Author Topic: foreshortening effect  (Read 1347 times)

which way

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foreshortening effect
« on: April 15, 2016, 04:40:01 AM »
Can anyone explain a simple way of working out the foreshortening effect. This topic seems important but not often referred to.
Any help would be appreciated.

captain paranoia

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Re: foreshortening effect
« Reply #1 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?

captain paranoia

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Re: foreshortening effect
« Reply #2 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:

<quote>
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]
</quote>

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

Lost Soul

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Re: foreshortening effect
« Reply #3 on: April 18, 2016, 09:33:58 PM »
Ah ha.  As you say CP spurious precision and how does one access tables etc when out on the hill.  The ability to climb a hill and the time taken is variable from person to person.  Condition of surface, weather conditions, snow cover, personal fitness, tiredness levels, stopping to get your breath back, weight of pack and even weight of foot ware all have an impact too.  A lb on the feet is equal to 5 lb on your back.  Muddy boots vs dry ones.  Which all has to be hauled upwards.

Methodologies for compensating for the foreshortening effect are Naismith's Rule which is really what the UNM is dealing with and then there is Tranters variations to it.  Interestingly Hugh Westacott in his "The Walkers Handbook" 5th Edition deals with the subject.  In essence he say the same as you CP. 

Its all a load of academic tosh.  And only relevant to the fit and experienced in defined conditions.  As far as I am concerned best to do your own trials with your normal gear and pack weight over different slopes and see what you get in terms of pacing and time.  Both for dry and slippery muddy slopes and snow cover.  As a staring point Hugh suggests 15 min to walk 1 km plus 1.5 minutes for every 10m contour crossed and 2.5 minutes for 15m contour intervals.

Pacing in my own case is 72 steps per 100m on the flat reasonable surface.  Get to a bit of a steep slope then its at least 88 paces and if muddy much steeper approaching a 100.  As to precision timing more or less forget it in those types of conditions.

Hope this helps
« Last Edit: April 19, 2016, 12:23:00 PM by Lost Soul »

which way

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Re: foreshortening effect
« Reply #4 on: April 19, 2016, 09:03:28 AM »
The reason I was asking was to try and work out beforehand hand how long a walk may take. Having read with interest the last two comments I think I'll have to go along with good old Nasmsmith. Haven't got the brain power for anything else.
Thanks very much for the answers gents