Author Topic: Navigation accuracy  (Read 2700 times)

sammyh20

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Navigation accuracy
« on: February 03, 2014, 05:42:02 PM »
Hi folks I made it out this weekend learning the basics..just to finish the day I turned on my gps ..marked my position on the map then took a bearing to a track about 400mtrs away(I took a grid reference for this track).Earlier in the day I measured 100 mtrs out using a couple of tent pegs and a 30 mtr line,my pace count was 44....I do walk fast and am tall but not sure I've been accurate enough..anyway i decided to walk on the bearing and used my pace count ,after 176 paces looked at my gps and plotted my position on map..I was about 85 mtrs out from my track...what kind of accuracy am I looking for?....cheers

captain paranoia

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Re: Navigation accuracy
« Reply #1 on: February 03, 2014, 06:47:27 PM »
Assuming you did walk 400m in a straight line, and you ended up 85m to one side* (perpendicular to your intended track), that gives a bearing error of 12 degrees.  You should certainly be aiming for better than that, and, ideally, close to the bearing error of your compass.  Otherwise, use smaller legs, or use 'aiming off' technique to help you get to your leg target.

If your pace count is 44 per 100m, that means each leg stride is 1.14m (and each pace 2.28m).  That's quite a stride; do you have very long legs?  Note Lyle's comment in the book ('PC typically varies from 55, for very tall people, to 75 for people with short legs').

How did you determine the desired bearing?  What technique did you use to maintain the bearing?  You say you took a bearing to a track; could your aiming point be uniquely identified (linear features such as tracks can be hit at any point along their length, so usually require a secondary reference to provide a resection to give a unique position along the track)?  Could it be that your bearing was taken to a point on the track that was different to the grid ref you thought it was?

Are you sure your GPS readings were sufficiently accurate?  Were there any hills or buildings that may have perturbed the received signals?  Was there any tree cover?  Had you let the receiver acquire a good constellation; you say you turned your GPS on, and the position can take a while to settle as additional satellites are acquired and added to the position solution.

* or did you end up 85m short of your target?  In which case, this would give a PC of 100*176/(400-85) = 56, which might be a bit more reasonable.

As you can see, there are many ways in which measuring and following a bearing can go wrong!  Good for you for getting out and practising.

Hugh Westacott

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Re: Navigation accuracy
« Reply #2 on: February 03, 2014, 07:05:22 PM »
Sammy

Welcome to this forum!

There is something not quite right in the statistics that you quote. With experience, pace-counting can be remarkably accurate. When pacing, the usual practice is to count your double-paces. if right-handed count the number of times that your right foot strikes the ground, if left-handed count the number of times that your left foot strikes the ground.

I suggest that you try to find a sports complex or school with a running track where you should find an exact measurement for the 100-metre sprint. Next, count the number of double-paces it takes to cover the 100 metres at your normal walking speed (for most walkers this will be within a range of 65 and 70 double-paces). Repeat the exercise four times and then average the results. If you can't find a running track I suggest that you borrow a surveyor's tape and measure 100 metres as accurately as possible.

This method will provide accurate results on level ground, but remember that your stride shortens a little when going uphill, and tends to lengthen when descending, although on steep descents your stride may actually be slightly shorter than when on the level.

Suggest that you practise pace-counting until you are confident of your accuracy. If you live in a lowland area of England and Wales, find a right of way that runs beside a straight field boundary (hedge, wall or fence) on an Explorer map. Using the romer on your compass, carefully measure the distance between the first field boundary and the next field, then count the number of double-paces it takes to cover the distance. Then compare the measured distance and the result you got from pacing.

Hugh

The proper pursuit of accuracy should not be confused with pedantry. Horace

Hugh Westacott

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Re: Navigation accuracy
« Reply #3 on: February 04, 2014, 07:12:44 AM »
Sammy

I posted my contribution before I read that of Captain Paranoia.

Unfortunately, there are many terms used by walkers that do not have a generally accepted definition. CP is using the term 'stride' to mean one step and 'pace' to mean two steps.

In my post I use the term 'pace' to mean one step or stride and 'double-pace' to mean two steps or strides. Apologies for the confusion!

I am a geriatric dwarf 5' 6" (167 cm) tall with an inside leg measurement of 29" (74 cm) and I take 68 double-paces to cover 100 metres on level ground.

Hugh

The proper pursuit of accuracy should not be confused with pedantry. Horace

captain paranoia

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Re: Navigation accuracy
« Reply #4 on: February 04, 2014, 12:47:24 PM »
Hugh, I used 'stride' to mean one step, because Pace Count usually refers to a a two-stride/step 'pace' (that's what Lyle uses in UNM on p109).  In a 'normal' context, I think of a pace as one stride, but in the navigation context, have to think of it as two...

Of course, provided you use a counting system that makes sense to you, it doesn't matter whether you use a Pace Count to refer to one stride or two, provided you count appropriately...

According to Larousse, we have the Romans to blame for this confusion:

pace pas,
noun a stride; a step; the space between the feet in walking, about 76 centimetres, or (among the Romans) the space between two successive positions of the same foot, approx 1.5 metres

(c) Larousse plc.  All rights reserved

Although I thought that pace in Latin meant 'peace', and a pace is a passus...

Hugh Westacott

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Re: Navigation accuracy
« Reply #5 on: February 04, 2014, 01:45:11 PM »
CP

I was composing my response to Sammy when you posted your reply so had not read your contribution when i pressed the 'Post' button.

However, I think using the term 'pace' to refer to a double stride is a bit out of step (ho-ho!) with generally accepted terminology. I have eighteen books that cover British navigation techniques for walkers and mountaineers, and of the fourteen that describe this method of covering a fixed distance, all (including Langmuir, Keay and the Mountain Leader Training Handbooks), use the term 'double-paces'. UNM is the exception.

It's worth noting that 'pace' in certain contexts can also refer to speed.

Hugh

The proper pursuit of accuracy should not be confused with pedantry. Horace

Angle of Repose

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Re: Navigation accuracy
« Reply #6 on: February 04, 2014, 09:05:33 PM »
FWIW, I am tall as well coming in at 195 cm (6'5"); my pace count (I start on my left, even though I am right handed, go figure) on flat gravel (no pack or snow) is around 60. Of course that is the best it will ever be, especially once I start adding additional factors in.
"You can't get there from here"

boogyman

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Re: Navigation accuracy
« Reply #7 on: February 09, 2014, 11:31:09 AM »
According to Larousse, we have the Romans to blame for this confusion:

pace pas,
noun a stride; a step; the space between the feet in walking, about 76 centimetres, or (among the Romans) the space between two successive positions of the same foot, approx 1.5 metres

(c) Larousse plc.  All rights reserved

The word mile is directly derived from the Latin as well as French mille (passuum) -- which means one thousand (paces).
In other words, back then they left no doubt: one mile = thousand paces.
Could be used to agree on the definition of pace.

See here: http://en.wikipedia.org/wiki/Mile
« Last Edit: February 09, 2014, 11:36:30 AM by boogyman »

MoonMan

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Re: Navigation accuracy
« Reply #8 on: February 12, 2014, 12:35:32 PM »
What I did was to use Google  Earth/Maps to measure the length of some local town streets, from building corner to building corner; as these turned out to be of equal distance apart, I happily set off, & worked my measure out within two hours. Works just as good for any stretch of road or point-to-point, off road. We drag our feet as the distance increases.
Keeping Track of where Here is in relation to There.