gps software for walking

On Mon, 14 May 2007 13:40:07 +0100, <[email hidden]> wrote:

|!I've got a ipaq pda and would like to add gps software to it for use when
|!walking. I find my magellan explorist 200 a bit basic, and wonder if anyone
|!could advise on affordable (sub ?100) software/maps.

Remember that you will need National Grid references for walking, which all
the walking map software use.
--
Dave Fawthrop <sf hyphenologist.co.uk> 165 *Free* SF ebooks.
165 Sci Fi books on CDROM, from Project Gutenberg
http://www.gutenberg.org/wiki/Main_Page Completely Free to any
address in the UK. Contact me on the *above* email address.

"Dave Fawthrop" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

On Mon, 14 May 2007 13:40:07 +0100, <[email hidden]> wrote:

|!I've got a ipaq pda and would like to add gps software to it for use
when
|!walking. I find my magellan explorist 200 a bit basic, and wonder if
anyone
|!could advise on affordable (sub ?100) software/maps.

Remember that you will need National Grid references for walking, which
all
the walking map software use.
--
Dave Fawthrop <sf hyphenologist.co.uk> 165 *Free* SF ebooks.
165 Sci Fi books on CDROM, from Project Gutenberg
http://www.gutenberg.org/wiki/Main_Page Completely Free to any
address in the UK. Contact me on the *above* email address.

I have used an Ipaq with a Bluetooth GPS receiver (£30 EBay) and Anquet
mapping (National parks can be had cheaply). It looks pretty but is not as
functional as even a basic mapping GPS which can probably be had for less
than £100. Battery life is also a problem although you can get a battery
extender that takes NiMh AA's for a fiver.

Jon

On Tue, 15 May 2007 08:11:36 +0100, "JRS" <[email hidden]> wrote:

|!
|!"Dave Fawthrop" <[email hidden]> wrote in message
|!news:[email hidden]...
|!> On Mon, 14 May 2007 13:40:07 +0100, <[email hidden]> wrote:
|!>
|!> |!I've got a ipaq pda and would like to add gps software to it for use
|!> when
|!> |!walking. I find my magellan explorist 200 a bit basic, and wonder if
|!> anyone
|!> |!could advise on affordable (sub ?100) software/maps.
|!>
|!> Remember that you will need National Grid references for walking, which
|!> all
|!> the walking map software use.

|!I have used an Ipaq with a Bluetooth GPS receiver (?30 EBay) and Anquet
|!mapping (National parks can be had cheaply). It looks pretty but is not as
|!functional as even a basic mapping GPS which can probably be had for less
|!than ?100. Battery life is also a problem although you can get a battery
|!extender that takes NiMh AA's for a fiver.

I am looking to get a GPS including PDA this year, but need the ability to
use at least 1 50,000 maps, EGNOS, and to enter 10 figure OSGB grid refs.
--
Dave Fawthrop <sf hyphenologist.co.uk> 165 *Free* SF ebooks.
165 Sci Fi books on CDROM, from Project Gutenberg
http://www.gutenberg.org/wiki/Main_Page Completely Free to any
address in the UK. Contact me on the *above* email address.

JRS said:

I have used an Ipaq with a Bluetooth GPS receiver (£30 EBay) and
Anquet mapping (National parks can be had cheaply). It looks pretty
but is not as functional as even a basic mapping GPS which can
probably be had for less than £100.

Really? Are mapping units they that cheap nowadays? I thought they still
cost a lot more than that. And of course, you have to buy the map software
as well, and that's not cheap. Also, when you say "basic mapping", isn't
that a contradiction in terms? Basic units don't have mapping, if they do,
then they aren't basic.

But more to the point, I'm curious as to what you mean by a mapping unit
being more functional. Do you mean that it's more convenient to use (i.e.
self-contained) with better waterproofing and battery life, or that the GPS
has more usable features, like waypoints, routes, goto buttons and so on?
I've never used a palmtop/Anquet type GPS so I can't really comment on them.

Or are you referring to the map details themselves being more accurate? This
is something I've noticed with the road mapping. At complex junctions like
motorway roundabouts, with lots of slip roads, it's amazing how accurately
the car is positioned on the map if you zoom right in. When following dual
carriageways, they are shown as two separate roads on the GPS and the car is
always on the correct one (good signal permitting of course). OS maps aren't
that accurate. The width of roads is exaggerated to make reading easier, if
they were shown at their correct widths they'd appear as really thin lines
on the map.

Whether you need that degree of accuracy is another matter though, many
might argue that simply being able to see where you are on an OS map is good
enough. Another good feature of mapping units is the ability to zoom in and
out. While some may criticise the relative lack of detail on GPS mapping,
being able to zoom out is great for an overview, especially when the detail
is thinned out as you do so. Likewise, being able to zoom in, far more than
you can on an OS map, is great when you have complex contours to deal with.

Paul
--
http://www.wilderness-wales.co.uk

Dave Fawthrop said:

I am looking to get a GPS including PDA this year, but need the
ability to use at least 1 50,000 maps, EGNOS, and to enter 10 figure
OSGB grid refs.

I see no reason why you wouldn't be able to enter 10 figure grid refs, but
you won't get that accuracy on 50k maps. Having placed a lot of waypoints on
50k and 25k maps, I can tell you that the accuracy of specific points, like
trigs, spot heights and so on, is significantly better on the 25k maps. I
know we all know that anyway, but I thought I'd mention it since 10 figure
grid refs and 50k maps don't go well together. On 50k mapping I've found
some points to be in error by 50m or more, whereas 25k mapping is rarely
more than 10m out.

What you may not realise if you haven't used OS map software is that due to
the resolution of the maps, each pixel on a 50k map equals 5m, while each
pixel on a 25k map equals 2.5m. So even if your mouse clicking is pixel
perfect, you can't get better than 5m accuracy on a 50k map.

Problem is, most spot heights on 50k maps are actually wider than 1 pixel
and can cover a 4 pixel square, so that's really only 10m accuracy, and even
that assumes that the spot is in exactly the right place, which it often
isn't. Spot heights are often very sloppily positioned on 50k maps.

In comparison, not only are 25k maps higher in resolution, but the features
are smaller. Spot heights are about the same size, giving 5m accuracy, but
they're much more precisely positioned. Lines and other features tend to be
thinner and smaller. Roads are thinner. I've just done a quick comparison
and found a main road on the 50k map to be about 35m wide, while the same
road on the 25k map is only 12.5m wide.

Also, cairns tend to be represented quite differently. On the 50k maps, not
only is the cairn symbol much larger, but they often use a single symbol to
represent multiple cairns, so if there's a number of cairns close together,
the 50k map often shows fewer cairns and the positions are much more
inaccurate, since they place them further apart to avoid the symbols
overlapping.

Of course, even 25k mapping isn't perfect, but given the choice, I'd much
sooner trust it than 50k mapping. I'd say that it's reasonable to expect say
10m accuracy with 25k, but more like 20m accuracy with 50k (and sometimes
far worse).

I may do some screenshots.

Paul
--
http://www.wilderness-wales.co.uk

"Paul Saunders" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

JRS said:

I have used an Ipaq with a Bluetooth GPS receiver (£30 EBay) and
Anquet mapping (National parks can be had cheaply). It looks pretty
but is not as functional as even a basic mapping GPS which can
probably be had for less than £100.

Really? Are mapping units they that cheap nowadays? I thought they still
cost a lot more than that. And of course, you have to buy the map software
as well, and that's not cheap. Also, when you say "basic mapping", isn't
that a contradiction in terms? Basic units don't have mapping, if they do,
then they aren't basic.

But more to the point, I'm curious as to what you mean by a mapping unit
being more functional. Do you mean that it's more convenient to use (i.e.
self-contained) with better waterproofing and battery life, or that the
GPS has more usable features, like waypoints, routes, goto buttons and so
on? I've never used a palmtop/Anquet type GPS so I can't really comment on
them.

Or are you referring to the map details themselves being more accurate?
This is something I've noticed with the road mapping. At complex junctions
like motorway roundabouts, with lots of slip roads, it's amazing how
accurately the car is positioned on the map if you zoom right in. When
following dual carriageways, they are shown as two separate roads on the
GPS and the car is always on the correct one (good signal permitting of
course). OS maps aren't that accurate. The width of roads is exaggerated
to make reading easier, if they were shown at their correct widths they'd
appear as really thin lines on the map.

Whether you need that degree of accuracy is another matter though, many
might argue that simply being able to see where you are on an OS map is
good enough. Another good feature of mapping units is the ability to zoom
in and out. While some may criticise the relative lack of detail on GPS
mapping, being able to zoom out is great for an overview, especially when
the detail is thinned out as you do so. Likewise, being able to zoom in,
far more than you can on an OS map, is great when you have complex
contours to deal with.

Paul
--
http://www.wilderness-wales.co.uk

http://www.gpsw.co.uk/details/prod3429.html

I have seen these go 2nd hand for not much more than £50 and national parks
on anquet can be had for £20

I would call it basic compared to this:

http://www.gpsw.co.uk/details/prod3041.html

which is what I use.

I can only speak for Anquet on an Ipaq but in terms of it navigation
abilities and facilities its functionality (route / waypont management, etc)
is limited alongside a dedicated GPS unit as well as being fragile with poor
battery life and the need for a seperate receiver as mentioned. I don't use
it any more. Following your position on an O/S map on screen looks pretty
for a while but I didn't find it a very effective navigational tool. perhaps
spending a bit more time on it would have helped but the 60csx was so much
easier to use.

I didn't use it long enough to identify accuracy issues but I have noticed
that when I draw a route on a road in anquet it does not always lie exactly
on the road that the 60csx has in its memory. The 60csx is very accurate in
this respect and when I stand on the road it shows me on the road whereas my
line from the o/s mapping may be slightly off. I put this down to not
drawing accurately at high enough zoom but sometimed however accurately I
draw a route on a footpath the GPS will occasionally show me off course even
though I'm on the footpath. This happens even with an excellent signal.

I only use routes and waypoints downloaded to my 60csx from anquet in
conjunction with european street level mapping (metroguide v8 almost whole
of europe on 2gb SD card) - I occasionally like to check position with a
road or know how close I am to one if anything goes wrong. Its also useful
if I am trying to drive to the start of a walk. My mate has the garmin topo
s/ware and it lacks detail for me - particulalry with respect to footpaths.
Jon

Quoted message said:

In message <[email hidden]>, Dave Fawthrop
<[email hidden]> writes

Quoted message said:

On Tue, 15 May 2007 08:11:36 +0100, "JRS" <[email hidden]> wrote:

|!
|!"Dave Fawthrop" <[email hidden]> wrote in message
|!news:[email hidden]...
|!> On Mon, 14 May 2007 13:40:07 +0100, <[email hidden]> wrote:
|!>
|!> |!I've got a ipaq pda and would like to add gps software to it for use
|!> when
|!> |!walking. I find my magellan explorist 200 a bit basic, and wonder if
|!> anyone
|!> |!could advise on affordable (sub ?100) software/maps.
|!>
|!> Remember that you will need National Grid references for walking, which
|!> all
|!> the walking map software use.

|!I have used an Ipaq with a Bluetooth GPS receiver (?30 EBay) and Anquet
|!mapping (National parks can be had cheaply). It looks pretty but is not as
|!functional as even a basic mapping GPS which can probably be had for less
|!than ?100. Battery life is also a problem although you can get a battery
|!extender that takes NiMh AA's for a fiver.

I am looking to get a GPS including PDA this year, but need the ability to
use at least 1 50,000 maps, EGNOS, and to enter 10 figure OSGB grid refs.

I used my Bluetooth PDA with Holux Bluetooth GPS. I use Memory Map and
have both 1:50,00 and 25:000 maps of the Beacons NP and Pembs BP coastal
path.

MM require 10 fig grid refs even if you only have 6 fig ones. You have
to add to two noughts to each half of the ref. As for EGNOS - I don't
know.

HTH
--
Bill Grey

In message <[email hidden]>, Paul Saunders
<[email hidden]> writes

Quoted message said:

Or are you referring to the map details themselves being more accurate? This
is something I've noticed with the road mapping. At complex junctions like
motorway roundabouts, with lots of slip roads, it's amazing how accurately
the car is positioned on the map if you zoom right in. When following dual
carriageways, they are shown as two separate roads on the GPS and the car is
always on the correct one (good signal permitting of course). OS maps aren't
that accurate. The width of roads is exaggerated to make reading easier, if
they were shown at their correct widths they'd appear as really thin lines
on the map.

Whether you need that degree of accuracy is another matter though, many
might argue that simply being able to see where you are on an OS map is good
enough. Another good feature of mapping units is the ability to zoom in and
out. While some may criticise the relative lack of detail on GPS mapping,
being able to zoom out is great for an overview, especially when the detail
is thinned out as you do so. Likewise, being able to zoom in, far more than
you can on an OS map, is great when you have complex contours to deal with.

Recently after an exercise I had a short flight in a Sea King off the
hill to where our cars were parked. My Holux GPS was in my rucksack as
was my PDA. The fight track was recorded so was the drive back to
Brecon.

All on 1:25,000 OS map in MM
--
Bill Grey

Bitstring <[email hidden]>, from
the wonderful person JRS <[email hidden]> said
<big snip>

Quoted message said:

I didn't use it long enough to identify accuracy issues but I have noticed
that when I draw a route on a road in anquet it does not always lie exactly
on the road that the 60csx has in its memory. The 60csx is very accurate in
this respect and when I stand on the road it shows me on the road whereas my
line from the o/s mapping may be slightly off. I put this down to not
drawing accurately at high enough zoom but sometimed however accurately I
draw a route on a footpath the GPS will occasionally show me off course even
though I'm on the footpath. This happens even with an excellent signal.

Nope, I've seen exactly the same thing with Tracklogs, and even with
digitised OS paper maps used with Oziexplorer. While the OS went to
amazing lengths to get the trig points and heights right to amazing
accuracy (for the era they were done in) I suspect that many of the
minor roads have been pencilled in from aerial photographs or whatever,
and in some cases incorrectly adjusted for angle of view, or height, or
something.

I'm talking about roads here - footpaths move around on the ground, and
the OS representation just can't be trusted.

The maps in the 60CSX (of roads) are Navteq data, which I believe they
actually collect(ed) more recently than the OS .. maybe even by driving
along them using a GPS as they went (although some minor roads are too
simplified in the 60CSX mapping).

I pretty sure the OS is wrong, since I've done some sections on
different days in different years even, and got answers +/-5m from my
GPS, whereas the OS 1:25k map has the road 50m off to one side. Some OS
1:25k mapping seems to have not been updated for at least 50 years
('revised' maybe, but not completely re-surveyed), which means it
definitely was NOT done with a GPS.

--
GSV Three Minds in a Can
8,630 Km walked. 1,677Km PROWs surveyed. 30.5% complete.

JRS said:

http://www.gpsw.co.uk/details/prod3429.html

I have seen these go 2nd hand for not much more than £50

Yeah, but the RRP is £250. Bloody good deal though!

Quoted message said:

and national
parks on anquet can be had for £20

But that's not GPS mapping, which is I was referring to.

Quoted message said:

I would call it basic compared to this:

http://www.gpsw.co.uk/details/prod3041.html

which is what I use.

I've got the 60CS myself, which is virtually the same thing.

Quoted message said:

I can only speak for Anquet on an Ipaq but in terms of it navigation
abilities and facilities its functionality (route / waypont
management, etc) is limited alongside a dedicated GPS unit as well as
being fragile with poor battery life and the need for a seperate
receiver as mentioned. I don't use it any more. Following your
position on an O/S map on screen looks pretty for a while but I
didn't find it a very effective navigational tool. perhaps spending a
bit more time on it would have helped but the 60csx was so much
easier to use.

Yeah. Once the novelty wears off you don't find yourself walking around
staring at the GPS screen all the time, it's just something you check now
and again. If you have some preloaded waypoints it's simply a matter of
looking at the map or compass screen to check which direction you should be
walking in and how far it is to the next waypoint.

Quoted message said:

I didn't use it long enough to identify accuracy issues but I have
noticed that when I draw a route on a road in anquet it does not
always lie exactly on the road that the 60csx has in its memory. The
60csx is very accurate in this respect and when I stand on the road
it shows me on the road whereas my line from the o/s mapping may be
slightly off. I put this down to not drawing accurately at high
enough zoom but sometimed however accurately I draw a route on a
footpath the GPS will occasionally show me off course even though I'm
on the footpath. This happens even with an excellent signal.

It's not a problem with inaccurate drawing, the GPS is spot on. It's the OS
maps that are inaccurate. We've discussed this many times in the past. The
features drawn on OS maps are often positioned relatively rather than
absolutely. Few positions on an OS are spot on. Naturally trig points are
(or should be). The thing with roads is that they're usually drawn wider
than they really are, to make reading the map easier, but when you widen a
road, that can push other features out of position, including other roads.
Hence the relative positioning I mentioned.

If a lot of details, say a road, a river, a footpath and a line of trees all
occur in close proximity, some will get pushed out of the way. Relatively
speaking, the map looks correct, but in absolute terms, it's wrong. Nobody
ever noticed this until GPS came along, but now we usually have better
accuracy that the maps, reception permitting. Obviously GPS doesn't
understand relative positioning! ;-) If there's a difference between the
map and the GPS, then provided reception is good, trust the GPS not the map.

Quoted message said:

I only use routes and waypoints downloaded to my 60csx from anquet in
conjunction with european street level mapping (metroguide v8 almost
whole of europe on 2gb SD card) - I occasionally like to check
position with a road or know how close I am to one if anything goes
wrong. Its also useful if I am trying to drive to the start of a
walk.

Indeed. With some country lanes I've had more trouble navigating to the
start of the walk than during the walk itself!

Quoted message said:

My mate has the garmin topo s/ware and it lacks detail for me -
particulalry with respect to footpaths. Jon

I find it strange that you should say that. It obviously gives you more
detail than you'd have without it, yet you complain that it's not detailed
enough!

A lot of people have complained about the lack of footpaths, but what about
the superbly detailed contours and streams? I think they are far more
important IMO. But then, I'm not a great fan of following footpaths anyway.
Besides, if you want to follow footpaths you can always trace them and
upload them as saved tracks. Or create detailed routes.

If you don't have contours, why not download the free ones? They're not as
detailed and accurate as the Garmin ones, but they're infinitely better than
nothing at all. I find them to be extremely useful and are often all I need
to help pick my way across trackless terrain.

Paul
--
http://www.wilderness-wales.co.uk

GSV Three Minds in a Can said:

I pretty sure the OS is wrong, since I've done some sections on
different days in different years even, and got answers +/-5m from my
GPS, whereas the OS 1:25k map has the road 50m off to one side.

Yep, same here. Did you look at the screenshot of the forest track I gave a
link to a short while ago? I think that proves the point pretty well.

Paul
--
http://www.wilderness-wales.co.uk

"Paul Saunders" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Quoted message said:

My mate has the garmin topo s/ware and it lacks detail for me -
particulalry with respect to footpaths. Jon

I find it strange that you should say that. It obviously gives you more
detail than you'd have without it, yet you complain that it's not detailed
enough!

Paul
--
http://www.wilderness-wales.co.uk

I meant relative to OS mapping that has many more footpaths off the beaten
track that I like to explore. Ideally you could use both but if budget means
its one or the other I would go with road mapping plus UK OS which was my
choice. My mate has garmin topo and no OS which I do believe is an
incomplete solution.

Jon

"JRS" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

"Paul Saunders" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Quoted message said:

My mate has the garmin topo s/ware and it lacks detail for me -
particulalry with respect to footpaths. Jon

I find it strange that you should say that. It obviously gives you more
detail than you'd have without it, yet you complain that it's not
detailed enough!

Paul
--
http://www.wilderness-wales.co.uk

I meant relative to OS mapping that has many more footpaths off the beaten
track that I like to explore. Ideally you could use both but if budget
means its one or the other I would go with road mapping plus UK OS which
was my choice. My mate has garmin topo and no OS which I do believe is an
incomplete solution.

Jon

off down the opticians now. just looked quickly at post below this and
thought it said

"Fettish walks of 3- 5 miles in Lakes?"

!

Quoted message said:

JRS said:

Quoted message said:

Quoted message said:

My mate has the garmin topo s/ware and it lacks detail for me -
particulalry with respect to footpaths. Jon

I find it strange that you should say that. It obviously gives you
more detail than you'd have without it, yet you complain that it's
not detailed enough!

Quoted message said:

I meant relative to OS mapping that has many more footpaths off the
beaten track that I like to explore.

True, but I was thinking of it relative to a GPS road map with big blank
areas in between. Given that I practically never look at a map whilst out
walking anymore, any detail shown on my GPS is more useful to me than all
the extra detail on the OS map which is packed away in my rucksack.
Obviously I'll take the map out if I have to, but it's rare that I really
have to do that.

Quoted message said:

Ideally you could use both but
if budget means its one or the other I would go with road mapping
plus UK OS which was my choice.

You mean on a palmtop? I think it would be way too fiddly to faff around
with two GPS's. For me anyway, since I'm already faffing around with cameras
and walking poles in addition to the one GPS.

Quoted message said:

My mate has garmin topo and no OS
which I do believe is an incomplete solution.

Well it's incomplete if you don't carry a map as well, but I guess it all
depends how you like to navigate. If your idea of planning a walk is
thinking "let's go to Hill 123", jumping straight in the car and driving
there, getting out of the car, looking at your GPS and thinking "now then,
which way do we go?", then obviously a lack of paths in your GPS might be a
problem.

I don't plan that way. I study the maps in detail beforehand, plan possible
routes, mark waypoints, maybe even trace the odd path or cliff edge if I
think it's necessary, then upload to the GPS. When I start my walk I already
know exactly where I'm going and which way to go to get there, and my GPS
contains all the information I need to do that. The map in my rucksack is
basically redundant, it's just like my first aid kit, for emergency use
only. It's something that I don't want to look at.

In fact, if the terrain is unfamiliar to me, I'll probably fly around it in
my flight sim first, to get a really good idea of the shape of the
topography and to assess the likely ground conditions - the photoreal
terrain gives a pretty good idea of whether the ground is boggy or covered
in bracken or heather or whatever.

Actually, which version of UK Topo has your mate got? I've just had a look
and noticed they're selling version 2 now, so I'm wondering if the paths
have improved at all?

I've just had a look at it on the MapSource Map Viewer and it seems to have
quite reasonable path coverage for a few areas I've looked it. Not complete
of course, and there are some very prominent paths that are missing, but
it's significantly better than having no paths at all. Check it out if you
haven't seen the online map viewer before.
http://www8.garmin.com/cartography/ontheTrail/

Paul
--
http://www.wilderness-wales.co.uk

Bitstring <[email hidden]>, from the wonderful
person Paul Saunders <[email hidden]> said

Quoted message said:

GSV Three Minds in a Can said:

I pretty sure the OS is wrong, since I've done some sections on
different days in different years even, and got answers +/-5m from my
GPS, whereas the OS 1:25k map has the road 50m off to one side.

Yep, same here. Did you look at the screenshot of the forest track I gave a
link to a short while ago? I think that proves the point pretty well.

Nope, care to repeat the link? (I may have lost the thread before I got
that far).

--
GSV Three Minds in a Can
8,630 Km walked. 1,677Km PROWs surveyed. 30.5% complete.

"Paul Saunders" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

JRS said:

Ideally you could use both but
if budget means its one or the other I would go with road mapping
plus UK OS which was my choice.

You mean on a palmtop? I think it would be way too fiddly to faff around
with two GPS's. For me anyway, since I'm already faffing around with
cameras and walking poles in addition to the one GPS.

Quoted message said:

Quoted message said:

Paul
--
http://www.wilderness-wales.co.uk

No, I meant OS digital mapping on the PC and garmin topo mapping on the
60csx. He has v2 and I've used it and the viewer and it is still badly
deficient in the areas I walk. I always plan my walks using anquet on the PC
and d/load onto the GPS. The added detail of topo on the GPS screen over and
above my road mapping would be nice but not at the price they are currently
asking and the level of detail provided.

Jon

GSV Three Minds in a Can said:

Quoted message said:

Yep, same here. Did you look at the screenshot of the forest track I
gave a link to a short while ago? I think that proves the point
pretty well.

Nope, care to repeat the link? (I may have lost the thread before I
got that far).

Maybe the post didn't get through. Sometimes that happens, I don't know why.
I'll repeat the whole post below.

Okay, I've taken a few screenshots to demonstrate the difference in accuracy
between the 50k and 25k maps.

Exhibit 1: Cairns

I marked a group of 4 cairns on the 50k map, then switched to the 25k map
and took a screenshot at 200% zoom. They're not only in different positions
but also in different directions. Some are even shown in different fields.
The distances between the 50k waypoints and the positions on the 25k map are
38m, 24m, 20m and 30m respectively, an average error of 28m (assuming that
the 25k map positions are accurate).
http://www.wilderness-wales.co.uk/misc/cairns.gif

Exhibit 2: Fan Fawr

I've marked the summit spot height and the trig on both map scales, the red
waypoints represent the 50k map, the orange waypoints the 25k map. It's also
at 200% zoom. The difference in the trig position is quite small, only 7.5m
(although I've seen much bigger differences in some cases). The spot height
at the summit is a different matter though, there's a whopping difference of
49.5m! Of course, you must bear in mind that I'm only comparing spot heights
on maps here, I'm not comparing the waypoints to the actual summit position.
But I'd sooner trust 25k spot height than 50k ones anyday.
http://www.wilderness-wales.co.uk/misc/fan-fawr.gif

Exhibit 3: Craig Cerrig Gleisiad

So what about the actual summit position? Well on this map (300% zoom) I've
marked three waypoints, the yellow one is a GPS waypoint I marked at the
summit cairn back in March, with excellent reception and EGNOS active,
reporting an EPE of just 1-2m. Of course, there are co-ordinate conversion
issues to consider as well, but that rarely amounts to more than 5m.
Comparing the 25k waypoint to the GPS waypoint, there's a difference of
7.8m, whereas the 50k waypoint is a massive 96.3m away! One gets the
impression that the 50k mappers just marked an arbitrary spot in the middle
of the highest contour ring. This sort of thing is quite common where the
highest point is right next to the edge of a cliff. The 25k map usually
marks the summit in the correct spot, but the 50k map usually marks it some
distance away from the edge of the cliff.
http://www.wilderness-wales.co.uk/misc/ccg.gif

Exhibit 4: A recorded track through a forest path

This is a different comparison, between a 25k map and an aerial photo. As
you can see, the GPS track doesn't follow the map track very well at all. At
first you might assume bad reception due to the trees, and that's probably a
factor, but the track still doesn't follow the map even when it exits the
forest on the west side, where the reception was probably quite good. I'd
first noticed this particular error many years ago when using a 50k map, but
although bad reception seemed the most likely cause, repeated walks produced
consistent tracks, which were consistently inconsistent with the map!

Of course, that was 50k mapping so I wasn't unduly concerned, but more
recently it bothered me that the 25k map didn't match either (although it's
actually a much better fit than the 50k map).

But when you superimpose the track onto the aerial photograph, it suddenly
becomes surprisingly accurate. Granted it's still not perfect, but it's not
far off, some bits are dead on. It's worth noting also that I wasn't walking
in the centre of the track, I was usually walking close to one side or
another, hence the "racing line" that I took around many of the corners. In
particular, notice that the track is dead accurate outside of the forest.
Clearly the mappers were very sloppy in drawing this forest path, which
proves the point that even 25k maps are not always very accurate (although
spot heights, trigs and cairns usually are).
http://www.wilderness-wales.co.uk/misc/forest-path.jpg

So, do you still want 10 figure grid refs with 50k mapping? Clearly
25k mapping is far superior, but even that isn't perfect. IMO the most
accurate waypoints are those that have been marked in the field, with good
reception conditions of course. Such waypoints also eliminate coordinate
system conversion errors. So even allowing for typical reception errors, a
field waypoint will usually be more accurate than one marked on a map.

By the way, when you do mark field waypoints, it's best to hold the GPS
above your head so that your body doesn't block the signals, and let it
settle for a short while first (to avoid "momentum" errors). Averaging the
waypoint helps too, even if only for a short time (even a 30 second average
should eliminate any spurious position blips).

Paul
--
http://www.wilderness-wales.co.uk

On Tue, 15 May 2007 16:22:25 +0100, "Paul Saunders"
<[email hidden]> wrote:

|!GSV Three Minds in a Can wrote:
|!
|!>> Yep, same here. Did you look at the screenshot of the forest track I
|!>> gave a link to a short while ago? I think that proves the point
|!>> pretty well.
|!>
|!> Nope, care to repeat the link? (I may have lost the thread before I
|!> got that far).
|!
|!Maybe the post didn't get through. Sometimes that happens, I don't know why.
|!I'll repeat the whole post below.
|!
|!
|!
|!Okay, I've taken a few screenshots to demonstrate the difference in accuracy
|!between the 50k and 25k maps.

Ok I'll believe you.

So my requirement, to find my rocks in a featureless moor, should now read
"able to enter a 10 figure (1 m) OSGB grid reference on the fly and use the
correct algorithm to transform it into something else.

Just got the software from:
http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/furtherinfo/questsoftware.html
but have not tried it out
--
Dave Fawthrop <sf hyphenologist.co.uk> 165 *Free* SF ebooks.
165 Sci Fi books on CDROM, from Project Gutenberg
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Bitstring <[email hidden]>, from the wonderful
person Paul Saunders <[email hidden]> said

Quoted message said:

GSV Three Minds in a Can said:

Quoted message said:

Yep, same here. Did you look at the screenshot of the forest track I
gave a link to a short while ago? I think that proves the point
pretty well.

Nope, care to repeat the link? (I may have lost the thread before I
got that far).

Maybe the post didn't get through. Sometimes that happens, I don't know why.
I'll repeat the whole post below.

<big snip>

Nope, hadn't seen any of that before. Yes, the errors in the forest
track are exactly the sort of thing I sometimes find with unclassified
roads out in the sticks. You have to wonder if they left the drafting of
those to some very junior oik in the OS, by way of an apprenticeship (as
I said, it was probably done 50 or more years ago) and whether they did
it from aerial photographs, or some other ground based method.

--
GSV Three Minds in a Can
8,630 Km walked. 1,677Km PROWs surveyed. 30.5% complete.