/ Hillwalking GPS advice

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Coel Hellier - on 17 Jun 2013
I'm thinking of getting a GPS advice for hillwalking. I'd want it to have a grid-reference readout and an altimeter (either GPS or barometric), but don't care about other functions (e.g. programming waymarks etc is much too much faff).

So, other than the readout of grid reference and altitude, as small and light as possible with long battery life. Any advice on what I should get?

(Or should I go the whole hog and get one with built-in mapping?)
roddyp on 17 Jun 2013
In reply to Coel Hellier:

For something simple and bombproof to stuff in the bottom of a pack for emergency use, a Garmin Etrex H or 10 should do fine.

For GPS with mapping, try ViewRanger on your 'phone. The usual caveats about ruggedness, battery life can be addressed easily enough. It's a cheap way of finding out what capabilities you actually need.

Never leave the map and compass at home, though.

Darren Jackson - on 17 Jun 2013
In reply to Coel Hellier:

I've got the Garmin Etrex-10. No altimeter, but it does the job for me.

Cheap, robust and waterproof. It covers both GPS and GLONASS and has decent battery life.
Coel Hellier - on 17 Jun 2013
In reply to Darren Jackson:

> I've got the Garmin Etrex-10. No altimeter, but it does the job for me.

How about the wrist-watch size things, Garmin 210, 410 or fenix? Are they a better option if all I want is a readout of grid reference and altitude?
alooker - on 17 Jun 2013
In reply to Coel Hellier: I have an etrex 10 you can have for 55? I've upgraded to the fenix, which is much more complex - probably overkill if you just want GR and altitude
tony on 17 Jun 2013
In reply to Coel Hellier:
> (In reply to Darren Jackson)
>
> [...]
>
> How about the wrist-watch size things, Garmin 210, 410 or fenix? Are they a better option if all I want is a readout of grid reference and altitude?

They're really expensive compared with a cheap hand-held GPS. Getting a grid reference from a 410 is a bit of a faff, and I'm not sure if it's possible with a 210. Both those give the altitude based on the GPS, which may or may not be as accurate as a barometric altimeter.
Coel Hellier - on 17 Jun 2013
In reply to tony:

Thanks everyone, I may need to wander into a shop and play with these before deciding.
Mungo Shuntobox - on 17 Jun 2013
In reply to Coel Hellier:

the wrist watch size ones are getting better but....

bottom of the range eTrex that I use takes two AA batteries just like me head torch, whereas wrist GPS don't and will need recharging after 7-8 hours use (if my Garmin 205 is anything to go by)

Basic eTrex does give you GPS altitude rather than barometric but it's usually more accurate than my wrist altimeter.

Not sure how the latest wrist GPS units do for accuracy but my eTrex is good to a couple of metres on a good day.

Happy shopping
Darren Jackson - on 17 Jun 2013
In reply to Mungo Shuntobox:

I've only got the basic eTrex too; I wasn't aware that it has a GPS altimeter: I've seen the elevation profiles, when I've imported tracks into Memory Map, but always assumed that the software was responsible for that.
angry pirate - on 17 Jun 2013
In reply to Coel Hellier:
Foretrex are basically etreks for your wrist. My v old foretrek 100 uses two aaa batteries for a decent days use. Gives all the basic data: grid ref, altitude etc.
If you want to interface with a pc you may want to look at the better models, 300 or 400, as they have mini usb.400 has a barometric altimeter too.
Gps triangulate position vertically as well as horizontally hence the height reading though the bloke at garmin told me it was 3x less accurate, I.e. if the unit tells you it is accurate to 7 metres, the height will be plus or minus 21
PopShot on 17 Jun 2013
In reply to angry pirate:
> (In reply to Coel Hellier)
>
> Gps triangulate position vertically as well as horizontally hence the height reading though the bloke at garmin told me it was 3x less accurate, I.e. if the unit tells you it is accurate to 7 metres, the height will be plus or minus 21


I've often been told that too. However I've had 100% accuracy for altitude on a simple eTrex 10 many times oddly enough. What I mean is I've been at the summit of a hill and after a few minutes the eTrex 10 has displayed the same height as the one marked on the OS map.
captain paranoia - on 19 Jun 2013
In reply to PopShot:

> I've often been told that too. However I've had 100% accuracy for altitude on a simple eTrex 10 many times oddly enough.

HDOP and VDOP (Horizontal and Vertical Dilution Of Precision; unit-less 'goodness measures') vary constantly with the satellite constellation; the satellites are constantly moving relative to the Earth, so the geometry between satellites and receiver is constantly changing. Since VDOP is rather more sensitive to geometry than HDOP, it tends to be a) worse and b) larger.

Thus, even if you're standing still, the reported altitude can vary quite dramatically, whereas the position wanders by a few metres.

Then, of course, there's the issue with of map accuracy, and your ability to place yourself at an exact spot where a height is quoted.

So, yes, it's possible for the GPS to give exactly the same height as the map says it should be. Look again in a few minutes, and it might be different...

http://en.wikipedia.org/wiki/Dilution_of_precision_%28GPS%29
tony on 19 Jun 2013
In reply to captain paranoia:
> Thus, even if you're standing still, the reported altitude can vary quite dramatically, whereas the position wanders by a few metres.

Which presumably is why, when the Munro Society have been remeasuring assorted Scottish hills, they've taken GPS measurements over quite a long period of time (hours) to get an averaged height.
Darren Jackson - on 19 Jun 2013
In reply to tony:

I suspect that they've relied on multiple GPS measurements due to ongoing problems with sourcing a decent-sized tape measure.
Mark Bull - on 19 Jun 2013
In reply to tony:

The Munro Society surveys use a dual channel receiver which gets the raw vertical error down to about 3m, but the final accuracy of 2-3cm comes from post-processing in combination with data from fixed points. Some more info here: http://www.themunrosociety.com/assets/fisherfieldreport.pdf

Jim C - on 19 Jun 2013
In reply to Darren Jackson:
> (In reply to Mungo Shuntobox)
>
> I've only got the basic eTrex too; I wasn't aware that it has a GPS altimeter: I've seen the elevation profiles, when I've imported tracks into Memory Map, but always assumed that the software was responsible for that.

You should have a screen that displays both height and position data Darren.
If not, you should be able to select what data is displayed on the screens .

On a basic GPS that choice of fields is minimal, so not having height and position on the main screen seems strange. Have you got the setup guide?
PopShot on 19 Jun 2013
In reply to Mark Bull:
> (In reply to tony)
>
> The Munro Society surveys use a dual channel receiver which gets the raw vertical error down to about 3m, but the final accuracy of 2-3cm comes from post-processing in combination with data from fixed points. Some more info here: http://www.themunrosociety.com/assets/fisherfieldreport.pdf

The Munro Society are crackpots. They build huge mounds of earth and stone on top of hills which are a few feet off 3000 and then claim it to be a 'Munro'.
tony on 19 Jun 2013
In reply to PopShot:
> (In reply to Mark Bull)
> [...]
>
> The Munro Society are crackpots. They build huge mounds of earth and stone on top of hills which are a few feet off 3000 and then claim it to be a 'Munro'.

No they don't.
PopShot on 19 Jun 2013
In reply to tony:
> (In reply to PopShot)
> [...]
>
> No they don't.

It's what I was told by a chap in my club. Maybe he misunderstood haha!
Darren Jackson - on 20 Jun 2013
In reply to Jim C:
>
> On a basic GPS that choice of fields is minimal, so not having height and position on the main screen seems strange. Have you got the setup guide?

Thanks; it's probably there and things are just down to me being a bit dim.
ads.ukclimbing.com
Jonny2vests - on 20 Jun 2013
In reply to PopShot:
> (In reply to angry pirate)
> [...]
>
>
> I've often been told that too. However I've had 100% accuracy for altitude on a simple eTrex 10 many times oddly enough. What I mean is I've been at the summit of a hill and after a few minutes the eTrex 10 has displayed the same height as the one marked on the OS map.

For a good height, you need some satellites near the horizon, geometry is the key, like with a map resection. The summit of a mountain is often very good for that, you may even track satellites below your local horizontal. In general, height is around 1.5 times worse than plan rather than 3.
Martin W on 20 Jun 2013
In reply to Darren Jackson:

> I've only got the basic eTrex too; I wasn't aware that it has a GPS altimeter

Every GPS has some idea of what it thinks its altitude is.

Even on the basic eTrex the unit has more data fields available to display than can actually fit on the screen. There will be a section in the manual somewhere which explains how to configure which information you want it to show you.
Martin W on 20 Jun 2013
In reply to Darren Jackson:

> I've seen the elevation profiles, when I've imported tracks into Memory Map, but always assumed that the software was responsible for that.

Some software can compensate for the inherent shortcomings of GPS altitude cf position. For example, the Garmin Connect online service which takes data uploaded from their GPS sport watches like the 310XT has a setting allowing you either to display the raw altitude data from the device, or to apply elevation corrections. Here's what they say about it on the web site:

What are Elevation Corrections?
Elevation Corrections cross reference the horizontal position (latitude/longitude) provided by the GPS with elevation data that has been acquired by professional surveys. When corrections to elevation data are made, each trackpoint of your activity now contains the elevation from the web service, not the elevation provided by your GPS device.
Garmin Connect selectively applies corrections to depict a more realistic representation of your elevation experience. Activities recorded from devices without a barometric altimeter are enabled with Elevation Corrections by default. Alternatively, activities recorded by devices with a barometric altimeter generally contain accurate elevation data and therefore Elevation Corrections are disabled by default. For those users who are familiar with the MotionBased Gravity service, this is the same service.


(Note the statement about the accuracy of elevation data from devices with a barometric altimeter!)
captain paranoia - on 20 Jun 2013
In reply to Jonny2vests:

> For a good height, you need some satellites near the horizon, geometry is the key

Acually, for good height, you want satellites away from the horizon; consider two satellites on the horizon, one W and one E. Now think about how the range spheres intersect; for a small range ambiguity, you get a very large intersect circle, because your intersect points climb up the steep (near vertical) slope of the intersecting sphere. Geometry, as you say, is the key.

Not only that, but satellites on the horizon are subject to much larger iono and tropo degradation, because the signal passes through a lot more atmosphere, and is further away. The receiver computation engine should be weighting out low elevation satellites from the solution.

The best satellite position for good height values is one directly overhead, since the height error it gives is simply the range error.

Generally speaking, the best constellation would be one in which satellites were mutually orthogonal...
Darren Jackson - on 20 Jun 2013
In reply to Martin W:

Interesting. Thanks.
Martin W on 20 Jun 2013
In reply to PopShot:

> However I've had 100% accuracy for altitude on a simple eTrex 10 many times oddly enough. What I mean is I've been at the summit of a hill and after a few minutes the eTrex 10 has displayed the same height as the one marked on the OS map.

You shouldn't confuse (or conflate) accuracy and precision. Note that captain paranoia's HDOP and VDOP are about precision, and how it varies over time. The GPS altitude calculation is less precise than the position calculation, but by averaging a number of imprecise readings over time - as you have observed - it can get quite an accurate result. That assumes that the errors in the various readings roughly cancel out. If there's a constant error factor in there then it's always going to be wrong - as can happen with position readings, for example, if you set the wrong map datum for the grid you're using (yup, been there, done that).
Jonny2vests - on 20 Jun 2013
In reply to captain paranoia:
> (In reply to Jonny2vests)
>
> [...]
>
> Acually, for good height, you want satellites away from the horizon; consider two satellites on the horizon, one W and one E. Now think about how the range spheres intersect; for a small range ambiguity, you get a very large intersect circle, because your intersect points climb up the steep (near vertical) slope of the intersecting sphere. Geometry, as you say, is the key.

Hmmm, you appear to be right now I think about it. This is a little embarrassing considering my background :-)

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