UKH

I’ve not been in the mountains for 2-3 years,  and whereas in the past I would have defaulted to taking map, compass and my Sunto altimeter watch, I’m wondering whether the time has come that I can at least leave the last item behind? Ie even if I wouldn’t want to trust the compass on my mobile, I can probably wing it on the altimeter front. 

From experience, how accurate are phoned these days?

As long as you have GPS signal, in my experience very.

I have my Sunto GPS watch with me on the sort of days I might need altimeter and GPS, saves getting the phone out, wasting battery and the distance measuring is super useful (I'm sure view ranger etc do that too)

Don’t do it! Stick with what you know. You have 3 tools there that are  vital and you’re still here to post so you probably know how to use them. 

Ive had bad experiences with phone apps. Shite.

Don’t become the Gemini Man!

Perhaps I've been lucky but I have never been in a position when I've felt I needed an altimeter.

Even though I have Viewranger and OS maps on my phone, mainly for fun, I find the combination of a map, compass and the free OS locate (https://www.ordnancesurvey.co.uk/shop/os-locate) on my phone covers what I need.

Dave

If you're in the UK I would say that an altimeter is a luxury item. I can only think of one occasion in over 30 years where I have used one for navugation and even then it was not essential.

The OS app on my mobile phone says I'm currently minus 34m. This is a bit worrying as I'm sat on an oil rig in the middle of the North Sea. The location is correct and compass points in the correct direction, however the altimeter is way off as I know I'm 57m asl at the moment and my feet are dry.

Your mobile phone will be using GNSS for altitude; it is unlikely to have pressure sensors (though if it does, it will almost certainly have a GNSS receiver...).

If you have GNSS altitude, you have GNSS lat/long. So the altitude is pretty pointless, as it is only used to confirm lat/long position in conventional navigation.

GNSS altitude is less accurate than horizontal position, due to geometry of the constellation.

Reflections and shadowing are the main problem with GNSS in mountainous areas (and the 'urban canyon').

My iPhone (6) has a pressure sensor, and a number of other phones do. 

There are a surprising number of sensors in a modern mobile.

I bet it has a GNSS receiver, too...

A pressure sensor is sometimes part of an integrated silicon IMU; a ten axis IMU includes compass, gyro and accelerometer for each xyz axis, plus pressure. Add temperature for an 11-axis IMU...

If by GNSS receiver you mean a GPS chip, then yes. And it has a motion co-processor to integrate all the data - I assume that's what you mean by an IMU.

GNSS is the generic term; global navigation satellite system. Your device will have GPS, but it may also include GLONASS and BDS.

An IMU is a sensor device. For a phone, it will be a single silicon device.

https://en.m.wikipedia.org/wiki/Inertial_measurement_unit

Thanks - informative thread this. 

Using the phone is a downward spiral. Perhaps initially you will use it for the height, but then you might use it to check you absolute position more and more often (invaluable in an emergency), but if you are like me, your map skills will suffer and I have had to make a conscious decision to get my map-reading skills back up to a decent level. An OS map on the phone with a target dot at you position is not the same as map reading. 

Of course you may be hugely disciplined and this might not be an issue, but getting a bit lost or feeling uncertain with a map in your hand makes you really work out what is going on.

Using GPS doesn’t have to be a downward spiral. You can use it constructively. One way to approach this is to use the GPS/ phone as a kind of back-up and training tool.  I prefer to use real maps because I get a better sense of scale and distance from them. But then I can check with a phone to confirm my position, and how accurate my technique is.  It’s a good way of checking things like pacing and time/distance too.

Another aspect of this - I started using my phone for navigation in the first place because I realised I’d got a bit sloppy and complacent with my map-reading. It became a useful corrective and a way of keeping me on my toes. But I think your general point still holds - real maps should take priority, and a bit of discipline is necessary.

I have a Garmin 76 which has a barometric sensor as well as a GPS receiver.

It says: The GPS altitude is 120m
The barometric altitude is 111m
The iPhone 7 compass app says 100m.

Google Earth gives me 108m, so with suitable pressure calibration I'm happy with the barometer, and the GPS is within 10-20m which is what a brief search tells me I'm likely to expect.

Also the manual informatively tells me that the device needs reception from at least 4 satellites to give a reliable GPS altitude, whereas it will give me a position using only 3. I guess this ties in with captain paranoia's helpful posts.

I have seen some debate in the past about whether fog / cloud etc can degrade GPS signals to the point where they are unreliable, and various explanations put forward with varying degrees of technical and scientific expertise. I have never had a problem with this, but am wondering if anyone has on here?

Edit: added iPhone reading.

The mainmissue in using technology of any type is to understand its strenghths and its weaknesses. FogFor rain will impact the ability of the reciever to lock on to the signals. Theses come in two forms for GPS, not sure about the other systems but I,d expect a similar architecture. The C/A codes are easier to acquire and give a 30 m CEP error depending on the satellite geometry you can see. The P/S code is harder to acquire and gives a sub 10 m CEP. So in rain cloud if you can get a fix it should be good to within around 30 meters, which is pretty good for most situations. However the weather may be so crappy you can't get a fix, phones then drop back to location via cell reception. This can be very poor  to non existent in wilderness areas. Wet altitude, what you get from a GPS Rx is height above the speroid, which is a mathematical concept which approximates the shape of the earth. The actual surface, including that of the sea, can and is in most places either above or below this. I seem to remember that the worst deviation at sea level is around 50 m. But it can be much worse in some places on the earth.

Rain, cloud and fog can attenuate the received signal. This was a problem with older, less sensitive receivers, reducing accuracy. But a modern receiver is so sensitive that ground weather conditions shouldn't cause a problem.

Tree cover, especially wet, broadleaf tree cover, can reduce accuracy, due to attenuation and scattering.

As I said above, the biggest problem you need to be aware of is reflection in 'canyon' environments. These involve obstruction of the direct path, and reflection from large faces. These conditions can conspire to give a position with an error of the order of hundreds of meters. I've seen this happen in the mountains. It also happens in high rise urban environments; the 'urban canyon'. Look for a small number of visible satellites, and a position that is changing quickly, especially snapping about, or changing when you are static.

That sounds like information from before the turn off of Selective Availability (pre-2000); a military presentation?. P-code is not available to civilian receivers; it is encrypted, and remains for military use.

You should achieve a CEP (95%) of around 3m today, in ideal conditions. A quick Google will confirm. And a modern receiver will report its computed estimate of CEP.

Position is computed by intersecting the flight time radii from satellites to receiver. Our position is at the intersection of these two circles. However, there are two intersection points with two circles... Thus, with only two satellites, we have an ambiguous 2-D position. Adding a third satellite allows us to resolve the ambiguity. For the same reason, we need four satellites to resolve a 3-D position.

A modern phone will receive both GPS and GLONASS satellites, and fuse them into a single position. This gives you more chance of seeing four satellites. It may even receive the Chinese BDS.