- Yeah! Believing anything you read in the Guardian is tantamount to believing the earth is flat. Add the fact that Landsat 7 images are about as reliable as a child's scribbling pad for navigation purposes, and this whole excercise becomes derisory.

Agreed, the positions quoted are relative and therefore would show differences on even an inaccurate map. I would find it difficult to understand, if there was more than one handheld GPS on the British boats, why someone did not hit the "mark" key or the "man overboard" keys at the time of the attack, thus putting their position beyond all doubt. Maybe the Navy should have a special course aimed at teaching it's recruits how to use a GPS. It would sure relieve the boredom of playing Monopoly.

Sounds like Garmin could exploit this one to their advantage, whichever way you look at it!

Dennis.

----- Original Message -----

From: David McKinnon

To: OziUsers-L@yahoogroups.com

Sent: Saturday, March 31, 2007 10:57 PM

Subject: RE: [OziUsers-L] Re: Shatt-al-GPS

Oh yeah, don't trust what you read in the newspapers, no matter how well

regarded they are.

Just a typo, but has a significant effect on matters

_____

From: OziUsers-L@yahoogroups.com [mailto:OziUsers-L@yahoogroups.com] On Behalf

Of egdlym

Sent: Sunday, 1 April 2007 6:11 AM

To: OziUsers-L@yahoogroups.com

Subject: [OziUsers-L] Re: Shatt-al-GPS

Thanks to J.G. and Davo on the datum issue, which is pretty important

stuff for ozie users.

First, the UK newspaper "The Guardian" which I quoted from in

starting this thread, has re-published the coords using the same

numbers but now changing it to deg/min/sec whereas their earlier

effort was deg/min.mm, thus 29 50.36' N 048 43.08'(they did use the

minute apostrophe)is now replaced by 29 50' 36"N 048 43' 08" E ,

Adding this to the Landsat/MrSid image I was using, after editing

config/maps/lat-lon, gives a third position !! It is 451 metres North

of the ddd mm.mm version. No wonder bombs go astray.

Second, The Guardian showed a satpic at low tide and the navigable

channel is clearly seen with a meander which is not followed by their

overprint of the Iraq/Iran offshore boundary. Therefore any ship

would have to curve into Iraqi waters on its way up the channel if

this is correct. If on the other hand the boundary goes mid-channel

round the bend, the ship's position would be much closer to the line.

If anyone says they want it, I could post some images on the file

section of this usegroup.

regrda,

Richard

--- In OziUsers-L@yahoogro <mailto:OziUsers-L%40yahoogroups.com> ups.com,

"rwcx183" <lgalvin@...> wrote:

>

> --- In OziUsers-L@yahoogro <mailto:OziUsers-L%40yahoogroups.com> ups.com,

"egdlym" <rpgosnell@> wrote:

> >

> > Quite right Davo, we all use greenwich/equator for the origin.

> > However, I understand that choosing different mathematical models

> > (called spheroids)for the Earth's MSL shape causes lat/long, and

> > grid, lines to appear to be 200 or more metres shifted among

> > different models/sheroids. Obviously, using the true, exact

shape

> of

> > the Earth would remove all ambiguity, but I gather that is too

much

> > for a whole-earth model. WGS84 is therefore a simple whole-earth

> > compromise that has significant errors compared to the 'Truth'

> > locally. Most countries have a local spheroid (UK uses the Airy

> > Spheroid) which gives a much better match than WGS 84, but only

> > within its assigned national coverage. If the Iranians' nautical

> or

> > topo maps were using a local accurate datum there would be a

> similar

> > discrepancy compared to WGS 84

> >

> > The Degree Covergence Project had to decide, for expediency, to

use

> > WGS 84 to keep everone sane. But going to say 52 00' 00"N, 002

00'

> > 00"W using OSGB 36 would put you 200 metres away from DCP

> position,

> > evn though the OSGB is in fact more accurate.

> >

> > Richard

>

> Richard,

>

> Actually, once the location of the center of the earth is

> established, the ellipsoid can be changed at will, with no effect

on

> lat/lon coordinates. Lat/lon are just angles. The model of the

> earth's shape cannot change these angles. Different ellipsoids

were

> indeed used with different datum definitions, but mostly this was

> done to make altitudes accurate for a given local area. It's also

> absolutely necessary when using non lat/lon grid coordinates like

> UTM. To transform lat/lon coordinates from one datum to another,

> mostly requires a simple x/y/z offset correction for the origin

> (center of the earth) and it's not necessary to consider a

different

> ellipsoid. This is the so-called 3 parameter transform used by

> OziExplorer. Ozi of course does take into account differing

> ellipsoids for the purpose of displaying positions in non lat/lon

> grid coordinates where ellipsoid really does matter.

>

> To anyone that thinks they understand all this, it will come as no

> shock that the ground offset between 2 datums, is different for

> lat/lon coordinates than it is for UTM or other grid coordinates.

>

> J.G.

>

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[Non-text portions of this message have been removed] - --- In OziUsers-L@yahoogroups.com, "Matti Gronroos"

<matti.gronroos@...> wrote:>

are

> --- In OziUsers-L@yahoogroups.com, "rwcx183" <lgalvin@> wrote:

> >

> > I think that you're missing the point. Latitude and longitude

> > angles. An angle is formed by 2 intersecting lines. A line is

measurement

> > defined by 2 points. It does not matter if the earth was square

> > shaped. The angles remain the same. You cannot make a

> > of an angle of a line without an intersection to another line.

This

> > intersection must be at the implied center of the earth. Simple

angle

> > basic geometry.

>

> No.

>

> The latitude of a point P on the Earth is defined as a geodetic

> latitude, which is the angle between the equatorial plane and the

> normal to the reference ellipsoid surface on that position. That

> is NOT the same as the angle between the equatorial plane and the

line

> drawn from the center of the mass to the point P. The difference

degrees,

> depends on the latitude and it varies from zero to about 0.2

> i.e. to about 20 kilometres.

Yes, you're right. I don't know what I was thinking. I did know

>

> Unfortunately, is not just simple basic geometry, because are not

> talking about spherical coordinates. Thus, forget the geocentric

> angles in this context.

>

this at one time, but seem to have lost this knowledge over the years.

As a practical reality, for a given point on the earth, I can define

2 lat/lon coordinate systems, using different ellipsoids, but both

with the same origin. Both ellipsoids provide a reasonable fit to

the shape of the earth in a given local area. Converting lat/lon

coordinates from one to the other, results in zero change in

longitude coordinates and an extremely tiny change in latitude

coordinates. I'll grant you that the extremely tiny change may be

very important for land surveying at the millimeter level, but it's

less than negligible for navigation purposes. The point I'm trying

to make here, is that a far larger source of error between datums, is

the location of the origin and not the difference in ellipsoids for 2

datums.

> > They would do just as well if the shape was a dodecahedron.

Lat/lon

> > angles have nothing to do with the an arbirtrary model of the

earth's

> > shape.

metrics

>

> Yes, they do. If you need to find a position of a point P in a 3D

> coordinate system relative to a 3D shape, you need to know the

> of that shape.

and

>

> In the GPS positioning (and other positioning), that 3D shape is the

> reference ellipsoid, which is a approximation of the irregular shape

> of the Earth. The position lat/lon given by a GPS device consist of

> the following coordinates:

>

> 1) geodetic latitude (which is not equal to the geocentric angle)

> 2) geodetic longitude (which is equal to the geocenteric angle)

> 3) vertical distance from the sufrace of the reference ellipsoid

>

> To calculate 1) and 3), you need to know the position, orientation

> the size of the ellipsoid.

Not to nitpick here, but the prime result of a standalone GPS

>

position calculation, is a position in 3D ECEF XYZ coordinates. Any

translation to other coordinate systems, is not really anything to do

with the GPS system per se, but more to do with putting the

coordinates into a more human friendly form.

> > The position of these ellipsoids vary because they could not

much

> > accurately determine the mass center of the earth. They very

> > did want to do that.

the

>

> This definitely is not true. They wanted to find an ellipsoid best

> matching to the area they were mapping.

>

> > Tectonic plate motion can happen with very negligible change to

> > shape of the earth. 2 different things.

implementing

>

> No. The movement currently is about 2.5 centimeters a year. That is

> why WGS84 is complemented by terrestial reference systems

> a non-moving coordinate system.

I'm not following your thinking here. The plates can move around and

that may change the x/y shape of the land mass, but the shape of the

earth's surface doesn't change much because of it. True there are

very minor changes in the geoid, but these are much smaller than the

simple x/y ground position changes.

J.G.