Author Topic: navigation accuracy part 2  (Read 2109 times)

sammyh20

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navigation accuracy part 2
« on: February 07, 2014, 10:12:06 AM »
Hi  thanks for the replies...been pacing a few times this week and got to 64 paces,so going out to work on this weekend to test it...just invested in a watch with built in altimeter ...I need to input the current sea level pressure...does this constantly change ?..is it different around the uk coast? ..can anyone recommend a good weather book so I can get a bit more understanding please...thanks

Lost Soul

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Re: navigation accuracy part 2
« Reply #1 on: February 07, 2014, 01:17:22 PM »
Sea level pressure changes constantly.  With a storm moving through it can change by a mili-bar (or hectopsacal) or more an hour which equates to 10 metres in altitude.  Yes it does change  around the coast just a few 10s of miles can give you a mili-bar difference. What you need to set is the equivalent sea level pressure for your location not sea level pressure at the nearest bit of coast to you, which could be hundreds of miles away.  Sea level pressure range in the UK can be between 960 mb and 1035 mb giving an height variation of 750 metres. 

The best way to set the altimeter is to set a reference height for a known position.  Do this by reference to your map.  Go stand on a spot height and set the altimeter to the value given on the map plus 1 metre.  The map is telling you the height of your boots, the altimeter is telling you the height of your wrist!!.  Those wrist altimeters are not particularly stable items and can drift very quickly as you go up and down hills, and be influenced by a passing storm, So you will need to reset it every half hour or so as described above. 

Easy way to check its  consistency is in your car. Drive to the top of a big hill.  At the top of said big hill set the altimeter, drive to the bottom, take reading, drive to the top take reading drive to the bottom take reading.  Compare 2 readings made at the top with each other and ditto for those made at the bottom.  Determiner errors work with that in mind.

In respect of publications I suggest you start with the Met Office Web site lots of good basic and more specific information there.  http://www.metoffice.gov.uk/learning

If you are interested in the practical theory and practice of altimetry then you need to look at a text book written for pilots.  A good basic one that serves all is the Air Pilots Manual Vol 2 Aviation Law and Meteorology (Pooley's Air Pilot Publishing).  The air law bit (sic) contains information on altimeter setting and the continual effects moving weather systems have on altimeters.  Also have  look at these 2, Hill Walking and Winter Skills both from Mountain Leader Training UK.  Neither addresses altimetry.
« Last Edit: February 07, 2014, 02:00:15 PM by Lost Soul »

Lost Soul

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Re: navigation accuracy part 2
« Reply #2 on: February 09, 2014, 11:41:46 AM »
Just to add to the above post I made. 

1.  To illustrate the point about rapid pressure changes.   Just now, 11:00 hrs Sunday 9 Feb 2014 atmospheric pressure at Gatwick Airport is rising at he rate of  1mb (hP)  per hour.  Earlier this morning it was rising at the rate of 2 mb (hP) per hour.  That earlier rise is an error in reading on your altimeter of 20 meters per hour - depending on your OS map contour intervals that gives an error of of 2 or 4 contour lines per hour and increasing by that rate per hour.  That can give you quite a position error if you are using contours for navigation / position fixing.

Hence the need for half hourly recalibrations.  Particularly in highly unstable weather conditions as we have been suffering in recent weeks.


2.  These wrist altimeter gadgets can be quite flakey in terms of consistency and accuracy hence my recommendation to do and error check going up and down hill in a car.  Walking up and down on a day like today will be totally pointless.  By the time you have gone up and down and up again the air pressure change will likely as not have created a 10 m error on its own and you wont know anything about it.

3.  Because of inherent flakiness and consistency issues with these altimeters then even on days where we have stable air masses - nice summer days of continuous high pressure where there is little or no change in pressure for very long periods of time - errors can and will manifest them selves so half hourly recalibration is essential.  I speak from experience on these matters!

4.  If you genuinely want to know your height with accuracy then you need to use a GNSS unit that is using OS maps.  Here the unit fixes your positing in 2 dimensions on the surface and then gives you the height of the surface above mean sea level by interpolating  contour information from the mapping data base.  A good unit to use for this purpose is Satmap Active 10.

5.  WARNING do not rely in height information being derived by GNSS itself from a hand held GNSS unit.  Whilst they are very accurate (when given a clear uninterrupted line of sight view of the total sky) in terms of 2D positioning on the surface height accuracy is another story.  Errors can be in the order of several hundred metres.  And that is also true of the more complex overpriced portable units sold to the aviation community.  To get height accuracy you need a unit that is able to, at a minimum, receive and manipulate correction signals provide by 3D WAAS. (Wide Area augmentation Systems).  Better still if it can work with more localised 3D correction systems.

6.  Finally if you want to know what the equivalent sea level air pressure is below your boots then calibrate the altimeter as described above; then switch it to read reference barometric pressure.  The value displayed is sea level air pressure for your location.

Lyle Brotherton

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Re: navigation accuracy part 2
« Reply #3 on: February 22, 2014, 05:12:35 PM »
Sammyh20, measure a distance of 500m, walk it then determine you 100m pace count :)

Plus check out the videos on this site http://micronavigation.com/learning-centre/ there is one on pacing and timing.
« Last Edit: February 22, 2014, 05:14:37 PM by Lyle Brotherton »
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Callum

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Re: navigation accuracy part 2
« Reply #4 on: February 23, 2014, 08:05:51 AM »
A couple of pointers from my experience in teaching navigation: People's initial pace count is frequently higher than when they relax into their stride and walk naturally, plus, remember to take account of the wind and terrain, it is surprising how much difference it can make.