Hacker News Reader

Comment by walrus01 | Hacker News Reader

Imagine if Pokemon go used the elevation data from gps. It's just lat long. People would be climbing trees, using prybars to open the locked roof access doors of tall office buildings, etc.

View on HN

GPS provided elevation data is bad, like really really bad. Often tens of meters out of sync. Unlikely this would work well in practice without additional location data sources (barometer, camera based tracking, etc.).

GPS elevation data on a modern device with a sufficiently good view of the sky is often, but by no means guaranteed, to be a lot better than that. On a really cheap smartphone with GPS+GLONASS receive ability you can regularly see 12 to 14 satellites. The more satellites, the better the Z axis precision.

Anecdotally, when standing next to FCC licensed radio systems on roofs, which are supposed to have accurate license data in their database entries for height MSL and AGL, I have seen elevation data from my late-2019 vintage smartphone just as accurate as on a $5000 surveying GPS.

At least to within 2-3 meter Z axis of reality.

I was intrigued by this, so here's a quick explanation by Garmin:

https://support.garmin.com/en-CA/?faq=QPc5x3ZFUv1QyoxITW2vZ6

Important quote:

"It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft."

Wow.

Yes, this is why most backcountry navigation apps don't display GPS elevation to the user, instead they use the lat/long to look up the position on a topographical map and report that elevation instead. In very steep terrain, it's always a good idea to employ a conventional (barometric) altimeter.

Genuinely curious - what would you need moment by moment readings for? My experience with back country camping is the Rockies, so I've never been at any sort of altitude danger zone, and it's usually pretty binary - you're either on top of the mountain or still not. For something like a GPS watch I'd just want to look back on it later to be able to review my route, but I can't see myself taking barometric measurements for accuracy.

In steep terrain, elevation is a great way to find your location. Suppose I know that I'm on a certain lateral feature - a ridge, canyon, or watercourse. By knowing my precise elevation, I can determine my exact location. This helps me navigate as well as quickly guesstimate my current pace / progress. If I know I will spend all day ascending gradually on a valley glacier from 4000 m to 5000 m, in 1 second I can check my altimeter and know how far I've come when a handheld GPS will spend 30 seconds trying to get a fix.

If someone is trying to state the location of, for instance, a good bivy site on a rock climbing route, "1890 m on the SE ridge" is equally as precise as the GPS coordinates, but much more descriptive.

Ah, I didn't think of rock climbing.

Internet points, really. A more accurate elevation read gives you a more accurate ability to boast about how brutal your hike was :)

The negative part is also very true --- I've had a GPS tell me I was deep underground, when in fact I was on top of a hill.

... which seems like a good premise for a Black Mirror episode!

/u/cstross's Halting State features people in an ARG being used for cover for attacks. Well worth a read. https://en.wikipedia.org/wiki/Halting_State

Reply on news.ycombinator.com