Waas

[+Bill D (wwh)]

Nellies Knackers wrote:

So what is the comparison coords to metres? I guess this changes at every location but is there a rough guide that would cover the UK for us amateurs? I'm all in favour of learning new gps tricks.

A minute of latitude is always 1.9 kilometres, so a change of one digit in the last decimal place of the minutes represents 1.9 metres on the ground.

 

With longitude it depends where you are, but in the UK a minute of longitude is about 1 kilometre, so a change of one digit in the last decimal place of the minutes represents about 1 metre on the ground. At the equator the respective figures are 1.9 kilometres and 1.9 metres.

 

N 51 33.331 and N 51 33.332 are 1.9 metres apart, if the W is the same in each case.

 

W 001 55.551 and W 001 55.552 are about 1 metre apart, if the N is the same in each case, and the waypoint is in the UK.

 

Oh, and there is no cache at the above coordinates, which happen to be just north of the M4 near Wootton Bassett.

 

Edited for what I hope is clarity. Doh, and a second time for a typo.

Edited November 24, 2004 by Bill D (wwh)

[+Team Ullium]

Nicely explaned Bill D

 

I don't mean to insult anyone's intelligence here so just ignore this if I'm trying to teach my Granny to such eggs

 

But I know as a young man who was used to working with X-Y axis concepts which the British OS system fits into quite easily....I found it just that bit difficult to come to terms with Latitude and Longitude for navigating.

 

So for anyone not accustomed to thinking in terms of Latitude and Longitude the analogy of hulla hoops springs to mind where the distances you have quoted are the distances of hulla hoops of increasing and decreasing diameters are laid flat and stood on top of each other and separated by the distance of Latitude ... in the case above 1.9 metres in the last decimal point of the minutes apart! (and stays the same distance apart whether one moves north or south as the distance between the hoops are positioned like the spokes on a bicycle wheel radiating out from the earth's center.).

 

Whereas the hulla hoops for Longitude are all the same diameter and are stood vertical and cross each other at the north and south poles and their distance apart in the last digit of minutes are 1.0 metres (and getting narrower the further north or south one moves away from the equatore because the hoops are all converging on the poles).

 

It's a bit like and orange....if you peel it the segments depict the lines of Longitude

and if you slice across the orange at right angles to the segments you get the lines of Latitude.

 

I don't know if I have explained myself clearly...but it is the way I have explained to first time map readers who are having difficulty in the concept of slices through the earth's globe in different ways for Latitude and Longitude....the intuitive way is to think of a grid laid on the ground such as the OS system and work on an X-Y axis concept and Lat and Long are just that bit different.

 

Ullium.

[+Flyfishermanbob]

Nicely explaned Bill D 

 

I don't mean to insult anyone's intelligence here so just ignore this if I'm trying to teach my Granny to such eggs 

 

But I know as a young man who was used to working with X-Y axis concepts which the British OS system fits into quite easily....I found it just that bit difficult to come to terms with Latitude and Longitude for navigating.

 

So for anyone not accustomed to thinking in terms of Latitude and Longitude the analogy of hulla hoops springs to mind where the distances you have quoted are the distances of hulla hoops of increasing and decreasing diameters are laid flat and stood on top of each other and separated by the distance of Latitude ... in the case above 1.9 metres in the last decimal point of the minutes apart! (and stays the same distance apart whether one moves north or south as the distance between the hoops are positioned like the spokes on a bicycle wheel radiating out from the earth's center.).

 

Whereas the hulla hoops for Longitude are all the same diameter and are stood vertical and cross each other at the north and south poles and their distance apart in the last digit of minutes are 1.0 metres (and getting narrower the further north or south one moves away from the equatore because the hoops are all converging on the poles).

 

It's a bit like and orange....if you peel it the segments depict the lines of Longitude

and if you slice across the orange at right angles to the segments you get the lines of Latitude.

 

I don't know if I have explained myself clearly...but it is the way I have explained to first time map readers who are having difficulty in the concept of slices through the earth's globe in different ways for Latitude and Longitude....the intuitive way is to think of a grid laid on the ground such as the OS system and work on an X-Y axis concept and Lat and Long are just that bit different.

 

Ullium.

 

hloody bell ....think this Bruichladdich needs to be moved to class A !

[+Team Ullium]

hloody bell ....think this Bruichladdich needs to be moved to class A !

 

My apologies to everyone if I was carrying coals to Newcastle!

I just thought there might be some newcomers who might appreciate some form of clarification....but maybe I was wrong?

 

Ullium.

[+The Forester]

So what is the comparison coords to metres?

I don't understand the question.

 

Co-ordinates can be expressed in Lat/Long or in grid format.

 

A degree of Latitude is defined as a 90th of the angular distance from the equator to the pole(s).

 

A minute of Latitude is defined as a 60th of a degree.

 

A Nautical mile is defined as 1,852 metres, ie a minute of Latitude.

 

What, exactly, was the question?

 

Are you asking how to convert from Lat/Long to Ordnance Survey grid co-ordinates?

 

Cheers, The Forester

Seek and ye shall find

[JackiePenn]

So for anyone not accustomed to thinking in terms of Latitude and Longitude the analogy of hulla hoops springs to mind where the distances you have quoted are the distances of hulla hoops of increasing and decreasing diameters are laid flat and stood on top of each other and separated by the distance of Latitude

What flavour hulla hoops?

[+Team Ullium]

So for anyone not accustomed to thinking in terms of Latitude and Longitude the analogy of hulla hoops springs to mind where the distances you have quoted are the distances of hulla hoops of increasing and decreasing diameters are laid flat and stood on top of each other and separated by the distance of Latitude

What flavour hulla hoops?

Bruichladdich flavour obviously

 

And also obvioulsy not to everyone's taste

 

Ullium.

[+Nellies Knackers]

So what is the comparison coords to metres?

I don't understand the question.

 

Co-ordinates can be expressed in Lat/Long or in grid format.

 

A degree of Latitude is defined as a 90th of the angular distance from the equator to the pole(s).

 

A minute of Latitude is defined as a 60th of a degree.

 

A Nautical mile is defined as 1,852 metres, ie a minute of Latitude.

 

What, exactly, was the question?

 

Are you asking how to convert from Lat/Long to Ordnance Survey grid co-ordinates?

Bill D got the qestion spot on,as did he the answer, basically it was how many diced oranges to the milli-minute of arc, and which way should they be peeled.

I think Ullium got the second bit too.

[+The Forester]

the distances you have quoted are the distances of hulla hoops of increasing and decreasing diameters are laid flat and stood on top of each other and separated by the distance of Latitude ... in the case above 1.9 metres in the last decimal point of the minutes apart! (and stays the same distance apart whether one moves north or south as the distance between the hoops are positioned like the spokes on a bicycle wheel radiating out from the earth's center.).

I think that's what Sir Humphrey Appleby would call 'mathematically a terminological inexactitude'. In other words, it's not true.

 

In fact, the distance between minutes of Latitude do *NOT* stay the same distance apart whether one moves north or south.

 

An arcsecond of Latitude at the equator is 311mm shorter than at the poles. This is because the Earth is neither flat nor round. It approximates to an oblate spheroid which is represented geodetically as an ellipse of rotation which has flattening ratio of about 1/298.257223563.

 

311 millimeters isn't much, but when you take into account the fact that there are 3,600 arcseconds of Latitude in each hemisphere is adds up to more than a couple of kilometres of error.

 

Converting co-ordinates from Lat/Long to grid and back again is a pig of a job without a computer or a calculator. I once had to do it with 7 figure logarithm tables on a ship which had suffered total electrical failure and had run out of batteries. The conversion took more than four hours with a single pair of co-ordinates and then I had to convert back the other way to check for mistakes. That was UTM grid, which is relatively straightforward. Accurately converting to Ordnance Survey grid is monstrously computer-intensive because the OS network is non-homogeneous. I'm told that the algorithm used by the Hydrographic Office and the Ordnance Survey has over 200 polynomials! Doing that calculation by hand is unthinkable

 

I'm not at all surprised that so many GPSrs make an error of 5 or 6 metres in the conversion. They have to use 3 or 7 parameter datum shift calcs which are necessarily limited to a specific area of the UK. That error was irrelevant in the bad old days before the abolition of Selective Availability and the advent of WAAS, but now it is much greater than the easily achieved accuracy of our GPSrs.

 

Cheers, The Forester

[+Team Ullium]

Sometimes Forester the general concept is more important than the actual reality of the fact

 

I must admit on re-reading my explanation I have made some misleading omissions...apart from my old fashioned way of putting things....but that aside...although you are undoubtably correct in everything you've stated there, can you see that what most cachers might be after is, just a general rule of hand to get some idea of much one co-ord differs from another??

 

Ullium.

[+The Forester]

As usual I agree FFB with all your comments above 

 

And you are correct that there is very little information on what procedure one should apply to be reasonably certain that the co-ords one posts are at least reasonably accurate!

 

Now I know this has been discussed before and what I understood from that was to walk away and return a few times to a specific location and take a different waypoint on each return....then average the results!

 

I've read what appears to suggest that some GPSr's can do this averaging on the spot?? Or am I mistaken ??

 

In any event....is there anything else I can do to ensure I get as accurate an estimate of location co-ords as I can?? 

(In language everyone can understand  )

 

Apart from switching WAAS on that is       

 

Ullium.

Ullium,

 

There are several things you can do.

The first thing to do is to check and recheck that you have selected WGS84 and not OSGB36. That's an error which many of us have made and it causes an error of about a hundred metres.

 

Switching on WAAS is the next thing to do. Be patient when waiting for it to start augmenting the fix. It can take 10 to 30 minutes sometimes. It might not come on at all if they are not transmitting from the Atlantic Ocean Region East satellite.

 

Next thing to do is take a look at the signal strength. Are you getting good signals?

 

Then take a look at the page which shows you the azimuth and altitude of the GPS sats. Are you getting a well conditioned fix. In other words, are the sats reasonably well spread out around you and not bunched together in one corner of the sky.

 

Take look at the quality indication figure. It can't tell you how accurate the fix is, but it will tell you if you've got a major problem of multipath.

 

If you are trying to fix the position of an object, such as a cache, take one fix and then walk away several tens of metres. Then walk back to the indicated position. Are you now standing at the same spot? This is a test of repeatability, not accuracy, but it will give you an idea of how stable the satellite pseudoranges are. Repeat that walk-in from several different direction if possible. This will help to identify multipath as it is extremely variable over quite short distances.

 

Above all else, take as many fixes as you possibly can. A single fix is almost impossible to assess. Two fixes puts you in the situation of the man with two watches. Which one is telling the right time? You need at least three fixes to be able to assess the stability of a GPS fix. The more fixes, the better. The power of averaging is truly amazing. In the offshore survey industry we take a minimum of 200 fixes (preferably 400) when 'boxing in' acoustic transponders. With EHF beacons we achieve accuracies of under 5 centimetres on the seabed in a thousand feet of seawater. We do it by averaging lots and lots of fixes. When measuring the position of oilwells or drilling rigs we log anything up to a thousand fixes. Even in the old days of optical theodolites, when doing high accuracy work we used to measure angles 8, 16, 32 or even 64 times to get that last little bit of accuracy.

 

The Magellan has a wonderful feature that it automatically starts averaging a fix as soon as it detects that it is not moving (defined as a measured speed of less than half a knot). Some Garmins have a feature that enables the user to select a logging interval and store consecutive fixes in memory for later uploading to a computer.

 

Log as many fixes as you possibly can any way you can. If possible, go back and log fixes a couple of hours later. You will have a completely different suite of satellites. Ideally, take three or more sets of averaged data should be used, just in case one of them has incorporated junk pseudoranges.

 

Make check fixes from three different witness points. To do this, measure out a suitable distance such as 10 metres and position yourself North, then 120°, then 240° from the target point and log the GPS fix and the compass bearing taken with a sighting compass (the type which you hold up to your eye and read off the bearing to the nearest degree). Plot the co-ords of the three station on a piece of graph paper and draw your recorded sightlines lines using a protractor or using a suitable graphics package. The three lines won't intersect perfectly, but the size of the 'cocked hat' of the three intersecting lines will give you a good idea of the accuracy you have achieved. Plot the fix of the target onto your drawing. It should be either inside the cocked hat or quite close to it.

 

If you are measuring the co-ords of a structure which has a regular shape, such as a rectangular building, use the transit technique. Log fixes along the extended line of the sides of the building some distnce away from the building to avoid multipath. You should have 8 fixes if the building is rectangular. Plot those fixes on your graph papaer or graphics package and draw a line between each pair. You should have something that looks like a noughts and crosses frame. You can assess the quality of the fixes by seeing whther the lines are parallel and square to eachother. It's an extremely accurate technique and very useful for measuring co-ords of a structure to which you do not have physical access.

 

Plot your derived co-ords onto the largest scale map you can find. If there is a significant gross error, it may show up quite clearly. For example one of the caches on the Water of Leith trail had ludicrously inaccurate co-ordinate when first published. They were actually on completely the wrong side of the river. A little bit of commonsense and five minutes with a map would have saved people from wasting their time looking in entirely the wrong place. Another cache on that trail was actually located on the opposite side of the path from the co-ordinates. Again, a few minutes spent plotting the co-ords onto a large scale map such as the excellent Canmap on thr RCAHMS website (where you can plot a fix to single metre precision) would have immediately shown the cache-placer that there was a blunder in the co-ords. Interestingly, that cache was found before the co-ords were corrected by someone who searched using only a map, not a GPSr!

 

In England, they have http://www.magic.gov.uk/website/magic/ which is an excellent plotting/mapping resource.

In Scotland we have http://www.rcahms.gov.uk/search.html#canmore which also allows mapping and plotting at metre level.

 

With a wee bit of effort and some patience (and WAAS!) it is quite easy to obtain an accuracy from a GPSr which is of similar magnitude to the precision of geocaching.com ddmm.mmm format (2.13 metres in Govan!) and the 2.8m accuracy of Ordnance Survey mapping in rural areas.

 

Cheers, The Forester

[+Team Ullium]

It's funny how sometimes you need someone to draw your attention to a situation for you to make special note of it's relevance

 

Most of what you said there Forester I was pretty well aware of myself...however...the point about observing whether or not there was a good spread of satellites is one that I subconsciously felt happy about if they were well spread out and uneasy if they weren't ... without giving it too much thought as to why

 

I'm glad you stated that four or more recorded waypoints from different directions was in the main reasonable for an average to be taken....as 200 to 400 was beginning to worry me

 

Also, with Angela having her own GPSr which has a different aerial...it means we can both take readings and average the lot....on the principle of the more the merrier

 

Thanks for the explanation ;-)

 

Ullium.

[+The Forester]

just a general rule of hand to get some idea of much one co-ord differs from another??

I presume you mean from Lat/Long co-ords?

 

Bear in mind that a minute of Latitude is a nautical mile (1,852 metres) and that at our Latitude(ish) a minute of Longitude is about half that.

 

If you think distances in statute miles then just knock 10% off the distance in nautical miles. That's easy to do with mental arithmetic.

 

There are several other quick and easy tricks in navigation, such as the 1 in 60 rule. If you want to know far off track you are with a 10 degree error after going 60 metres in to dense forest without a useable GPS signal then it is simply a matter of 1 times 10, ie your cross-track error is 10 metres. Or you want to know how far off track you will be after 180 metres with a 5° error, then its 3 times 5 because 180 is 3 times 60. It's a very handy method of doing mental trigonometry and only needs a knowledge of the 6 times table.

 

Yesterday I spent happy afternoon in a forest doing a recce for a multi that I'm compiling. I've made one of the locations quite deep into dense forest, just to make the cacher do a bit of thinking without the aid of a GPSr. Compulsive arrow-followers will probably lose the signal, especially if the tree canopy is wet. Thinking navigators will take a fix before leaving the path and will use a compass and count their steps. It's an elementary navigational technique but a very useful one. Any land navigator ought to know the length of his/her double stride: how far you will travel with 100 double steps. Knowing that, and holding your course to within 5° or so of the required heading, you can keep track of your position or home in into a required position with a surprisingly high accuracy. I found that 120 metres into the forest I was within 4 or 5 metres of the target, even when I approached from three different poitions. That's pretty similar to (or perhaps better than) an unaugmented GPS fix.

 

During the Great Survey of India in the 1880s (I think) they traversed the entire continent with pacers. Those guys had to be able to measure distance by disciplined pacing and the absolute minumum accuracy required of them was 1 in 1,000. The better ones achieved an amazing 1 in 10,000 accuracy.

 

A competent land navigator should also know his/her average speed of walking. Your watch then becomes an odometer, in effect. Knowing how long you've been walking in a certain direction should give you a reasonably good approximation of your position.

 

I think that GPS is one of the greatest things since sliced bread, but I hope that navigation doesn't become totally deskilled by it. I'm appalled by how long it I since I last measured position by a theodolite a stopwatch and the stars. Almost 20 year, I think. The thrill of extracting a Lat/Long from the calculations after making observations on a bright starry night is hard to beat.

 

Sorry 'bout the thread drift! Perhaps I should start a thread about land navigation techniques! It's a fascinating subject and one which can be very useful out on the hills when those pesky batteries run out or the GPSr spontaneously fails. I'm constantly amazed by the number of people who don't know even the basics such as the difference between True North, Grid North and Magnetic North. I don't blame them, I blame the schools for not properly teaching navigation as part of the geography syllabus.

 

Old fashioned navigation techniques fall into disuse at our peril.

[+Team Ullium]

Sorry 'bout the thread drift! Perhaps I should start a thread about land navigation techniques! It's a fascinating subject and one which can be very useful out on the hills when those pesky batteries run out or the GPSr spontaneously fails.

 

Go for it Forester

 

With more and more caches being placed in wild places...this kind of knowledge could well save a life

 

BTW took note of the wee tips

 

Ullium.

[+The Forester]

With more and more caches being placed in wild places...this kind of knowledge could well save a life

Back in late February or early March of this year I started to put together some course materials for a course to teach a geocacher some intermediate and advanced level land navigation. Unfortunately, my student lost interest and so my easy instructional course never came to fruition.

 

I agree that a bit of knowledge about how to navigate with and without without GPS could usefully supplement the use of GPS by any geocacher out on the hill.

 

Even the simple stuff, such as what immediately to do if a blizzard hits and the GPSr batteries fade, is unknown to some geocachers. That's appalling.

 

I don't think that this forum is the place or the right medium to give such a course -- even though it might save a life.

 

Mebbe I should gather together some interested parties and do a correspondence course thingy using the medium of the internet and mobile phone to while away the long and dark winter evenings. Could be fun. Might even save a life.

 

Cheers, The Forester

Pardon the extended thread drift. WAAS won't be there to help you if you don't know how to navigate without it.

[+Team Ullium]

Well how about a series of articles for for one of the sister websites ?

Or even the whole course to be attached to Lactodorum's information link?

 

Ullium.

[+The Forester]

Well how about a series of articles

The idea of of the course was entirely to be interactive and interpersonal. It was never intended to be postal and impersonal.

 

Without instructor/student 2-way interaction it never could have worked.

[+snaik]

Afternoon Forester

 

Was at the Trig point on Kinoull Hill and mananed to get a WAAS lock on Magellan but not on Garmin but both had same reading

 

 

 

Snaik

[+Flyfishermanbob]

Re Rich Snaik and his 2 GPSr's ....

 

I'm sure I've read an article which said that if you operated 2 receivers together , you could /would get interference / noise ..... obviously not !

 

Picture begs the question ........ is current Waas correction data credible.

[+The Forester]

Good evening,

 

Trigpointinguk.com lists the co-ords of that trig point as:

56° 23.37666N 3° 24.00079W

 

Your measured co-ords are almost exactly a metre (1.02m) from that point.

 

That's pretty dadgum good!

 

Thanks for the datapoint, Snaik. I think it demonstrates how accurate GPS can be.

 

 

Cheers, The Forester

[+snaik]

I think testing your GPS unit against a known location like a Trig Point builds confidence on the accuracy of the wee beastie's

 

To get with 1 metre to me is accurate enough for me

 

Thanks Snaik

[+The Forester]

To get with 1 metre to me is accurate enough for me

An interesting learning point is that the Garmin "accuracy" figure was completely unrelated to the realworld accuracy figure.

[+The Forester]

Another WAAS datapoint:--

 

I bagged the previously unlogged trigpoint at Little Hill near Comrie in Perthshire.

 

The co-ords listed by Trigpointinguk.com equate to a Lat/Long of 56° 19.7951N 3° 59.2937'W

 

I took two WAAS fixes, in the standard geocaching DDD.MM.MMM format, at the trig pillar:

 

56° 19.796N 3° 59.293'W (Computed - Observed = 1.82m)

and;

56° 19.795N 3° 59.294'W (Computed - Observed = 0.36m)

 

The first fix was averaged over a period of a little over 2 minutes; the second fix was averaged over just under 5 minutes.

 

Both fixes are well within the claimed accuracy for WAAS/EGNOS of <3m.

 

 

Cheers, The Forester

[+snaik]

How long does the magellan need to sit to compute, to get a good average coord

 

Snaik

Edited November 30, 2004 by Snaik

[+The Forester]

How long does the magellan need to sit to compute, to get a good average coord

 

Snaik

Good question.

 

My answer would be that it doesn't take many minutes to average a decent fix, but that you should take at least three fixes to see if the fixes are stable,

 

Repeatability is not at all the same thing as accuracy, but if you have an external indicator of the true value of co-ordinates such as we have at trigpoints, you can get by with just two averaged fixes.

 

If you have good agreement amongst two or three averaged and augmented fixes, then you have a good indicator of the probable accuracy of your derived co-ordinates. That will be much more representative of the actual accuracy than the "accuracy" or "EPE" figures shown on Garmins and Magellans.

 

An interesting study of the accuracy of WAAS-averaged fixing is at: http://users.erols.com/dlwilson/gpswaas.htm

 

 

My advice would be to take at least three averaged fixes if you are trying to get a good accurate fix on a location such as a newly placed cache. The three fixes should be taken as far apart in time as you can be bothered with, as that will produce a different spatial geometry of the observed satellites and will consequently be more representative of what a finder can reasonably expect to observe on his/her GPSr when seeking the cache.

 

Cheers, The Forester

[+Alibags]

Oh, my brain hurts!

[+Fangsy]

I think testing your GPS unit against a known location like a Trig Point builds confidence on the accuracy of the wee beastie's

 

To get with 1 metre to me is accurate enough for me

 

Thanks Snaik

Also, when I was up at the sumit of Pen Y Fan which the OS maps show as 886 Meters my Garmin Venture gave me an elevation of 887 Meters, which when you take into account the fact I was holding it at the time is spot on.

I always had assumed the horizontal accuracy was greater than vertical but it seems with good coverage both can be pretty pin sharp.

 

Oh and I didn't have any WAAS or EGNOG either.

Edited November 30, 2004 by Fangsy

[+snaik]

Also, when I was up at the sumit of Pen Y Fan which the OS maps show as 886 Meters

 

Ah "Pen Y Fan" run up that hill many a time always felt a lot higher than that!

 

 

Snaik

[+Fangsy]

Also, when I was up at the sumit of Pen Y Fan which the OS maps show as 886 Meters

 

Ah "Pen Y Fan" run up that hill many a time always felt a lot higher than that!

 

 

Snaik

Depends where you start from I suppose

 

As a wise Irishman once said when asked for directions, "If I were you, I wouldn't be starting from here."

[+The Forester]

I always had assumed the horizontal accuracy was greater than vertical

Correct.

 

The reason for this is that satellite pseudoranges low to the horizon tend to cancel the random errors of satellites on the other side of the sky and vice versa.

 

Satellites high above you obviously do not have any satellites below you which can have that compensating effect. It's the sats high above your head which give you the height data and the ones low to the horizon which give you your Lat/Long data.

 

I've found that the vertical error of a GPS fix (with or without WAAS) is about 1.6 times greater than the horizontal error.

 

Fortunately for us in geocaching, we are seldom interested in the accuracy of the height amsl of a fix.

 

when you take into account the fact I was holding it at the time is spot on.

Oh and I didn't have any WAAS or EGNOG either.

With or without WAAS/EGNOS, if you plot numerous instantaneous fixes on graph paper or screen, you will see a scatter plot which resembles the scattered pellets of a shotgun hit. Some of the them will be closer to the true centre of the target than others.

 

It is very satisfying to score a direct hit and sooner or later we will look at the screen and record a fix which just happens to be that middle pellet, but we have to remember the existence of the other 99% of fixes!

 

I'd be interested to see some more datapoints such as Snaik's one. I believe that a WAAS/EGNOS GPSr is capable of extraordinary accuracy(<3m), but I'd like to see some more data from the real world experiences of geocachers.

 

Cheers, The Forester

[+Lance Ambu]

The reason for this is that satellite pseudoranges low to the horizon tend to cancel the random errors of satellites on the other side of the sky and vice versa.

 

<snip>

Do you remeber the days when we used to write <snip> on everything? I digress.

 

When I went to school my physics teacher would always emphasise the need to use units correctly. Hence his oft asked question " ... apples, oranges, what are you talking about?"

 

Now I see that it isn't even oranges - it's pseudoranges! Bring back real oranges, I say!

[+Fangsy]

I'd be interested to see some more datapoints such as Snaik's one. I believe that a WAAS/EGNOS GPSr is capable of extraordinary accuracy(<3m), but I'd like to see some more data from the real world experiences of geocachers.

Hmm is this what you mean? I took this photo on top of a trig point, number S2024 on top of Y Cribarth in Wales. I don't know what the -known- possition of this trig point is, but the GPSr was showing what you see in the picture there. Datum at the time was UK OS Grid.

 

 

I'd be interested to find out more about trig points. I saw the MapMan show about the OS maps earlier this evening incidentaly.

[+The Forester]

trig point, number S2024 on top of Y Cribarth in Wales.

I'm wary of the accuracy of the internal conversion by GPSrs from WGS84 to the British National Grid.

 

However, the listed co-ords of your trig are SN 82838 141191.

 

That's a distance of 6.4 metres from the co-ords indicated on the screen of your GPSr in the photo.

 

I don't know how much of that difference is due to the crude conversion by the GPSr from WGS84 and how much is due to the actual accuracy or inaccuracy of the fix, so I'm not sure what it tells us about WAAS accuracy, but I appreciate the datapoint. Thanks.

 

Cheers, The Forester

[+John Stead]

I'd be interested to find out more about trig points.

If you don't know about it already you will find Teasel's Trigpointing site very useful. http://www.trigpointinguk.com/

If you want more detail on a particular Trigpoint it is available on the Ordnance Survey's database which is accessible if you register.

[+Snosrap]

I've mentioned before on this thread that I cannot seem to get a lock on the WAAS sats on my Garmin GPSV. The receiver finds the sats and shows them as grey bars as if it is trying to analyse the information it's receiving but no matter how long I leave it they never become solid.

 

Has anyone else managed to get an augmented fix using a Garmin V??

[+Team Ullium]

I've mentioned before on this thread that I cannot seem to get a lock on the WAAS sats on my Garmin GPSV. The receiver finds the sats and shows them as grey bars as if it is trying to analyse the information it's receiving but no matter how long I leave it they never become solid.

 

Has anyone else managed to get an augmented fix using a Garmin V??

Sounds like maybe you should get your Garmin checked out Snosrap

 

If the bars stay grey and never turn blackish...then that would suggest to me that the Garmin is not receiving useful data from those satellites??

 

Ullium.

[+Snosrap]

Sounds like maybe you should get your Garmin checked out Snosrap 

 

If the bars stay grey and never turn blackish...then that would suggest to me that the Garmin is not receiving useful data from those satellites?? 

 

Here's a clip from the GPSV firmware update history:

 

"Improved WAAS/EGNOS satellite selection algorithm to select the satellite with the most beneficial corrections given the unit's current position. A unit will not use a WAAS/EGNOS satellite if the unit's current position is outside of a given WAAS/EGNOS satellite's service volume. "

 

Perhaps this is the case for my location (N Yorkshire). Can any of you learned chaps make sense of this for me?

[+The Forester]

A unit will not use a WAAS/EGNOS satellite if the unit's current position is outside of a given WAAS/EGNOS satellite's service volume. "

 

Perhaps this is the case for my location (N Yorkshire).

"Service volume" is Garminese for the space within which the augmentation data is readable and valid.

 

North Yorkshire is well within the footprint of the AOR-E sat, so no problem there, I think.

 

I would suggest three steps:

 

Step 1:- Recycle the enabling of WAAS on your GPSr. In other words, switch WAAS off and then on again.

 

Step 2:- For this you will need to contact a chum who has a WAAS-capable Garmin GPSr. Check whether EGNOS signals are being sent and received and used at the current time. They are sent most of the time on most days, but are certainly not yet 24*7 for us in Northern Britain.

 

Step 3: Has already been suggested by Ullium. Make good use of Garmin's excellent customer service.

 

Cheers, The Forester

LearnING, not learnED!

[+Team Ullium]

One other thing occurred to me Snosrap!!

 

I had a problem with my Geko 201 at one point and I had to do a reboot of the software which corrected the fault!

 

If all else fails this might be worth a try?

 

Ullium.

[+Snosrap]

Thanks for the input guys. I'll keep trying various 'solutions' and see if something comes up.

[+Team Ullium]

Sorry I used the word 'Reboot' when I meant 'Reset' and more accurately 'Master Reset'

 

Now this resets everything held on one's GPSr (as I only have experience of Garmin units I can't comment on other makes!) and one has to go through the whole setup procedure of entering time zone etc etc.

 

This procedure is instigated via a small hole in the back of the GPSr and should only be used as a last resort! Do a search on 'Master Reset' on the Garmin home site to read more about it before attempting this procedure!

 

Also I would suggest downloading and installing (or re-installing if you already have the latest update of software for your particular unit) the latest update of Garmin software (also to be accessed on the Garmin home site!)

 

I don't know if any the other Garmin users have used this Master Reset option apart from myself...but I found it cured my particular problem

 

Ullium.

P.S. sorry if this is a bit off topic ... but I thought it important enough to bring to attention here having brought the subject myself

I will start a separate thread!

Edited December 3, 2004 by Ullium

[+The Forester]

'Reboot'; 'Reset'  'Master Reset'

For those of us who have GPSrs which don't have the reset hole thingy, could we do the same thing just by leaving the batteries out overnight?

 

Wouldn't that restore the machine to the factory set default condition?

 

Cheers, The Forester

[JackiePenn]

.... and another thing !

[+The Forester]

I think testing your GPS unit against a known location like a Trig Point builds confidence on the accuracy of the wee beastie's

 

To get with 1 metre to me is accurate enough for me

This afternoon I plonked my GPSr on the Wallacestone trig pillar.

 

The stated co-ords for that pillar are:

55° 58.482N 3 44.038W

 

My Magellan, with WAAS, showed:

55° 58.483N 3 44.037W

 

Not bad! That's within a couple of metres ans well within the manufacturers claim of 3 metre accuracy with WAAS/EGNOS.

 

Edited February 1, 2005 by The Forester

[Documentally]

It has taken me hours to read all these posts and try to comprehend the do's and don'ts, truths and misgivings of WAAS.

 

...And still no one has the same WAAS system in place that i do...

 

Should i know i am close to a cache and want to narrow down the possible minor errors and indescrepancies talked about here, WAAS always helps me...

 

I Walk About Accurately Strimming

 

 

Never fails 100% accurate

 

(edited due to immense embarrassment and a head full of wine...)

Edited February 17, 2005 by 3rdeye

[+Birders]

Ahhh... that's how all those plastic boxes got minced!!

[+Stuey]

WASS.

 

I Walk About Strenuously Strimming 

But it is WAAS (Wide Area Augmentation System), so maybe you are:

 

Walking About Aimlessly Strimming hehe

[Documentally]

See this post had driven me mad!! drove me

 

i think i meant to say Walk About Accurately Strimming