IGN maps and etrex

In article <[email hidden]>, Dean
<[email hidden]> writes

Quoted message said:

Is it possible to use basic etrex with 1:25,000 and 1:50,000 french IGN
maps?

The newer series (introduced in1999) maps have "GPS compatible" or
similar on the front and have a grid on them. The older series do not
have a grid.

The grid on the new maps is UTM on the WGS84 datum - which your eTrex
has.

Quoted message said:

I haven't seen any of these maps as yet and wondered what the grid
was. If the grid is not available from the etrex as a preset, is it
possible to use the custom grid option to get it to output coords that I can
plot on the map? Also is the appropriate datum available so the heights tie
up?

The datums in your GPS are only for horizontal position and do not
affect the heights reported so there is nothing to change.

--

Dominic Sexton

Dominic Sexton said:

In article <[email hidden]>, Dean
<[email hidden]> writes

Quoted message said:

Is it possible to use basic etrex with 1:25,000 and 1:50,000 french IGN
maps?

The newer series (introduced in1999) maps have "GPS compatible" or
similar on the front and have a grid on them. The older series do not
have a grid.

The grid on the new maps is UTM on the WGS84 datum - which your eTrex
has.

Many thanks. Just need to buy the maps now!

Quoted message said:

Quoted message said:

I haven't seen any of these maps as yet and wondered what the grid
was. If the grid is not available from the etrex as a preset, is it
possible to use the custom grid option to get it to output coords that I can
plot on the map? Also is the appropriate datum available so the heights tie
up?

The datums in your GPS are only for horizontal position and do not
affect the heights reported so there is nothing to change.

Are you sure?
With map datum set to 'Ord srvy GB', would I see altitude above OS Newlyn
datum or does the etrex always show it relative to a fixed datum regardless
of selection? Looking at the custom datum setup, it has a DZ which leads me
to believe datum selection does determine altitude reading.

Cheers,

Dean

Dean said:

Is it possible to use basic etrex with 1:25,000 and 1:50,000 french
IGN maps? I haven't seen any of these maps as yet and wondered what
the grid was. If the grid is not available from the etrex as a
preset, is it possible to use the custom grid option to get it to
output coords that I can plot on the map? Also is the appropriate
datum available so the heights tie up?

As I understand it French maps use a Lambert Conical projection, not a
Transverse Mercator projection like UTM, OSGB, Irish and a whole
shedload of other local grid systems. The GPS switches between these
by just changing a few constants for each grid system but the French
grid system is based on completely different maths and this usually
isn't supported.

Chris
--
Chris Eilbeck
MARS Flight Crew http://www.mars.org.uk/
UKRA #1108 Level 2 UYB
Tripoli UK Member #9527 LSMR

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

Quoted message said:

With map datum set to 'Ord srvy GB', would I see altitude above OS Newlyn
datum or does the etrex always show it relative to a fixed datum regardless
of selection? Looking at the custom datum setup, it has a DZ which leads me
to believe datum selection does determine altitude reading.

Nope, afaik (and I believe Garmin have said it) GPS handsets output
height above WGS84 Geoid, =only=. And a simplified WGS84 Geoid at that.
The calculations to get OSGB Newlyn (or any of the other OSGB datum
heights) would fill the machine up, since the OSGB use a dirty great big
lookup table.

--
GSV Three Minds in a Can
SC recommends the use of Firefox; Get smart, or get assimilated.

In article <[email hidden]>, Dean
<[email hidden]> writes

Quoted message said:

Dominic Sexton said:

In article <[email hidden]>, Dean
<[email hidden]> writes

Quoted message said:

Is it possible to use basic etrex with 1:25,000 and 1:50,000 french IGN
maps?

The newer series (introduced in1999) maps have "GPS compatible" or
similar on the front and have a grid on them. The older series do not
have a grid.

The grid on the new maps is UTM on the WGS84 datum - which your eTrex
has.

Many thanks. Just need to buy the maps now!

If you are getting them in the UK you could try Stanfords (in London or
Manchester) http://www.stanfords.co.uk/. If you French is good you can
also buy them online from the IGN http://www.ign.fr/

Quoted message said:

Quoted message said:

Quoted message said:

I haven't seen any of these maps as yet and wondered what the grid
was. If the grid is not available from the etrex as a preset, is it
possible to use the custom grid option to get it to output coords that I can
plot on the map? Also is the appropriate datum available so the heights tie
up?

The datums in your GPS are only for horizontal position and do not
affect the heights reported so there is nothing to change.

Are you sure?

Yep ;-)

Quoted message said:

With map datum set to 'Ord srvy GB', would I see altitude above OS Newlyn
datum or does the etrex always show it relative to a fixed datum regardless
of selection?

It shows it relative to the model of the Geoid that the receiver has
inside it. That is a pretty good approximation to mean sea level
everywhere. The difference between the modern receivers' models and the
Newlyn datum is well within the accuracy of a stand alone receiver. Some
Garmins from more than 5 years ago had some systematic errors which
caused them to over read height by about 8 metres. They were designed
and released when the US gov still degraded the accuracy with Selective
Availability which swamped the systematic error so it was not noticed.

If you hook a computer up to the GPS and set it to output NMEA data you
can see the height above the Geoid and the Geoid / ellipsoid separation
at that location which you can compare with the local vertical datum is
you wish.

More info
http://www.gps.gov.uk/guide2.asp
http://earth-info.nga.mil/GandG/tr8350.2-a/Chapter%206.pdf
http://earth-info.nga.mil/GandG/datums/vertdatum.html
http://www.colorado.edu/geography/gcraft/notes/datum/datum.html

Quoted message said:

Looking at the custom datum setup, it has a DZ which leads me
to believe datum selection does determine altitude reading.

The DX, DY and DZ figures are the differences in the three axes between
the location of the centre of the earth for the datum compared to that
for WGS84. Higher precision conversions can be done with an additional
three rotations (one about each axis) and a scale factor. That technique
is only necessary for high accuracy surveying transformations and often
requires a special survey just to determine the parameters.

--

Dominic Sexton

In article <[email hidden]>, Chris Eilbeck
<[email hidden]> writes

Quoted message said:

Dean said:

Is it possible to use basic etrex with 1:25,000 and 1:50,000 french
IGN maps? I haven't seen any of these maps as yet and wondered what
the grid was. If the grid is not available from the etrex as a
preset, is it possible to use the custom grid option to get it to
output coords that I can plot on the map? Also is the appropriate
datum available so the heights tie up?

As I understand it French maps use a Lambert Conical projection, not a
Transverse Mercator projection like UTM, OSGB, Irish and a whole
shedload of other local grid systems. The GPS switches between these
by just changing a few constants for each grid system but the French
grid system is based on completely different maths and this usually
isn't supported.

That was the case with the pre 1999 maps but they now have GPS
compatible maps with a 1km UTM grid on them based on WGS84.

--

Dominic Sexton

Dominic Sexton said:

In article <[email hidden]>, Chris Eilbeck
<[email hidden]> writes

Quoted message said:

Dean said:

Is it possible to use basic etrex with 1:25,000 and 1:50,000 french
IGN maps? I haven't seen any of these maps as yet and wondered what
the grid was. If the grid is not available from the etrex as a
preset, is it possible to use the custom grid option to get it to
output coords that I can plot on the map? Also is the appropriate
datum available so the heights tie up?

As I understand it French maps use a Lambert Conical projection, not a
Transverse Mercator projection like UTM, OSGB, Irish and a whole
shedload of other local grid systems. The GPS switches between these
by just changing a few constants for each grid system but the French
grid system is based on completely different maths and this usually
isn't supported.

That was the case with the pre 1999 maps but they now have GPS
compatible maps with a 1km UTM grid on them based on WGS84.

IDR (I dimly recall) some User Datum setup figures for the Lambert
system.

Indeed, here is a page in French http://latronche.free.fr/gps.html

or Google's machine translation into a semlance of English
http://translate.google.com/translate?hl=en&sl=fr&u=http://latronche.free.fr/gps.html

Whether it is worthwhile setting up User Grid and Datum these days
with the WGS compatible maps available is a moot point.

--
Phil Cook looking north over the park to the "Westminster Gasworks"

In article <[email hidden]>, Phil Cook
<[email hidden]> writes

Quoted message said:

Dominic Sexton said:

In article <[email hidden]>, Chris Eilbeck
<[email hidden]> writes

Quoted message said:

Dean <[email hidden]> writes:

> Is it possible to use basic etrex with 1:25,000 and 1:50,000 french
> IGN maps? I haven't seen any of these maps as yet and wondered what
> the grid was. If the grid is not available from the etrex as a
> preset, is it possible to use the custom grid option to get it to
> output coords that I can plot on the map? Also is the appropriate
> datum available so the heights tie up?

As I understand it French maps use a Lambert Conical projection, not a
Transverse Mercator projection like UTM, OSGB, Irish and a whole
shedload of other local grid systems. The GPS switches between these
by just changing a few constants for each grid system but the French
grid system is based on completely different maths and this usually
isn't supported.

That was the case with the pre 1999 maps but they now have GPS
compatible maps with a 1km UTM grid on them based on WGS84.

IDR (I dimly recall) some User Datum setup figures for the Lambert
system.

Indeed, here is a page in French http://latronche.free.fr/gps.html

or Google's machine translation into a semlance of English
http://translate.google.com/translate?hl=en&sl=fr&u=http://latronche.fre
e.fr/gps.html

Or an English version

http://gpsinformation.net/main/igngrids.htm

The French page also has 0 for DF and DA which suggests that it uses the
same ellipsoid as WGS84. This is a bit strange because France
historically used either a modified Clarke 1880 or the International
1924 ellipsoid.

Quoted message said:

Whether it is worthwhile setting up User Grid and Datum these days
with the WGS compatible maps available is a moot point.

The older maps had no grid on them just marks in the margins so there
was not much point.

--

Dominic Sexton

Quoted message said:

Quoted message said:

Is it possible to use basic etrex with 1:25,000 and 1:50,000 french IGN
maps?

The newer series (introduced in1999) maps have "GPS compatible" or
similar on the front and have a grid on them. The older series do not
have a grid.

The grid on the new maps is UTM on the WGS84 datum - which your eTrex has.

This is true for the IGN 25k series but I didn't think the 50k IGN maps
had a grid other than Lambert. Or is there a new edition ?

--
Boo

Quoted message said:

That was the case with the pre 1999 maps but they now have GPS
compatible maps with a 1km UTM grid on them based on WGS84.

Note to the OP : better check this is true for the 50k maps before you
buy any of those. It's definitely true for any IGN 25k maps you are
likely to find in stock these days, but probably worth specifying
"GPS-compatible" just in case.

hth,

--
Boo

Dominic Sexton said:

Quoted message said:

Whether it is worthwhile setting up User Grid and Datum these days
with the WGS compatible maps available is a moot point.

The older maps had no grid on them just marks in the margins so there
was not much point.

Indeed. To get any useful positioning information off them you had to
draw your own grid. The results must have been about as useful as
Adny's famous navigation by the direction of the grass on summits.

--
Phil Cook looking north over the park to the "Westminster Gasworks"

Quoted message said:

Is it possible to use basic etrex with 1:25,000 and 1:50,000 french IGN
maps? I haven't seen any of these maps as yet and wondered what the grid
was. If the grid is not available from the etrex as a preset, is it
possible to use the custom grid option to get it to output coords that I can
plot on the map? Also is the appropriate datum available so the heights tie
up?
Cheers
Dean

I've posted below as well, but out of interest where in France to you
expect to visit ? There are some pretty good maps of parts of France
issued by the ICC and these cover a lot of Spanish territory too which
can be useful in the Pyrenees.

Hth,

--
Boo

Dominic Sexton said:

In article <[email hidden]>, Dean
<[email hidden]> writes

Quoted message said:

Dominic Sexton said:

In article <[email hidden]>, Dean
<[email hidden]> writes
>Is it possible to use basic etrex with 1:25,000 and 1:50,000 french IGN
>maps?

The newer series (introduced in1999) maps have "GPS compatible" or
similar on the front and have a grid on them. The older series do not
have a grid.

The grid on the new maps is UTM on the WGS84 datum - which your eTrex
has.

Many thanks. Just need to buy the maps now!

If you are getting them in the UK you could try Stanfords (in London or
Manchester) http://www.stanfords.co.uk/. If you French is good you can
also buy them online from the IGN http://www.ign.fr/

Quoted message said:

Quoted message said:

> I haven't seen any of these maps as yet and wondered what the grid
>was. If the grid is not available from the etrex as a preset, is it
>possible to use the custom grid option to get it to output coords that I can
>plot on the map? Also is the appropriate datum available so the heights tie
>up?

The datums in your GPS are only for horizontal position and do not
affect the heights reported so there is nothing to change.

Are you sure?

Yep ;-)

Quoted message said:

With map datum set to 'Ord srvy GB', would I see altitude above OS Newlyn
datum or does the etrex always show it relative to a fixed datum regardless
of selection?

It shows it relative to the model of the Geoid that the receiver has
inside it. That is a pretty good approximation to mean sea level
everywhere. The difference between the modern receivers' models and the
Newlyn datum is well within the accuracy of a stand alone receiver. Some
Garmins from more than 5 years ago had some systematic errors which
caused them to over read height by about 8 metres. They were designed
and released when the US gov still degraded the accuracy with Selective
Availability which swamped the systematic error so it was not noticed.

If you hook a computer up to the GPS and set it to output NMEA data you
can see the height above the Geoid and the Geoid / ellipsoid separation
at that location which you can compare with the local vertical datum is
you wish.

More info
http://www.gps.gov.uk/guide2.asp
http://earth-info.nga.mil/GandG/tr8350.2-a/Chapter%206.pdf
http://earth-info.nga.mil/GandG/datums/vertdatum.html
http://www.colorado.edu/geography/gcraft/notes/datum/datum.html

Quoted message said:

Looking at the custom datum setup, it has a DZ which leads me
to believe datum selection does determine altitude reading.

The DX, DY and DZ figures are the differences in the three axes between
the location of the centre of the earth for the datum compared to that
for WGS84. Higher precision conversions can be done with an additional
three rotations (one about each axis) and a scale factor. That technique
is only necessary for high accuracy surveying transformations and often
requires a special survey just to determine the parameters.

Thanks for all the advice from yourself and others, very useful. (I studied
a bit of geodesy ages ago but its all a bit distant and doesn't seem to
relate well to my etrex!) I had assumed the etrex didn't model global
geoid/spheroid seperation and just gave height above the selected spheroid.
But I guess national mapping inevitably uses stuff that approximates well to
the geoid locally. So will altitude (say +- 10m) as well as the position
tie in ok with appropriate etrex settings and these IGN maps?

Cheers

Boo said:

Quoted message said:

Is it possible to use basic etrex with 1:25,000 and 1:50,000 french IGN
maps? I haven't seen any of these maps as yet and wondered what the grid
was. If the grid is not available from the etrex as a preset, is it
possible to use the custom grid option to get it to output coords that I can
plot on the map? Also is the appropriate datum available so the heights tie
up?
Cheers
Dean

I've posted below as well, but out of interest where in France to you
expect to visit ? There are some pretty good maps of parts of France
issued by the ICC and these cover a lot of Spanish territory too which
can be useful in the Pyrenees.

Hth,

I want maps of western Pyrenees from Hendaye to the national parks at 1:50k
and 1:25k. I'd ultimately like to walk coast to coast but thats a bit of a
dream ATM. Apart from wanting maps compatible for use with my gps, I'm also
finding its revived my interest in maps in general. Thanks for the help.

Dean

In a recent message
<[email hidden]>, Dean
<[email hidden]> wrote.

[...]

Quoted message said:

I want maps of western Pyrenees from Hendaye to the national parks at 1:50k
and 1:25k. I'd ultimately like to walk coast to coast but thats a bit of a
dream ATM. Apart from wanting maps compatible for use with my gps, I'm also
finding its revived my interest in maps in general. Thanks for the help.

[...]

Just to mention that you can buy IGN mapping on CDs at 1:25,000. Each CD
covers about half of a departement and costs about £25. See the
'CartoExploreur 3' range on the IGN web site. They work with GPSs, see
the site for details. Pound for pound you get about ten times more area
with an IGN map on CD than you get from the OS! But then, the IGN knows
what it's *for*.

Regards,
Nick.
--
Nick Hopton and Anne Hopton
Caversham, Reading, England
<[email hidden]>