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Adjustment/Calibration of "ambient pressure" on Garmin GPS ?


webvan
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Having recently picked up a Moto X smartphone with a built-in barometer I pulled out my old Garmin handhelds and was happy to find that ambient pressure readings were (nearly) identical on the Moto X and the 60CSx (1021.1mb), however it's completely off on my Colorado 300 (1025mb) and Oregon 300 (1045mb) and I can't find a way to adjust/calibrate it (not talking about altimeter calibration here of course).

 

Anyone know if it's possible ? After searching the interwebs high and low it seems no one's had that problem/question, which is unusual ;-) Also it would be nice if the Colorado/Oregon could display one decimal...Thanks for any help !

Edited by webvan
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Thanks for the manual extracts but like I wrote I'm not talking about the Altimeter calibration, the problem here are the erroneous readings of the "raw" pressure, what Garmin call the "Ambient Pressure". It's probably just a "shift" in the value but it's annoying and it also means that you can't use a known QNH Pressure to calibrate the altimeter. There must be a hidden menu to adjust that value...

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No, you cannot change or adjust the ambient pressure reading. Nor do you need to....

 

The "Ambient Pressure", by definition, is the "surrounding" pressure that THAT gauge, "feels" at THAT time , at that place, under whatever existing circumstances ....

 

The ambient pressure number is used as an indicator as to what "this" gauge is feeling. You, as the operator, then use a "Known" elevation, or (Official)Barometric pressure reading to "Tell" this GPS gauge that the next time it feels this exact pressure(or right now ), that you want it to display that it is at "XX" elevation or the Baro Pressure is "XX".

 

When you calibrate, what you are doing is "adjusting " for whatever (ambient) pressure differences your unit is "feeling" vs another unit, another location, etc.

 

It can also be plotted but is a purely "relative" number......

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It's probably just a "shift" in the value but it's annoying and it also means that you can't use a known QNH Pressure to calibrate the altimeter. There must be a hidden menu to adjust that value...

 

While you can calibrate Garmin's barometric pressure for a known sea-level pressure reading or local elevation, AFAIK, there is no way to fine-tune the small plus or minus "shift" you are mentioning.

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Pressure sensors have different properties. As for any sensor, there is a complex tradeoff between price, sensitivity, accuracy, and stability.

 

When a pressure sensor is used as an altimeter, precision (sensitivity) and stability are more important than absolute accuracy. As a result, affordable GPS devices tend to have sensors that emphasize those parameters over accuracy.

 

Also, sensors have gotten better over time.

Edited by fizzymagic
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To the OP,

 

I understand the adjustment you're referring to and this thread struck a cord when I read it. The old Magellan Meridian used to allow the user to adjust pressure and elevation independently from one another so you could dial them both in perfectly. I appreciated that adjustment so much that about 6 years ago I wrote an email to Garmin outlining the procedure and benefits. Their reply was that this adjustment simply wasn't something they were concerned with (not many users really gave a $@#¥) so it probably wouldn't be implemented.

 

I'm always annoyed when I calibrate to the right pressure and the elevation is off or vise versa. Newer Garmin units that autocalibrate seem to work this disparity out over time and I haven't been as annoyed as I was with the Oregon 300 or 450. I'd give it a few days/weeks in continuous calibration mode and see what happens. I also emailed Garmin about an additional digit (hundredths) while using/calibrating inches of mercury units for the barometric pressure. I heard back asking me to clarify but after that I heard nothing but crickets. Only being able to adjust to the tenths when using inches of mercury puts you at a resolution disadvantage compared to using millibars. But, again, Garmin responds to volume of requests/complaints and since its lost on most users I'm not holding my breath.

 

Cheers,

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Thanks for the input and sorry I didn't reply earlier, I was only notified of the first reply.

 

So Garmin did get back to me and after initially failing to properly read my question replied with an extract from the manual where "calibrate using pressure" is mentioned... in a follow up they admitted that "Unfortunately there is no other way to calibrate the pressure on the device.". A rather neglectful omission if you ask me considering my Avocet Vertech from 1995 can be adjusted!

 

So you're stuck with either a wrong altitude/correct QNH pressure or a correct altitude/wrong QNH pressure. Since trends are what's important here you can live with a wrong QNH pressure but still, it's annoying and it shouldn't have to be like that.

Edited by webvan
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Unfortunately (or fortunately), Garmin isn't the only one with this problem. I have a Suunto Core watch that does the same thing -- set it for local pressure, and the elevation reading is off. Set it for elevation, and the local pressure reading is off. B)

 

Like Yogazoo, I've been using millibars of pressure instead of inches to get the most accurate calibration possible out of the Garmin. Also, if I'm in one location for a while, I'll go in to the settings and change them to "stationary" instead of "variable" elevation.

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Although I feel as though a simple firmware update could solve this problem, if only a small percentage of users find this to be a problem, I don't think Garmin will expend the energy. It would be simple to temporarily unchain the two numbers and allow for independent calibration of each. Just takes the motivation of us users who find it worthy of fixing to contact Garmin and place a request. I'm not putting the persistent problem on us, that's just the way it is. I will commit to contacting Garmin once monthly with the calibration request.

 

This is also not meant to detract from Garmin in any way. Their software engineers are overworked and underpaid and can't possibly respond to all the feature requests under the sun. The fact that they will if enough requests are made is a credit to them.

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If you search the web for information about altimeters,etc. you'll find alot of info on how hard it is to get accurate long term results.. too many factors change data fast. This is different then ground accuracy... elevation accuracy is not and never will be as accurate at any given time and place as ground accuracy.. It's just the nature of the beast.. it is very technical to achieve elevation pressure etc accuracy.

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So we went out to play tennis this morning with the court at 50' AMSL.

 

Now, the standard air pressure at that altitude is 29.87 inHg.

 

However, the actual air pressure was 30.35 inHg due to the weather effects.

 

Notice that using that pressure to determine elevation would put us 430 feet below Mean Sea Level.

 

Conclusion, one should not use a barometric device to determine elevation.

 

https://www.digitaldutch.com/atmoscalc/tableatmosphere.htm

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So we went out to play tennis this morning with the court at 50' AMSL.

 

Now, the standard air pressure at that altitude is 29.87 inHg.

 

However, the actual air pressure was 30.35 inHg due to the weather effects.

 

Notice that using that pressure to determine elevation would put us 430 feet below Mean Sea Level.

 

Conclusion, one should not use a barometric device to determine elevation.

 

https://www.digitaldutch.com/atmoscalc/tableatmosphere.htm

 

I agree completely!

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Another couple of posts that miss the point completely, did you even bother reading the first message?

 

Obviously you can't calculate elevation using standard air pressure since it's meant for use by planes flying at altitude so they have a common reference.

 

You CAN however calculate elevation if :

1. You know the local SLP (from METARS for instance)

2. Your barometer is calibrated to show the correct ambient pressure.

 

What is being discussed here is point 2 with devices unable to be calibrated.

 

@39_Steps - I dug out my old Delorme PN60 and can't even find a way to show barometer/altimeter readings. I was able to calibrate it in the menu though.

Edited by webvan
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Using a simple algorithm (^BP=^EL) I agree with the above statement. A more complex calculation however could help reduce some demons associated with the ever-changing nature of barometric pressure. A change in pressure doesn't always indicate a change in elevation, yet a change in elevation almost always results in a change in pressure. Soooo...

 

Garmin units can make moving rise/run calculations as reported in several data fields. Garmin handhelds also use an underlying DEM (if embedded in loaded map) to predict route elevation. Why not use either predicted or map based elevational data to help control for non-elevation related fluctuations in barometric pressure? The result wouldn't be perfect, but dadgum it would have to be better than the current method. I'm not sure what autocalibration does or how it does it, but perhaps something along those lines already occurs.

 

On a separate yet related note, speaking to cowboy hat wearing, tennis players under the sea. If allowed to adjust both baro and altitude independently, one could, at least in the short-term, experience more accurate numbers. This is what the OP discussed.

 

Before someone says it, I know I know, these are just consumer-grade units and there are other fish to fry. But, on the other hand, if Garmin doesn't deliver the best location information possible given the hardware, they may be missing a chance to appeal to those purely navigational/informational users. For example, the reported EPE data. Engineers wanted the EPE number to reflect a 95% confidence interval. Marketing wanted 50% to make that number smaller. As a result, we get less useful data sacrificed for the wow factor of a smaller error number.

 

I digress

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@39_Steps - I dug out my old Delorme PN60 and can't even find a way to show barometer/altimeter readings. I was able to calibrate it in the menu though.

If I knew where my wife dropped my PN-60 on the way to some mountaintop or other, I would look at it and suggest changing the viewable info fields display. We must have been in Tennessee or Colorado, or someplace like that looking for Davy Crocket's or some President's birthplace. Can't remember 'zactly.

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How I calibrate to ambient pressure:

1. I have a Casio watch (Module No. 3415) which displays atmospheric pressure in increments of 0.05inHg, e.g., 29.90, 29.95, 30.00, etc.

2. This morning about 6 AM, PST, it read 30.40inHg.

3. Looking at the weather data for my local airport, SNA, I saw 1029.6mb (adjusted to SL), or 30.41inHg.

4. Airport, Weather Conditions For:

Santa Ana, John Wayne Airport-Orange County Airport, CA. KSNA (NWS/FAA - SGX)

Elev: 52 ft.; Lat/Lon: 33.68000/-117.86639

Current Time: Jan 28 10:03 am PST, is about 12 miles from my house (2' AMSL) with very quiescent, no wind conditions.

http://www.wrh.noaa.gov/mesowest/getobext.php?wfo=sgx&sid=KSNA&num=72&raw=0

5. Consequently, I assume the SL adjusted pressure at the airport to be essentially identical to that at my location.

Edited by Team CowboyPapa
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