# Should I trust Google Earth or my altimeter ? (matter of elevation)
**URL:** https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826
**Category:** In My Humble Opinion
**Created:** [July 23, 2007, 10:41am UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826 "2007-07-23T10:41:04Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@Gymnopithys](https://boards.straightdope.com/u/Gymnopithys)
#### Post date: [July 23, 2007, 10:41am UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826/1 "2007-07-23T10:41:04Z")
I have a place with very good Google Earth definition. Elevation is 315 m. My altimeter shows 340 m, taken on two consecutive years. The area is quite flat, no mountains. I know an altimeter may vary by more or less 10 m according to the atmospheric pressure. But 25 meters (80 ft) ?
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### Author:  [@flyboy](https://boards.straightdope.com/u/flyboy)
#### Post date: [July 23, 2007, 1:40pm UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826/2 "2007-07-23T13:40:22Z")
I would trust your altimeter, although in my line of business, we’re usually happy if our barometric altimeter is within +/- 75 ft of a known elevation.
Google Earth probably uses some form of digital terrain elevation data, and in order to keep the size of the data down, the sampling is probably not as dense as it would be for other uses (e.g., targeting). So, what you may be seeing on Google may be the average of several near-by points (samples) of data and not the actual elevation you’re at.
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### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [July 23, 2007, 3:04pm UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826/3 "2007-07-23T15:04:41Z")
My heart rate monitor has an altimeter function. It has to be calibrated or when I go to ride at the bike path at the beach it tells me I am 70’ below sea level.
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### Author:  [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)
#### Post date: [July 23, 2007, 6:02pm UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826/4 "2007-07-23T18:02:20Z")
[QUOTE=Rick]
when I go to ride at the bike path at the beach it tells me I am 70’ below sea level.
[/QUOTE]
Swim!, Swim for your life, Rick!
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### Author:  [@butler1850](https://boards.straightdope.com/u/butler1850)
#### Post date: [July 23, 2007, 7:47pm UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826/5 "2007-07-23T19:47:47Z")
Of course, you could bypass both GE and your equipment, and trust the USGS (US Geological Survey), and get the exact info, within 10 feet or so, from a topographical map of your area. You don’t have a location posted, so I’m unable to find a link to one for you for your area, though there are multiple sites online:
[www.topozone.com](http://www.topozone.com) is a start.
FWIW, the topo map, and GE are within a foot or two of each other, depending on which spot I reference.
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### Author:  [@Gymnopithys](https://boards.straightdope.com/u/Gymnopithys)
#### Post date: [July 23, 2007, 7:52pm UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826/6 "2007-07-23T19:52:15Z")
No need to swim if he is pedalling in Death Valley. 🙂
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### Author:  [@Gymnopithys](https://boards.straightdope.com/u/Gymnopithys)
#### Post date: [July 23, 2007, 8:07pm UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826/7 "2007-07-23T20:07:14Z")
[QUOTE=butler1850]
Of course, you could bypass both GE and your equipment, and trust the USGS (US Geological Survey), and get the exact info, within 10 feet or so, from a topographical map of your area. You don’t have a location posted, so I’m unable to find a link to one for you for your area, though there are multiple sites online:
[www.topozone.com](http://www.topozone.com) is a start.
FWIW, the topo map, and GE are within a foot or two of each other, depending on which spot I reference.
[/QUOTE]
The area is in Sardinia where I’m doing some botanical research.
I’ll try to compare GE with some precise topo maps for some European countries.
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### Author:  [@Xema](https://boards.straightdope.com/u/Xema)
#### Post date: [July 24, 2007, 2:15am UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826/8 "2007-07-24T02:15:42Z")
[QUOTE=Gymnopithys]
I know an altimeter may vary by more or less 10 m according to the atmospheric pressure. But 25 meters (80 ft) ?
[/QUOTE]
It can vary a lot more than that. According to [this calculator](http://www.srh.noaa.gov/elp/wxcalc/altimetersetting.shtml) the difference between 29.5" hg and 30" hg is 470’ of altitude.
Thus, if you wish to measue altitudes accurately, you’ll certainly need a current local pressure setting. Alternately, you could take the altimeter to a place whose altitude is known (e.g. a local airfield), set it, and then take it to the place whose altitude you wish to measure.
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### Author:  [@Gymnopithys](https://boards.straightdope.com/u/Gymnopithys)
#### Post date: [July 24, 2007, 7:18am UTC](https://boards.straightdope.com/t/should-i-trust-google-earth-or-my-altimeter-matter-of-elevation/412826/9 "2007-07-24T07:18:09Z")
Good links Xema, Butler.
Thank you all.