Is there some tool, device, map or other thing that I can use so that if I am standing somewhere in the middle of nowhere I can say that this point is X number of feet above sea level? How accurate is such a device?
Thanks.
Google altimeter. You’ll get hundreds of hits, as well as a wide range of prices.
GPS can do this; this page implies that the accuracy of commercial GPS units is good to within 10-20 metres.
A lot of GPS include altimeters as a feature too. The ones I’ve seen for hiking are probably the most accurate, but the Garmin I use on my road bike has a basic altimeter too.
I’ve read about (but never bought nor used) small altimeters that you can put in the payloads of model rockets. They don’t give a digital or analog readout, but beep with the highest altitude reached. They beep for each digit. So 1,425 feet would go:
beep
beep beep beep beep
beep beep
beep beep beep beep beep
Supposedly they’re pretty accurate.
You’re going to love Garmin’s Forerunner 305. Mostly a jogger’s watch, but it has everything including an altimeter. It gets great reviews, and the price has really come down over the years. I haven’t checked the altimeter for accuracy, but everything else on it has shown to be very accurate.
Thanks for the answers. I guess I should clarify what I am looking for.
I am thinking of buying an old house near a lake (constructed flood control lake w/ a Dam). I want to see if the house will be underwater should the lake be held at maximum level. It is very close, perhaps I need to be accurate within ten or twenty feet…
Just look at a topo map of the area.
My older GPS unit would average if it was sitting still, and averaging greatly improves the accuracy of GPS. If you could find one that did this, that could be the answer.
Ten or twenty feet I think is pushing the accuracy of a pressure altimeter.
On the other hand, the topo map like beowulff suggested should be easy to find. Go to usgs.gov
This is a greater accuracy than you can probably get with commercial GPS, and substantially less than a typical barometric altimeter. As for topo maps, there are going to be few with contour intervals of less than 20 feet. If you want to be sure about such a critical issue, I think your only recourse may be having a survey done.
Also, GPS units usually give the height above the reference geoid, which is not necessarily (or even usually) equivalent to sea level. You can apply the correction, if you know how, and some more fancy GPS receivers can do this automatically.
If you know the current air pressure and have an accurate barometer, you can find your alititude to within about 20ft. My watch does this.
(Note that some of the reviews on that site are pretty clueless, complaining that “the altimeter needs recalibrating” - well duh, of course it does as the pressure changes, and that “you can’t use the barometer and the altimeter at the same time” - well double duh, how would that work, Einstein? The watch senses the change in pressure - you have to tell it whether that’s due to a change in height or a change in SLP.)
You may not need a well calibrated barometric altimeter, just a precise one (which isn’t too hard to find). For instance, if the lake is nearby and you can get to the physical location that would be the high water mark, then you can read the altimeter there and then head over to the house. If the house reads higher than that number (whatever it is), then you’re okay. Repeat a few times to verify reproducibility in the instrument and in the observed altitude difference (perhaps on different days).
As long as the reference point and the house are more or less in the same macroscopic air mass (so that the barometric pressure is the same in the two places), this is all you need.
It’s always easier to measure differences than it is to measure absolute values.
I wouldn’t take it as gospel (but frankly I wouldn’t take anything short of a survey crew if it’s really that important), but just find the spot on Google Earth and see what it thinks the elevation is. I’m not sure what elevation model they use, but it at least seems to be higher resolution than a topo map. For what it’s worth, it thinks my house is at the same elevation (within a meter) as all of my various navigational doodads think it is (and an expensive mobile mapping unit I borrowed for the weekend once).
Wouldn’t it have to beep really loud for me to hear it at 1425 feet?
may I clarify what you need?
not 10 or 20 foot accuracy…you need precise engineering accuracy(elevations measured in tenths of a foot). And you need engineering documents to define the flood plain.
In short, you need a professional land surveyor, and/or somebody from the city engineer’s office.
An old survey map should be available from the city…and if not, hire a professional surveyor yourself.
Following up on my post above…
More clearly: you never need to know any absolute height for your water/house question. That’s what’s hard to measure. But, any ol’ barometric altimeter should be able to tell you, precisely and reproducibly, the wrong answer, so you just need to measure the “apparent” altitude in two places – (1) at the house and (2) anywhere along the high water line – and see which is greater, even though you won’t ever know what the actual altitudes are.
(This would work with a barometric altimeter since the two locations (presumably in a valley) share an air mass. This wouldn’t necessarily work with a GPS altimeter, though, since the two locations may not share lines-of-sight to the satellites. If the GPS constellation is different, the bias in the altimeter reading could change, making relative comparisons suspect.)
GPS is not a good choice for this, it’s just not made for this application. Your best shot would be a altimeter, which would normally have to be calibrated, but that’s not so important in this case because you want elevation difference, not elevation above sea level.
Using the altimeter take a reading at the house, lake,house, lake back and forth to make sure there is no changes, then the difference should give you the answer.
It would be very strange if during the planning for this dam accurate surveys were not done to show exactly what land could be flooded. You should be able to obtain these by contacting the outfit that controls the dam (e.g. U.S. Army Corps of Engineers).
Not at all. You’ll have to be really tall, though.
1 - go to the dam (or the lowest point of lake outflow) and measure how far the water would have to rise before the dam overflowed. For example, if the water level rose 12 feet, the water would flow over the spillway. Now you know that the current water level is 12 feet below the maximum.
2 - Get a stick longer than 12-foot (or however high the water would have to rise to overflow the dam) and mark it at 12 feet and have your son hold it up at the water’s edge.
3 - Buy a pocket sighting level at Home Depot (or ebay) and sit on the front lawn. If your level is looking over the stick, then your OK.