[ale] WAY [OT] but geeky - how do I calibrate GPS barometric altimeter

[Ron Frazier (ALE)]

Hi Michael, and all,

I may have to look for some of those markers.  However, I found an 
alternate method.  I found out that the US Geological Survey has very 
detailed topographical maps, many with satellite imagery of pretty much 
the entire country.  You can buy them on paper but you can download them 
for free.

You can search for the very detailed maps by city name here:

http://store.usgs.gov/b2c_usgs/b2c/display/%28xcm=r3standardpitrex_prd&layout=6_1_61_50_2&uiarea=2&ctype=areaDetails&carea=0000000015%29/.do

Or you can search graphically from a map by placing a marker here:

http://store.usgs.gov/b2c_usgs/usgs/maplocator/%28isQuery=yes&xcm=r3standardpitrex_prd&query=*n+34+14+41+w+84+04+06*&layout=6_1_61_60&uiarea=2&ctype=areaDetails&carea=0000000562%29/.do

I downloaded the topo map for the area around my house, and around the 
trail that I walk at.

It turns out that my house is at 1160 ft, not 1210.  About the same 
time, the GPS estimate of my elevation was 1230, which is 70 ft too 
high.  I turned off the function that autocalibrates the altimeter based 
on GPS.  I then told the unit it was sitting at 1160 ft.  Once I did 
that, I was able to get a very accurate reading of barometric pressure 
on the GPS device.  It said 29.85 and WeatherBug said 29.84 for my 
area.  It looks like WeatherBug may be the best source of Barometric 
pressure for this location.  For whatever reason, The Weather Channel 
was .07 in hg higher.  Since I originally set the device, the indicated 
pressure has been drifting upwards even though the weather isn't 
changing.  That may be due to temperature differences since I was 
holding it in my hand.

As long as I calibrate it before going on the trail, and as long as 
there are no abrupt changes in the weather, I think I can get pretty 
accurate readings on the altimeter for several hours without the GPS 
calibration function turned on.

Sincerely,

Ron


On 2/25/2013 8:32 PM, Michael Potter wrote:
> Lake Lanier should have some elevation markers for water level.
>
> On Mon, Feb 25, 2013 at 8:07 PM, Ron Frazier (ALE)
> <atllinuxenthinfo at techstarship.com>  wrote:
>    
>> I don't think Cumming has an airport, but I'll keep it in mind.
>>
>> I took the GPS out with me to supper.  When I left, my house was at 1210 feet above sea level.  By the time I got back, it had sunk (on the display) to 1170 feet.  In the last 20 minutes of me sitting here, it has raised back up to 1185 feet.
>>
>> So, I guess I cannot expect too much from the elevation function of this device.
>>
>> Sincerely,
>>
>> Ron
>>
>>
>> Pete Hardie<pete.hardie at gmail.com>  wrote:
>>
>>      
>>> Go down to the airport - it's a well-specified lat/lon and alt, and a
>>> weather station.
>>>
>>> Pete Hardie
>>> --------
>>> Better Living Through Bitmaps
>>>
>>>
>>> On Mon, Feb 25, 2013 at 5:53 PM, Ron Frazier (ALE)<
>>> atllinuxenthinfo at techstarship.com>  wrote:
>>>
>>>        
>>>> Hi all,
>>>>
>>>> I just got my hands on a neat Garmin Etrex Vista H handheld GPS on
>>>> clearance for hiking trails and such.  It has a barometric altimeter
>>>>          
>>> that I
>>>        
>>>> wish to calibrate.  I have to either enter an accurate known
>>>>          
>>> barometric
>>>        
>>>> pressure; or I have to enter an accurate known elevation.
>>>>
>>>> I've checked Weatherbug (for Cumming, GA), The Weather Channel, and
>>>>          
>>> the
>>>        
>>>> Firefox ForecastFox plugin.
>>>>
>>>> The barometric pressure readings are: 29.85, 29.92, and 29.90.  As I
>>>> understand it, these variations would be enough to create altimeter
>>>>          
>>> reading
>>>        
>>>> differences of around 70 feet.
>>>>
>>>> I can't necessarily use my weather station as a good barometric
>>>>          
>>> pressure
>>>        
>>>> gauge, since it depends on elevation.  (I usually calibrate that
>>>>          
>>> instrument
>>>        
>>>> by putting in the barometric pressure from The Weather Channel.)
>>>>
>>>> I don't know the exact elevation of my house, nor the exact elevation
>>>>          
>>> at
>>>        
>>>> the trail at the park.
>>>>
>>>> I can calibrate to GPS elevation, but that's not very accurate.
>>>>
>>>> Garmin says the altimeter is accurate to 10 feet if properly
>>>>          
>>> calibrated.
>>>        
>>>>   Of course, it will change with the weather.
>>>>
>>>> So, how can I determine accurate barometric pressure for a location,
>>>>          
>>> or
>>>        
>>>> accurate elevation, so I can calibrate this thing?  Is there a way to
>>>>          
>>> get
>>>        
>>>> elevation from google maps, or something?
>>>>
>>>> Note, this is not critical, not life threatening if I don't find the
>>>> answer, not even exceedingly important.  But, I just want to get the
>>>>          
>>> most
>>>        
>>>> usage out of the device that I can.  And, you guys are a great source
>>>>          
>>> of
>>>        
>>>> geeky information of all sorts.
>>>>
>>>> Any help, as always, is appreciated.
>>>>
>>>> Sincerely,
>>>>
>>>> Ron
>>>>
>>>>
>>>>          

-- 

(PS - If you email me and don't get a quick response, you might want to
call on the phone.  I get about 300 emails per day from alternate energy
mailing lists and such.  I don't always see new email messages very quickly.)

Ron Frazier
770-205-9422 (O)   Leave a message.
linuxdude AT techstarship.com