gps positions...how to convert them?

[davem2005]

A very very nice man sent me a list of wrecks but some are in the format my gps uses (wgs84) i.e. 54, 56.123

But some are in a 6 digit format i.e. 54.55.56

Is there an easy way to convert them.

Cheers
Dave

[mark]

check before you worry about them. depends on the source, some chart datums use decimal notation so it might be the same co-ordinate just not as accurate ie 54.55.56 might actually be 54, 55.56

probably slightly older numbers with less accuracy (pre selective accuracy)from early gps systems

what I do with those is stick a 5 on the end - 54, 55.565 which is half a second either way then hunt about about. stick it in the plotter see if there's any wrecks or obstructions charted nearby

if they are definitely non wgs84, you need to know what format they are in - are they straight lat/lon, cartesian, utc, utm, derived loran, derived decca, nad 27, nad 83..... plus several others depending on where they came from

if they are straight lat/lon try http://www.jeeep.com/details/coord/

for utm:http://home.hiwaay.net/~taylorc/tool...hy/geoutm.html


most likely would be lat/long
just hunted the formula out its sort of like this:

a = 6378,137.0 metres (major semi-axis, ie equatorial radius)
Ellipsoid flattening ratio 1/f = 298.257 223 563 where f is flattening.

Flattening f = 0.00 335 281 0665)

Flattening distance (a × f) is 21.38468575 kilometres so Polar radius b is (a - a × f) = 6356,752.314 m (minor semi-axis)

At a given Lat and Long (Parallel / Meridian)


Meridian radius: M = a (1 - e ²) (1 - e ² × sin ²(lat))^(-3/2)
where a is the semi-major axis (equator radius) ; and,
e is eccentricity, where e ² = 2f - f ², so e = 0.08181919
Parallel radius: v = a (1 - e ² sin ²(lat))^(-½) Radius depends upon the azimuth of the vertical plane, except at the poles.

Radius at given latitude (Distance from Earth centre to Point concerned) = 1/(Cos lat × (root ((1/a ²) + (Tan lat ²/b ²))))


Geodesic

Geodesic distance = a × integral between Lat A and Lat B ((W/X + Y/Z)^½)

Where:

W = (1 - e ²) ²
X = (1 - e ² × sin ²lat)³
Y = (Long B - Long A) ² × (1 - e ² × sin ²Lat)
Z = ((1 - e ²) × (Tan Lat B - Tan Lat A - e ²(Lat B - Lat A)) × Cos ²Lat)
and e = eccentricity (0.08181919 for WGS 84)

[davem2005]

ROFLMAO.

Thanks mark...i'll stick that equation in me mobile and all will be sorted

Have to say I thought the suggestion of sticking a 5 in was probably easier.

I think you are right in that the first two sets of digits are right...the last two on the six didgit number are based on a factor of 60 as opposed 100.

So if im working this right a figure of 54.55.56 becomes 54,55.925...its a dam expensive job cruising around in a two stroke trying to check them all.

Dave

[codseeker]

check before you worry about them. depends on the source, some chart datums use decimal notation so it might be the same co-ordinate just not as accurate ie 54.55.56 might actually be 54, 55.56

probably slightly older numbers with less accuracy (pre selective accuracy)from early gps systems

what I do with those is stick a 5 on the end - 54, 55.565 which is half a second either way then hunt about about. stick it in the plotter see if there's any wrecks or obstructions charted nearby

if they are definitely non wgs84, you need to know what format they are in - are they straight lat/lon, cartesian, utc, utm, derived loran, derived decca, nad 27, nad 83..... plus several others depending on where they came from

if they are straight lat/lon try http://www.jeeep.com/details/coord/

for utm:http://home.hiwaay.net/~taylorc/tool...hy/geoutm.html


most likely would be lat/long
just hunted the formula out its sort of like this:

a = 6378,137.0 metres (major semi-axis, ie equatorial radius)
Ellipsoid flattening ratio 1/f = 298.257 223 563 where f is flattening.

Flattening f = 0.00 335 281 0665)

Flattening distance (a × f) is 21.38468575 kilometres so Polar radius b is (a - a × f) = 6356,752.314 m (minor semi-axis)

At a given Lat and Long (Parallel / Meridian)


Meridian radius: M = a (1 - e ²) (1 - e ² × sin ²(lat))^(-3/2)
where a is the semi-major axis (equator radius) ; and,
e is eccentricity, where e ² = 2f - f ², so e = 0.08181919
Parallel radius: v = a (1 - e ² sin ²(lat))^(-½) Radius depends upon the azimuth of the vertical plane, except at the poles.

Radius at given latitude (Distance from Earth centre to Point concerned) = 1/(Cos lat × (root ((1/a ²) + (Tan lat ²/b ²))))


Geodesic

Geodesic distance = a × integral between Lat A and Lat B ((W/X + Y/Z)^½)

Where:

W = (1 - e ²) ²
X = (1 - e ² × sin ²lat)³
Y = (Long B - Long A) ² × (1 - e ² × sin ²Lat)
Z = ((1 - e ²) × (Tan Lat B - Tan Lat A - e ²(Lat B - Lat A)) × Cos ²Lat)
and e = eccentricity (0.08181919 for WGS 84)

omg where am i .im lost at sea

[mark]

check em on a chart first, as the second 2 digits might also be 60 (minutes) not decimal, so could throw you even further out

I've generally found that numbers tend to be taken from a gps of some sorts which more often than not is decimal already just not displaying the last digit, measuring from a chart to get lat/long to that accuracy is pretty difficult, chart numbers tend to stop at 54.55 for example

studied for an yachtmaster ocean navigator in my yottie days, had to do all that with a slide rule, log tables and an almanac - no electronic instruments (calculators!) allowed. take reading with a sextant and then spend 20 mins working out where you are then another 5 mins correcting for how far and in what direction you've travelled since taking the reading

I've got a copy of the complete list of wrecks from UKHO and every now and then I'll send an hour converting a few and trying to chart them, this summers 'project' was then to go and confirm them, but hardly been out, managed to trace a couple of dozen wrecks off the tyne that aren't on any of the lists that circulate, be nice to find they are completely unfished and teeming with doublers

[Tadpoleman]

Yes Dave i just stick a zero on the end never thought of a 5 seems like a bit of logic to me ......................doh

[Tadpoleman]

"A very very nice man sent me a list of wrecks "
Do i know this person?

[davem2005]

Had a little mess about with the plotter this am.

Took the six digit number of one of the wrecks and entered it in (number 154).
xx.18.20 n
xxx.44.81 w

This showed a wreck to actually be about 260 yds SE of the point given at a location.

xx.18.146 n
xxx.44.686 w
This may be part of the same wreck.

Not bad but its a big ocean and they can be hard to find even when you know the exact location.

So I tried again (No 153 )

xx.21.66 n
xxx.52.03 w

Shows a wreck at

xx.21.621 n
xxx.52.063 w

Less than 100 yards away to the sw.

I cant see a mathmatical link for the change in figures but in both cases they are very close. I chose distant wrecks so there was less chance of one being close and in both cases they were very close and both slightly to the south.

Seems the numbers given are a good guideline but by no means bang on unless you happen to operate on the datum the numbers are in.

If anyone is strugling to find a particular wreck on that list let me know and I will see what I can do to help...but dont expect a fast reply as it takes ages to enter each one manually.

Cheers
dave

[mark]

could be a mixture of things. some of the marks could be from when SA was turned on so would be anything up to 100metres out, then there's the accuracy of one particular plotter versus another, some might be numbers from old 8channel non WAAS systems, yours will be a new 12 channel WAAS enabled system. 8channel non waas could add another 100 metres

just been sat on the boat for the last hour punchin numbers in. there's a lowrance mono plotter and the cobra colour one sat next to each other and just for good measure had me hand held there as well

so according to the 3 gps systems my front garden is either
N54 46.884 W002 05.728 (magellan hand held)
N54 46.879 W002 05.722 (lowrance)
N54 46.881 W002 05.732 (cobra)

and according to google earth is N54 46.54 W002 05.45
all there or there abouts

I've found the best thing to do is punch the numbers in, go tot hem, then sniff about till you find what you're looking for then edit the waypoint to your numbers

[braine]

Just to confuse matters somewhat, I and others in Seaham Harbour Boat club have found that most numbers show marks roughly 100 yards to the South East of the actual position in the north East. If you add initially 000.00.050 to the Northing and 000.00.085 to the Easting you should be very close to the actual position. In the north East, Latitude or Northing in WG84 (standard GPS format) 000.01.000 (one minute) equals one nautical mile or 6080 feet. Adding "5" on the very end equals @ 30 feet. (000.00.001 is approximately 6 feet). Satellite positions are accurate to about 3 metres. For Longtitude or Eastings (West) of Greenwich, the distances change. 000.01.000, one minute of longtitude equals about 0.575 nautical miles or around 3496 feet. So 000.00.001 is around 3.5 feet. Adding "5" to the last figure gives about 20 feet distance.
To put it all in perspective, If you have a standard 200KHz sounder with a 20 degree cone operating in 90 feet of water, the screen will show roughly 80 feet diameter of seabed. This equates to Northing 000.00.013 and 000.00.023 Easting. To search for a mark you can "Box Grid" with the numbers at the centre of the box and travel north /south at 000.00.040 spacing beween the tracks. If you travel +/- 000.00.200 East and West of the position and the same distance North / South, you will cover an area roughly 810 yards N/S and 466 yards E/W.
Old figures are usually 000.00, or 00.00.0 degrees, minutes and seconds. 000.00.1 northing is roughly 608 feet and 000.00.1 Easting is roughly 350 feet. With this rough accuracy you will easily miss the wreck. You need the second decimal 000.00.0-X-0 (the x) to be within +/- 2 Northing and +/- 4 Easting to find the mark quickly. I have a word format file made up with all this written down including an Excel spreadsheet of a box grid if anyone wants a copy. Just PM me.

[davem2005]

Thanks for a very educational reply Bill,

The first time I have seen the numbers expressed as feet and I did not realise how accurate they are.

Your comments regarding the size of the cone @ 200 hz set me thinking that switching to a 5 deg cone makes sense when you have actually locate your wreck and are drifting it, particularly in deeper water such as found at Whitby. It would give a better indication on a slow drift as to when you have left the wreck.

If I have understood what you say a 300 foot long wreck laying North/south could have a co-ordinate from 00.00.000 N up to 00.00.045 N (460ft) and the same wreck laying east west could be 00.00.000 w up to 00.00.090 w.

I feel better already and have some science to baffle the crew with when they ask why I cant find a particular mark

Cheers
Dave

[braine]

Hi Dave,
Yes, you got it in one. I fish a small well broken wreck of around 2,000 tons and the spread from the sounder shows the part of the wreck I fish covering from N 00.00.875 - 900 and E 00.00.385 - 405. The biggest part of the wreck, which may be cargo or the boilers / engine room / accommodation covers N 875 - 885 and E 397 - 405. showing around 3 - 4 metres high. Overall, the wreck part surveyed covers 150 feet by 76 feet. If the beam is 76 feet you would expect the ship to be around 3 - 4 times the beam so about 230 - 300 feet long. Depending on tide run and the position of the wreck, you could be across and past the wreck in less than a minute bearing in mind that a 1 knot tide run is equal to 6080 feet an hour or around 100 feet a minute. Slack water is when the fish rise above the wreck to reposition themselves facing into the tide to collect whatever the tide run brings them. They shelter behind anything that breaks/ eddies the run to drop the food. I know it is like teaching your grandmother to suck eggs (some new to the game may not realise this) but slack water occurs at different times as you progress out from the shore and north to south. With careful planning, you can cover up to 8 - 10 wrecks with 2 - 3 drifts each and all occurring at slack water, so giving you the best chance of good fish with the least tackle losses. Problem is when you are planning, you aint fishing! Them little diamonds on the charts dont look much but are the key to a good days fishing!