the cache???

 

I mean, wouldn't the GPSr figure that third dimension into the distance calculations...and inherently be wrong since no altitude is ever given for a cache?

[peter]

I mean, wouldn't the GPSr figure that third dimension into the distance calculations...and inherently be wrong since no altitude is ever given for a cache?

No, all GPS receivers that I know of measure distances strictly on the basis of the horizontal positions and ignore the altitudes of the waypoints. So if you enter a waypoint while on the ground and then get in a balloon that takes you straight up a few thousand feet, the GPS will still show you as being in the same place as the waypoint and the distance will be shown as 0' (or as close to that as reasonable given the inherent limits to accuracy).

[+JohnnyVegas]

I think the hunt is going to be more effected by cachers that only look in two dimensions

[+Rubberhead]

The deal is that you've got one sensor (ambient pressure) that can measure two parameters: Altitude and Barometric Pressure. This is the algebra equivelent of one equation with two unknowns.

 

If you stay at a constant elevation over time the ambient pressure meter is a barometer. There is one equation and one unknown so this is a solvable problem. In your by-the-sea example your ambient pressure meter was telling you that the barometric pressure was dropping.

 

If you stay at a constant barometric pressure (ie, constant weather pattern) the ambient pressure meter is an altimeter. Again, one equation, one unknown - problem solved.

 

If, over a fixed interval of time, both the barometric pressure and the elevation change then the ambient pressure meter can not solve for either barometric pressure or elevation: one equation with two unknowns. You can acutally solve this problem but you have to either know or assume a value for one of the unknowns. This is called calibration.

 

A GPS with a ambient pressure meter has an advantage over a stand-alone barometric pressure altimeter since it can always calibrate itself against elevation from the GPS satellites. Stand alone units have to have either a known elevation or a known barometric presssure entered to create the one equation with one unknown situation.

 

If helps to understand that the history of why the first ambient pressure meter was added to a GPS. It was done for hikers that wanted to keep track of elevation even when they lost a GPS signal for a little while. As long as the weather patterns were stable during the time the GPS signal is lost, then the ambient pressure meter is a very accurate altimeter.

 

It also works well as a weather forecaster when camped for the night since the elevation isn't changing the ambient pressure meter is a very accurate barometer.

 

Sorry for being so long winded. I hope this helps.

[+jacques0]

Sorry for being so long winded.  I hope this helps.

 

No need to apologize. That was a very good explanation. Thanks.

[+Deneye]

No, all GPS receivers that I know of measure distances strictly on the basis of the horizontal positions and ignore the altitudes of the waypoints.

Ok, thanks.