Do the mobile-app GPS apps actually triangulate your altitude, or do they simply note your position and retrieve the known altitude from a database? Clearly there’s a lot of wiggle room in mine, as it gives my altitude on my back porch at 1,277 feet, and at the back of my yard at 1,283 feet - which, considering that my yard slopes downward, is clearly wrong.
Is it possible to download an altitude app (or buy a real altimeter) that is accurate to within, say, less than a meter?
I’m pretty sure they calculate it from the GPS signals, but consumer level GPS isn’t that accurate, at least as far as implemented in mobile devices, and the elevation calculation is inherently less accurate than the position one- something like 1.5x the error of the horizontal error.
http://gpsinformation.net/main/altitude.htm
So from what I can tell, your 6 foot difference in the wrong direction is well within the error bands for mobile phone GPS.
What might help you without costing anything (or very much) is one of those GPS averaging apps that averages a bunch of measurements over time and gives you a much more accurate fix on your position, and a smaller error.
The iPhone 6 has a built-in barometric altimeter, that should be accurate to within about 1m. That is about as good as you’ll get on a consumer device. Any dedicated electronic altimeter you buy (e.g. those that come with some high-end hiking watches) will have about the same accuracy.
What elevation do you get when you set your pointer in your yard in Google Earth? Does that vary as you move about the yard?
Its been quite a while since I’ve been in the market for a GPS but as I understand it, your location can be determined by readings from a minimum of three satellite locations. To determine altitude, you need to add a fourth reading. The more the merrier (or more accurate).
Your average handheld GPS isn’t designed to giver you accuracy within a meter. They won’t give an accurate measurement of direction if your standing still for too long, unless the device has an internal compass.
Surveying GPS units are available that will provide the accuracy that you want but they can cost 10’s of thousands of dollars.
I don’t doubt you can measure atmospheric pressure that well.
But isn’t the problem going to be converting that pressure value to an altitude value?
How much variation is there in atmospheric pressure that are not due to actual changes in altitude?
Nope. A barometric altimeter might conceivably have a precision of 1m. Although I really doubt it.
To convert barometric pressure into an altitude requires a local weather station to have computed the local altimeter setting. Which is also not done to that level of precision. In the US, altimeter settings are computed to 0.01" Hg which corresponds to roughly 10 feet of altitude. And the true setting value varies continuously over time. But under typical conditions US weather stations only compute the altimeter setting once per hour. 50’ jumps from one hour to the next are not uncommon.
As well, the setting varies over space. Two official weather stations across town will determine different local settings even at the exact same time. So unless the OP’s backyard is adjacent to an official weather station there will be an unknown and unknowable delta between the locally reported altimeter setting and the one that applies to the OP’s backyard. This delta is not constant over time, but varies continuously as different weather patterns move through.
Bottom line: The baro altimeters we use in airliners are considered within tolerance if they read +/- roughly 50 feet. And they cost 10s of thousands of dollars apiece and involve a lot more tech than is in a smart phone pressure transducer. And they’re still subject to all the vagaries of weather-station supplied altimeter settings not reflecting immediate local current reality.
The delta between a serious low pressure day and a serious high pressure day is about 250 feet. More typical variations over the course of a month are 100-150 feet at any location.
At the other extreme, going from the low in a hurricane’s eye to a major Arctic high might be 450 feet. Although it’s unlikely any single spot on Earth would experience both conditions even over a hundred year span.
Lots.
Some Googling suggests that worldwide barometric pressure extremes range from around 25.7 to 32.2 inHg, which corresponds to an altitude difference over 1800 ft. The normal range of barometric pressure (say, over several weeks) is much less, but can still be the equivalent of a couple hundred feet.
[ETA: Beaten by LSLGuy]
You can. In my own testing I’ve found they can pick up differences down to 10 cm.
However, LSLGuy’s point is correct. The sensors are precise, but not inherently accurate. They can measure short-term altitude changes well but don’t have a great idea about the actual altitude.
Some sophisticated integration of the baro and GPS sensors could largely get you the best of both worlds, though.
My understanding is that the way GPS receivers with barometric altimeters work is that they’re constantly averaging the GPS altitude data and use that to calibrate the barometric altimeter. The barometric altimeter is more precise, but the GPS readings are more accurate over time. I assume a smart phone with a barometric sensor works much the same.
I’ve had a couple of GPS units like that and have found the altitude to be pretty accurate versus a known point on the topo map. Probably not within 1 meter, but within 2 or 3.
Which phone are you using? I’m sure the built-in iPhone compass app always used to give your altitude and GPS co-ordinates, but the altitude functionality seems to have been removed since iOS 7.
Mine was usually accurate to within about 5 metres when standing in the garden.
In any case there are numerous free apps that take altitude directly from the GPS.
Mine (a Garmin GPSMAP 276C) does not use a database. I know this because it reads the actual altitude when I power it up on a commercial airliner (i.e. 30K-40K feet) rather than the altitude on the ground directly beneath the aircraft. Clearly it’s not using barometric pressure, either, since the cabin is pressurized.
A smartphone could also easily access weather services and constantly update the local sea-level barometric pressure (from the nearest weather station).
There’s a cheat for those of us who’s experience is operating on the ground with a topographic map. It’s calibrating based on being at a clearly marked spot on the map and using that altitude. The device then tracks pressure changes as altitude changes from that point. That’s the typical method for the small devices like watches with altimeters built in. It may well give better relative differences in altitude for short periods (or periods without much weather induced pressure change) than having the GPS simply recalculating all the time. It pushes the relative error back towards the precision of the barometer instead of the precision of the calculation. Relative error would seem to answer the implied problem in the OP of measuring drop from porch to back of the yard. It doesn’t necessarily help with accurate elevation.
Note that GPS receivers do not do triangulation - they measure time difference between the arrival of signals from pairs of satellites.
Then is there a way to accurately measure the difference between the bottom of my yard and the surface of the porch, using backyard science (and not, you know, expensive surveying equipment)?
If you can attach a hose to a bucket you could use a water level and a tape measure.
Can you describe in more detail the geometry you are trying to measure?
One possibility is to run a length of string from the surface of your porch to the point on the ground you are trying to measure. You can then measure the angle of the string relative to horizontal (using a bubble level and a protractor), measure the length of the string, and then calculate the height from that.
can one rent a theodolite?
Brian