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Messages - captain paranoia

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First Aid / Re: Advanced Casualty Assessment
« on: July 21, 2016, 06:16:05 PM »
> I need to revise my aide memoire, and do some 'mental training'...

Well, three years on, and we have a refresher course. And I'm not sure I did much better in the exercises than last time... Still very focussed on me doing basic first aid, rather than dealing with the situation as a whole.

I've taken to watching the various emergency services TV programmes, and seeing how they deal with incidents. And the thing that strikes me most is the obvious (that I failed to do): say who you are, and ask who they are; ask them what happened. Then you quickly find out their basic state (AVPU), and get a clue to what injuries they may have, how many are in the party, whether there are other casualties, etc.

If only I could get my brain to react calmly like this. Back to the mental exercises to try to din this approach into my brain...

General Discussion / Re: bearing/GPS/Map differential
« on: April 20, 2016, 12:30:02 PM »
The diagram is trying to show attempts to plot a bearing of a 'leg' from a map, where there's uncertainty on the true position of points at each end of the 'leg'. This uncertainty can be represented as an error circle, which contains the 'true' point, somewhere within the bounds of the circle.

The red case shows a very short leg, and the blue case shows a longer leg. The error circles are the same on both cases.

The straight lines represent the edges of the compass in the worst case error situations, connecting the opposite outside edges of the error circles. One line takes one extreme pair, and the other line takes the other pair. The 'true bearing' lies somewhere between these two lines, so the angle between the lines is the potential error in reading the bearing.

You can see that the angle between the two red lines is much bigger than the angle between the two blue lines, showing that, with a short distance between the end points, the potential bearing error is larger. They're plotted to overlap in the centre, so the angles can be compared...

Then there's the error in aligning the compass bezel, but that's the same in both cases...

General Discussion / Re: bearing/GPS/Map differential
« on: April 18, 2016, 06:16:50 PM »
Attached is an exaggerated example, showing the error angles for two different distances, with identical error circles.

General Discussion / Re: foreshortening effect
« on: April 18, 2016, 06:06:25 PM »
I'm going to assume that you're referring to the 'foreshortening effect' discussed in Lyle's 'Ultimate Navigation Manual', specifically on p108, but also mentioned on p54/107/152/157.

This effect is the increase in the effective over-the-ground distance when ascending slopes, probably with the intention of adjusting the number of steps or time when using pacing/timing to track position.

I confess that I'm not convinced of the value of this technique. Here's the feedback I provided Lyle on this topic:

Okay; this is the first section that I have a real philosophical disagreement with.

What is the purpose of calculating the additional distance?  I assume it's to do with pacing/timing.

Have you ever used this technique in anger in the field?  Because I'm not sure there's any benefit to it:
  • for gentle slopes where you can walk at your normal pacing, the foreshortening effect is so small as to be 'lost in the noise'.
  • for steeper slopes where the slope hypotenuse is significantly longer than the horizontal adjacent, your stride will be significantly changed, and that will have a far greater effect on pace counting/time than the additional distance. (e.g. a 1 in 4 slope has an additional distance of just 3%). My stride length and speed will be significantly changed if I'm walking up a 1 in 4 slope.

For slopes of 45 degrees (1 in 1, with additional distance of 41%), you're likely to be zig-zagging, and, since the ground is pretty steep, you may not be able to concentrate on counting 'paces'; you'll be concentrating on not falling down...

I found the description of the technique hard to understand, especially point 1; it took me a while to figure out what you were saying (which is that the technique can only be used easily on slopes of relatively consistent gradient).

Now for the technical issues with the table:
  • You have assumed that the 1:25k contour spacing is 10m; as we've seen, it can also be 5m.
  • You haven't labelled the units for the horizontal contour spacing; it's millimetres.
  • What am I supposed to do with this table in the field?  Am I supposed to learn the numbers?
  • Do you really expect me to be able to measure to 0.01mm?  Even in the comfort of my office, I cannot do that.
  • You appear to be trying to go from contour spacing, to slope angle, to foreshortening compensation. However, the table calculation clearly starts with the slope angle, so the contour spacing figures are silly numbers (with 0.01mm precision). If this is how you intend the technique to be used (contour spacing to determine compensation), then start with the contour spacing, and calculate slope angle, and hence compensation.  If you think this has any merit; I’m not convinced it has.
  • The table is another example of spurious precision.  Whilst the figures are mathematically correct, they're of little practical use in the field.

Then there's the word itself; foreshortening usually refers to the visual effect of making slopes look steeper than they are when viewed from the front; I've never seen it used to describe the difference between horizontal and up-slope distances:

foreshort'en verb transitive to draw or cause to appear as if shortened, by perspective. [Larousse]

I'm very happy to hear others' thoughts on this technique.

General Discussion / Re: bearing/GPS/Map differential
« on: April 18, 2016, 01:33:47 PM »
No worries. It's always useful to everyone to think through challenges like these; we all learn something.

The other thing to consider is the distance between the two points; if they're very close together, then any small errors in choosing end points on the map (such as the church symbols Ian mentions) will cause a larger angular error; that's simple geometry. The larger the distance between the points, the more accurate the bearing measurement is likely to be (given identical error bounds on the end points).

To illustrate this, you can draw error circles at the end of lines of different lengths, and then draw lines between opposite sides of the error circles at each end of the connecting line, so that they cross in the middle. You'll see that the greater the separation of the end points, the smaller the angle between the crossed lines (which is the potential error angle).

General Discussion / Re: foreshortening effect
« on: April 17, 2016, 07:57:06 PM »
Could you clarify which technique you're referring to? Is there a reference in a book (UNM, perhaps?) that you could point us to?

General Discussion / Re: bearing/GPS/Map differential
« on: April 17, 2016, 07:54:23 PM »
Going from 6723 to 6038 (in that direction), you are heading 7km W and 15km N. Using grid angle, this is 335 degrees. (I calculated it using 270 + arctan(15/7))

That's assuming you're using 4 figure (1km) grid reference.

However, like Ian, I suspect you're not giving us the full GR, as you'd need a big compass to measure that bearing, and it comes out nowhere near either of your bearings...

General Discussion / Re: cattle
« on: September 03, 2015, 11:24:36 PM »
This thread, too:

I had encounters with young bullocks on Sunday, with some starting to run towards me. A 'stern word' stopped them in their tracks. A calm word soothed the skittish ones.

Always worth being sensibly wary of large animals.

Satnav (GPS GLONASS COMPASS Galileo) / Re: Handheld satnavs and GNSS
« on: August 25, 2015, 08:44:22 PM »
> for the 2nd impression, I'm taking the opportunity to correct one major factual and elementary error, and a couple of typos.

I missed this. Send me a doc or PDF of the chapter and I'll cast a more considered eye over it.

Satnav (GPS GLONASS COMPASS Galileo) / Re: Handheld satnavs and GNSS
« on: August 25, 2015, 08:42:04 PM »
Here are my comments.

> The Global Navigation Satellite System (GNSS) refers to the system of circulating satellites

There isn't a single GNSS; there are a number of systems, each of which uses a different signal and data protocol. They are designed not to interfere with each other.

> Note that signals from all four systems are compatible and can be received by most satnavs, smartphones etc.

As above, the different systems are not interoperable, and need a receiver capable of receiving and interpreting each of the systems. The receiver will be able to combine all signals it can can receive into a single fix.

GPS and GLONASS are the two systems most commonly available on smartphones. Expect BDS to grow in use due to Chinese dominance in consumer electronics manufacture, and their rapid constellation launch programme.

> WAAS, EGNOS, GLONASS and BDS are compatible systems.

WAAS and EGNOS provide corrections for GPS only. They can not be used to correct BDS or GLONASS.

> Providing that your handheld satnav is either WAAS or EGNOS-enabled then accuracy is likely to be as good as 15 metres for 99 per cent of the time

For a free-space environment. SBAS knows nothing about a local, ground-based environment, and so cannot be used to correct for multipath or reflection errors. These can cause significant position errors (10s - 100s of metres) in mountainous and high rise urban environments. Outdoor users must remain vigilant for these errors.

Hope this helps.

Maps / Re: Fascinating Soviet maps
« on: July 21, 2015, 10:59:01 PM »
I've heard references to Russian digital mapping as a source of 'free' maps. I guess this Cold War mapping must be the source.

New Techniques & Learning / Re: The Walker's Handbook - a review
« on: July 06, 2015, 06:04:49 PM »
Thanks for the review, Ian.  Glad to hear that Hugh's magnificent octopus* has finally been published; it's been a labour of love for a number of years now.

* excuse the Blackadder reference...

I would normally recommend GPSBabel, but the website seems unavailable at the moment.

You can still download it from SourceForge, though:
This is another useful utility:

Navigational Questions & Answers / Re: Break of Slope
« on: March 01, 2015, 07:23:18 PM »
There's nothing on either of those peaks that I'd call a plateau; as Hugh says, they're both gradually rising to rounded summits.

The col between then might be considered more plateau-like, as there's a section in the middle that's a few hundred metres wide, and judging by the contour lines cutting across it roughly at right angles, rather than rounded, the col appears to be fairly flat-topped.

It's not a great example of a plateau...

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