Is It Possible To Buy An *Accurate* Altimeter For A Mobile Device?

[Ale]

@LSLGuy

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 barometers used in flight controllers for multicopters and other hobby grade UAVs can be (AFAIK) precise to about ten centimeters variation in height. They are probably using more modern (and less tested) technology than ones used in airliners… besides I don’t think you’d need precision to the tenth of a meter to fly a big bird.

[doorhinge]

@N9IWP

can one rent a theodolite?

Brian

Survey equipment is available for rent.

(I hope the rest of my post isn’t against forum rules. If it is, please remove following and accept my apology in advance.)

Surveying Equipment Rentals from Duncan-Parnell

http://www.duncan-parnell.com/surveying-mapping/Rentals.html

Trimble S6 Total Station Series

http://www.duncan-parnell.com/surveying-mapping/Trimble-S6-Total-Station-Series-plu23882.html

GeoExplorer 6000 Series

http://www.duncan-parnell.com/surveying-mapping/GeoExplorer-6000-Series-pluTrimble_89000-01.html

[bump]

@N9IWP

can one rent a theodolite?

Brian

The usual way is with a level and Philadelphia rod- they’ll actually give you the actual elevation difference, while a transit or a theodolite will just measure an angle for you, and you’ll have to do other distance measurements and trigonometry to figure out your elevation.

[doorhinge]

@HeyHomie

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)?

Home depot has a Lufkin 1/2" x 200 ft Hi-Viz Orange Fiberglass Tape Measure for $27 and a CST/Berger Pro-Series Zip-Line 200 ft. PVC Coated Fiberglass Tape, also for $27.

You could also use a 3 in. Aluminum Line Level ($4) and a low stretch cord or a Empire True Blue Digital Laser Level ($160) to find the horizontal.

You could buy or make a hypsometer, which is basically a long stick, or pipe, which is alternately colored every inch (white, red, white, red).

Measure the distance from the deck top to the low point in your yard. Measure the distance from the deck to the horizontal point directly above the low point or the distance from the low point to the horizontal point on the hypsometer.

Then it just a matter of a(square) + b(square) = c(square).

[Dr.Strangelove]

@Ale

The barometers used in flight controllers for multicopters and other hobby grade UAVs can be (AFAIK) precise to about ten centimeters variation in height.

That’s correct. One that I have direct experience with is the Bosch BMP085. Although the absolute pressure accuracy is not that great (~1 hPa, which corresponds to ~7 m), it has much greater resolution than that. At its highest resolution setting, it has about 0.03 hPa RMS noise, which is about 20 cm. The noise is well distributed, so that you can average together even more samples for greater accuracy. At 10 Hz sample rate you get ~10 cm precision.

[Xema]

@Dr.Strangelove

One that I have direct experience with is the Bosch BMP085.

Interesting - what do they cost?

[Dr.Strangelove]

@Xema

Interesting - what do they cost?

Really cheap. Mine was a part of a 10 degree-of-freedom integrated sensor kit, very much like this one. That unit has 3-axis accelerometer, gyro, and magnetometer as well as barometer (it uses the BMP180, which AFAIK is just a slightly newer model of the BMP085). All for $10 and using a simple I2C interface.

[Absolute]

The iPhone 6 uses a Bosch BMP282, reportedly an Apple-specific variant of the BMP280, itself the successor to the BMP180.

http://www.bosch-sensortec.com/en/homepage/products_3/environmental_sensors_1/bmp280/bmp280