Decca to Lat Long conversions?

MINESAPINT2

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Is it possible to convert Decca co ordinates Chain 3B to Lat Long WGS84 and vice versa? Perhaps something available online?

Thanks

Mike
 

AntarcticPilot

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I'd guess not, in general. The reason is that Decca positions are subject to systematic offsets induced by coastlines and other terrestrial features. This can be accounted for in a DECCA map, as the offsets are systematic and constant. But they can't be accounted for algorithmically; you basically would need to digitize the entire map which is perfectly possible but a) probably a breach of copyright and b) time-consuming and of limited utility. I did a quick online search and one of the top results was a guy offering money for someone to write such a routine, with a BIG statement that the developer would have to source a means of doing it!

However, I have no direct experience of DECCA; I did use comparable systems back in the 1980s, though, and their accuracy over land was, to put it mildly, abysmal.
 

Davy_S

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Is it possible to convert Decca co ordinates Chain 3B to Lat Long WGS84 and vice versa? Perhaps something available online?

Thanks

Mike
It cannot be done with any accuracy! I used to have a shipmate Decca and a 2000D, we used the kingfisher charts for wreck fishing, the problem is, Decca wanders at different times of the day, you could be bang on a wreck at 12 noon, but 300yards away at 4pm, we tested this using GPS with differential, decca could easily be 400 yds out of position at different times of the day, forget it, it will drive you daft!
 

Wing Mark

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Is it possible to convert Decca co ordinates Chain 3B to Lat Long WGS84 and vice versa? Perhaps something available online?

Thanks

Mike
Decca receivers used to mange it with 'reasonable' accuracy using 1970s logic ICs, so it cannot have been that hard by modern standards.
The 'accuracy' was always less than the repeatability though.
AIUI, even the Decca charts tended to have significant levels of error in terms of the Decca 'lanes' actually being drawn in the exact right place.

So a fisherman looking for his lobster pot would find it by Decca because he laid it by Decca, but its Lat and Long were always wrong, but pretty constantly so.

Unfortunately, I can't remember the actual magnitude of the errors.
Could have been something like repeatability a few tens of metres, actual error in lat and long several times that?

As you changed from one 'chain' to another, you'd see big jumps in indicated position. ISTR Cowes was often a changeover point ?
I sailed a bit on a boat with a 'Navstar' Decca receiver.
Obviously a professional or military set would allow you to control which chain you were using and was probably more accurate.
 

srm

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Is it possible to convert Decca co ordinates Chain 3B to Lat Long WGS84 and vice versa? Perhaps something available online?

Thanks

Mike
Yes, but you will have to do it manually. Late Decca receivers for yachts used to calculate Lat/long but that was part of the display processing software.

You will need to find a chart of the area with the Decca lattice overlay, now long gone out of print but someone may still have one. Manually plot the positions that you require co-ordinates for to get lat long to decca or decca to lat long.
The lat long on the chart may be on a different datum, in which case you will have to convert to/from WGS 84. However, WGS84 was being used for new editions of admiralty charts while Decca was still in use around Europe so you may be lucky if you can find a late decca lattice chart of your area.

All the caveats given above regarding Decca errors are correct. There is no point in going in to the catalogue of possible errors now, but there were a lot of them, some fairly constant and others very variable. Near to the coast errors could be significant for navigation.

In the early 70's I was working as a surveyor for a government research organisation studying sediment and sea bed movements in the Bristol channel. We had to use Decca for positioning, so before every survey run I had the ship go to the end of a pier and set the decca plotter for that known position as following the charted lattice would in all probability have put us aground.
 

Juan Twothree

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It cannot be done with any accuracy! I used to have a shipmate Decca and a 2000D, we used the kingfisher charts for wreck fishing, the problem is, Decca wanders at different times of the day, you could be bang on a wreck at 12 noon, but 300yards away at 4pm, we tested this using GPS with differential, decca could easily be 400 yds out of position at different times of the day, forget it, it will drive you daft!
I seem to remember it was particularly inaccurate around sunrise and sunset, due to diffraction (or refraction?) of the radio signals.
 

Slowboat35

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"possible to convert"?
By algorithm? I doubt it.
But with a Decca overprinted chart dead easy of course, (insofar as plotting a decometer-derived position is dead easy).
Simply plot the decca position and read the lat and long off the chart as usual.

It only requires a chart, but something tells me this is not what the OP was after.

People seem all too eager to knock Decca for accuracy but a whole generations of pilots used it on the N Sea for navigation and instrument approaches, one of my bases had a Decca approach with, unless I am badly mistaken, a MDH (minimum descent height) of just 350ft, exceedingly low for a non-precision approach. That could not be possible without considerable confidence in its accuray and reliability.
My recollection (in its closing years)of standard Decca with its spiders and cheeses was of a ridiculously user-unfreindly system requiring considerable skill and practice to use and troubleshoot when it jumped lanes - or even worse when you changed chains - but one that was remarkably accurate and reliable otherwise. Accuracy varied according to where you were on the network, somewhere between a few tens of yards to maybe a third of a mile at the extremities. It suffered from hugely excessive opportunities for misreading and vast ability to distract from one's primary task which is not a good idea in the air. The moving map display, (DANAC) though archaic and somewhat bizarre by modern standards, however, was quite good as a first generation graphic navaid.
 
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Davy_S

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I seem to remember it was particularly inaccurate around sunrise and sunset, due to diffraction (or refraction?) of the radio signals.
Yes, it reminded me of the days of listening to radio Caroline, the signal would fade in and out, in the evening, but as Wing Mark said, the repeatability of decca could be quite good, but only at the same time, ie, if you drop gill nets at noon, you will find them again at noon, but not at 9pm, local fishermen would calibrate the decca from a known buoy before setting out.
 

srm

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People seem all too eager to knock Decca for accuracy but a whole generations of pilots used it on the N Sea for navigation and instrument approaches, one of my bases had a Decca approach with, unless I am badly mistaken, a MDH (minimum descent height) of just 350ft, exceedingly low for a non-precision approach. That could not be possible without considerable confidence in its accuray and reliability.
Now that is interesting. It would have been useful to know when I was teaching electronic nav aids to MN officers. However, for mariners working at sea level the Decca errors were mainly due to differences in the speed of propagation over land and sea. I suspect that at the heights aircraft were flying these errors were negligible.

An interesting aside is that the first use of what became the decca system was during the D Day landings. Routes through the minefields were swept using the system for navigation and lead boats guided the armada through the otherwise unmarked safe channels.
 

AngusMcDoon

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Is it possible to convert Decca co ordinates Chain 3B to Lat Long WGS84 and vice versa? Perhaps something available online?
I doubt there are many, if any, people left who know the details of the Decca system sufficiently well to know how to do it. My dad was chief surveyor at Decca & this was his stuff, but he's travelling the Decca lanes in the sky now having reached an age of 94. There were no former colleagues at his funeral; he outlived them all.
 

AntarcticPilot

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I doubt there are many, if any, people left who know the details of the Decca system sufficiently well to know how to do it. My dad was chief surveyor at Decca & this was his stuff, but he's travelling the Decca lanes in the sky now having reached an age of 94. There were no former colleagues at his funeral; he outlived them all.
The principles aren't difficult; if you know the location of the transmitters you can work out the theoretical positions without over much difficulty; it's a conventional hyperbolic navigation system. Decca "lanes" are simply lines of equal difference of radio path length from two transmitters. The problem arises because there are a lot of secondary effects that vary with position and time, and which can't be dealt with by an algorithm; someone has to go and measure them.

Way back in the 60s I did use Consol a couple of times. No automation - you had to count dots and dashes! Decca was a substantial advance on that - but you had to have a special receiver. Consol worked on a band that a transistor radio could be tuned to. And I'm trying desperately to remember the global system we tried to use in 1986 in Svalbard. Not Transit; it wasn't satellite based. Precision was good - about 100m - but accuracy was in the kilometre range!
 

jdc

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How accurately do you want the result of the conversion? As Antarctic Pilot says, If the Decca overlay you need is just the theoretical hyperbolae of constant carrier phase, and thus time, differences without topological effects being taken into consideration then an algorithmic approach is very straight-forward, especially if one assumes a spherical earth - which isn't that unreasonable over an area of only 100 miles or so across.

PS: Omega?
 

Bodach na mara

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And I'm trying desperately to remember the global system we tried to use in 1986 in Svalbard. Not Transit; it wasn't satellite based. Precision was good - about 100m - but accuracy was in the kilometre range!
Loran?

And why (OP) does anyone want to convert Decca to lat long? There are no Decca signals!
 

Hoolie

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By algorithm? I doubt it.
We used Decca to lat/long conversions on our airborne nav computers in the 1970's and they were probably significantly more accurate than the charts. However, the caveats on reduced repeatability at dawn and dusk of course still applied! However, on one system we had an algorithm that predicted the real-time accuracy of the conversion (in metres), taking into account geometry and daily and seasonal fluctuations.
 

MINESAPINT2

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Thanks for replies,

I used to use a navigator in the late 70's and 80's, I seem to remember it was called a Vigil RX. It used Decca but displayed co ordinates in Lat Long. I used to use chain 2A (still have a 2A chart) and found lots of wrecks with readings I used to beg, borrow and steal (and sometimes buy) from other fishermen. Had I not thrown the Vigil away it definitely would have done these conversions for me, I used to do them all the time.

Next I used GPS as soon as I could afford a set which was no where near as accurate as Decca due to "selective availability" until I purchased differential corrections (DGPS) for £1,500 PA. You young uns don't know you are born.

It was always an enormous thrill when I steamed sometimes over 100 miles offshore with reading in hand, arriving after 15 hours steaming, searching round for half an hour then BINGO! the wreck appeared on the sounder. As others have said generally for the rest of the day I could run back to the wreck to fish it with reasonable accuracy, obviously nothing like GPS today.

Although I now have a sailing boat I still do a bit of fishing, catching enough Cod to keep us fed and have recently spoken to a retired fisherman who has some readings where he used to catch lots of Cod, locally such marks are called (Cod Bangers). He has kept one particular reading secret for many years and is quite sure no one else has ever found it. It is a peak of rock 30 odd miles off. There are 2 problems, one is his reading is in Decca chain 3B and two, he has yet to agree to give me the reading but I am working on him.

I think that addresses most of the points above.

Mike
 

DownWest

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No; it was something else. It'll come back to me. Its advantage was global coverage; I don't think it was very widely used.
Consol? We tried in in the Channel in '63. Counting dots on long wave to get a bearing from the station in Norway (?) Not very precise..
 

srm

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OMEGA, which became operational in 1971, was the first global radio navigation system. It enabled aircraft to navigate using very low-frequency radio signals around the world. There were 8 OMEGA transmitters placed around the globe. The OMEGA system was shut down in 1997 due to the widespread use of GPS.
It was also used at sea, but it was neccessary to only all use satations and recieverthatwere in daylight or in night effect. Having some in daylight and some in night caused significant errors. It was basically an oceanic nav system of low accuracy. Because of this it was regaeded as difficult to use effectively at sea.

LORAN did not have global coverage, but did cover much of the North Atlantic and into the northern North Sea/Norwegian Sea during th cold war.
 
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