Garmin eTrex Vista 'Pressure Plot'

[Cache Canucks]

Does anyone know what governs the 'start' and 'stop' of the Vista's 12-hour barometric pressure plot? It appears that the data which makes up the displayed plot doesn't 'wrap' (ie: it doesn't always display a full 12 hours worth of previous barometric pressure). I thought that perhaps it might have had something to do with when I actually turn the Vista 'On'/'Off' (it doesn't), or when I change its batteries (nope, not that either). I also played with resetting the altimeter and trip computer data, thinking that 'zeroing' it might serve as the 'start' of the barometer's 12-hour data collection (it doesn't seem to). Unless I missed something, the Vista's manual doesn't make any reference to this. Any suggestions...?

[+apersson850]

Well, it actually is dependent upon when you turn the unit on and off.

 

The Vista has a display resolution of 5.9 pixels per mm, which means that the pressure plot display is 146 pixels wide (or so). Since it plots over 12 hours, that means that every pixel column is worth about five minutes. Just let's say it actually is five minutes. What I now will describe is not necessarily how Garmin's people have implemented it, just how I think they did it.

I'm a programmer myself, so I'm rather familiar with problems like this one.

 

When you turn the Vista on, for the first time, there is no data about the pressure stored in the unit. So it will start displaying the plot from the right side (now) of the display, moving it to the left side (past) one pixel column every five minutes.

 

Now, imagine the following scenario:

You have your Vista on for 12 hours, collecting a plot that fills the screen. This plot is then based upon a table with data pairs of time and pressure. A new data pair is sampled every five minutes.

 

Now you turn the unit off, and wait eight hours before you turn it on again.

 

Now, there is data available for the first four hours (to the left in the screen), but not for the last eight. So the Vista will interpolate, and make a straight line between the last known time/pressure pair, to the current pressure. In this case there is a real plot for four hours, then just a line for the last eight.

 

Now, let the unit stay on. It will plot the pressure, and the interpolated line will move to the left. After four hours, the start of the interpolated line will be at the left side of the screen, and a four hour plot will be at the right side.

 

But when the next data pair is collected, and inserted to the right, the last known combination of time and pressure shifts over the left edge of the plot. Now, the Vista doesn't create a new virtual point of origing, at the time represented by current time minus twelve hours, and with the pressure represented by the interpolated pressure at the left edge of the plot. It could have, if they had been a little smarter at Garmin, but they choose to simply start plotting at the oldest data pair, that is no older than twelve hours! In this case, the first eight hours of the plot disappears, and only the last four (which are really known) stay on the screen. It will take another eight hours to fill up the table of pressure values again, so that the plot will fill the window.

 

Anders

[Cache Canucks]

Thanks Ander, pleasure to 'meet' you!

 

I broke down and gave Garmin a call late last week with regard to this very question and their response was more or less the same ...although, it DID initially stump the customer service rep with whom I spoke (he had to seek the answer from another subject matter expert within Garmin). Judging by its functionality (and the lack of its descriptive text in the user's manual), it appears to me that the barometric pressure plot on the Vista is pretty much a 'biggy back' feature of the altimeter (something like getting a trip odometer on your car, mainly because the master odometer needs to be there anyway). Although the Vista's barometric pressure plot might be more useful if it enabled the user to manually reset the 'Start' of the 12 hour plot period and/or define the 'x' and 'y' axis of the plot itself (as can be done with the altimeter display), it's clear that the primary purpose of the barometer is to provide pressure data to the altimeter ...not to predict weather trends. Oh well, perhaps this is something that Garmin can address with a simple update to the software (...Garmin's R&D staff *do* read these posts, don't they?).

[+apersson850]

The new GPS 76S, which is also equipped with a pressure sensor, has a more developed software for handling the information from this sensor. There you can tell it that you are actually on the same elevation (i.e. at standstill, or on the sea), so that it will know that every pressure change is due to the wheather.

Or you can let it assume that the pressure is the same, it's the elevation that changes.

 

Actually, the Barometer pressure, which the Vista can report, is only correct directly after a calibration. As soon as the pressure changes, be it for any reason, the unit can't report the correct Barometer pressure until re-calibrated.

 

This shortcoming, which I discovered by some experimentation, was so unbelivable to me, that I had to ask Garmin's support about it. But they confirmed, and also stated that the 76S is better. Although they couldn't tell if the Vista firmware will ever be corrected to behave better.

 

So, your assumption that the pressure plot is something they did over a coffee break, just because it was easy, is probably correct.

 

By the way, nice to "meet" you too!

 

Anders

[+apersson850]

The new GPS 76S, which is also equipped with a pressure sensor, has a more developed software for handling the information from this sensor. There you can tell it that you are actually on the same elevation (i.e. at standstill, or on the sea), so that it will know that every pressure change is due to the wheather.

Or you can let it assume that the pressure is the same, it's the elevation that changes.

 

Actually, the Barometer pressure, which the Vista can report, is only correct directly after a calibration. As soon as the pressure changes, be it for any reason, the unit can't report the correct Barometer pressure until re-calibrated.

 

This shortcoming, which I discovered by some experimentation, was so unbelivable to me, that I had to ask Garmin's support about it. But they confirmed, and also stated that the 76S is better. Although they couldn't tell if the Vista firmware will ever be corrected to behave better.

 

So, your assumption that the pressure plot is something they did over a coffee break, just because it was easy, is probably correct.

 

By the way, nice to "meet" you too!

 

Anders