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

Mon Feb 25 23:59:58 EST 2013

The civilian rade GPS units by and large only claim to be accurate within 
50 feet or so -- in any direction. That certainly was proven true during a 
recent trip to Amelia Island, Florida -- just north of Jacksonville.  
the unit reported elevation on different days between -50 and +75 feet, 
when the car was parked ~maybe~ eight feet above mean high tide.

GPS units do exist that can pinpoint your location within a few small 
fractions of an inch in any direction, but they cost a LOT more than the 
Garmin on my dash.

Sean

On Monday, February 25, 2013 11:26:28 pm Ron Frazier (ALE) wrote:
> 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=r3sta
> ndardpitrex_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
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