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Comment by dholowiski | Hacker News Reader

The article claims that " GPS and GLONASS can measure altitude, but generally the data is inaccurate and too low-resolution (on the order of 10-25 meters" - which I'm fairly sure is totally un-true.

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I use GPS to fly UAVs and I've seen errors in GPS altitude up to 50ft compared to barometric altitude. Generally it's better than that, though. It's accurate enough for us to fly autonomous helicopters with the barometric sensor disabled. I'd say that disproves the inaccurate and low-resolution claims ;-)

No, it's true. The possible geometry of visible GPS satellites at any given moment (namely that they're all above you and none are below) means that the vertical error is pretty bad in comparison to the horizontal error.

GPS altitude readings seemed pretty fine on the numerous mountains I've climbed. I almost always cross-checked with a calibrated barometric altimeter.

Interesting. I wonder if the altitude readings were WAAS augmented, or if this is a case where theory doesn't translate in to reality.

Ah. You may be right, there. WAAS is enabled on my GPS (a Garmin 60CSx).

Link for bystanders: http://en.wikipedia.org/wiki/Waas

WAAS uses a network of ground-based reference stations, in North America and Hawaii, to measure small variations in the GPS satellites' signals in the western hemisphere. Measurements from the reference stations are routed to master stations, which queue the received Deviation Correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the corrections while computing their positions to improve [3D] accuracy.

And if you need even higher resolution (centimeter resolution), use use NOAA CORS sites (either with post-processing, or real time with their experimental datafeed)

http://geodesy.noaa.gov/CORS/data.shtml

http://beta.ngs.noaa.gov/CORS/NGSRealtimeGNSS/

It's true that this makes the VDOP generally higher than the PDOP, which leads to lower precision in the vertical direction. The 10-25m figure is a complete nonsense, though. It's more like 3-5m.

It has much more to do with how GPS receivers calculate a horizontal position when they have limited satellites in view.

I don't understand the math but essentially you are trying to solve four unknowns. Latitude, longitude, elevation, and the time offset between the satellite and device clocks. If only three satellites are in view the only way to solve latitude and longitude is to assume a value for elevation based on previous results. Without a forth satellite the GPS simply doesn't know what the altitude is. But that estimated value is sufficient to give a good estimate of horizontal position.

I'm sure most of the responses to your comment have technical reasons for their disagreement, but I agree with you for anecdotal reasons.

I have a Garmin GPS I wear when I run, it connects to 3 (at least?) satellites and I've never seen it be off by more than a few feet in altitude. I'm sure there are some random points in the graph with anomalous readings but the overall "smoothed" graph seems very accurate.

Granted, there are times it loses its signal altogether, but that is not a resolution issue.

I got a certificate in GIS back in 2002. Dusting off my brain: All GPS readings are only accurate to within about 10 meters if they are quick and dirty. To get accurate readings, you need multiple satellites in view for up to several hours.

I am assuming that has not really changed. Anyone with brand spanking new credentials or more current industry experience feel free to correct/update. Thanks.

If you're in the continental United States (or Canada), and have a GPS receiver that can handle the WAAS adjustment signal (http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System), you can get a much more accurate signal as quickly as a regular GPS signal.

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