Re: Accuracy of Coordinates
Posted by: Ken Dunn ()
Date: October 09, 2020 06:25PM

Hi Sergio,

What follows here is relative to WWII. I know nothing about WWI, though the merchant ship’s procedures must have been quite similar to those of WWII, assuming the merchant ships had radios.

The short answer is that the locations that went into the records were as accurate as the technology of the day and the fog of war permitted.

As Michael points out, the locations shown on are the “best available sources” for the location of a ship sunk by a U-boat. Unless the wreck is actually found and the latitude & longitude is reported you generally won’t find a better location. Even if the wreck is located, frequently the exact location isn’t reported just to keep divers off the wreck.

That said, how accurate are the “best available sources”? Well, unless the wreck has been found and the exact location reported they are almost always off due to the way they were acquired by the U-boat and / or the ship that was sunk.

There were a couple aids to navigation in those days in some cases. For example bearings taken on land features, bearings taken on known shore radio transmissions, and even the depth of the water where you were could sometimes be used. Some maps had water depth on them & if you thought you were here but the water wasn’t the depth it should be (based on your fathometer if you had one and all U-boats did) you could tell something was wrong. Of course these weren’t generally available in the middle of the ocean and aren’t taken into consideration in what follows here.

First as Mark points out, they were primarily determined by celestial navigation at some point. However, celestial navigation wasn’t as simple as looking at a GPS device and writing down the numbers. It was a rather complicated procedure that involved taking sightings with a sextant then based on the time of day where you were, the angles involved, at least one set of tables, and some math (and perhaps other things I am not aware of) you could come up with a latitude & longitude. However if it was a star sighting you had better be using the right star….

The correct procedure was to have at least two men take the sightings at the same time and compare notes before proceeding but frequently that wasn’t possible under wartime conditions. Getting this right on the rocking and rolling open bridge on a U-boat at sea wasn’t all that simple nor was it that simple on the bridge of a ship. The weather had something to do with it too. However, this was the best technology had to offer at the time.

Now to complicate matters, both the U-boat and the ship involved were moving at the time and they continued to move after the sightings were taken. It didn’t take long until the sightings were obsolete because the vessel wasn’t there anymore. What that left was dead reckoning navigation.

Dead reckoning navigation was based on the premise that: “from the most recent “navigational fix” (the sightings taken with a sextant) plotted on the map, we traveled on this course at this speed for this length of time so we should be right here on the map”.

Well right off there were generally problems with the premise. Currents and wave action and the prevailing winds during the time traveled probably made the premise somewhat invalid to one degree or another. Additionally, the longer dead reckoning was used, the more inaccurate it became. And again the weather could have a considerable impact on how long it was between navigational fixes. You couldn’t “shoot the stars” unless you could see them and completely overcast skies could keep you from seeing them. Raging storms, snow sleet, etc. made it impossible to get a reliable fix. This affected both U-boats and ships alike. However U-boats had to deal with being submerged as well, perhaps due to enemy action. They couldn’t surface to take a fix with a destroyer up there dropping depth charges on them etc. What’s more their course changes were both radical and frequent during the time they were being attacked and when they could finally surface they might find the weather didn’t permit taking a navigational fix. Lastly these storms / bad weather could go on for days nonstop.

Most U-boat KTBs don’t clearly identify when a navigational fix had been taken. They just logged their current location. But as navigational fixes weren’t normally taken very frequently, some U-boat positions in the KTBs were their dead reckoning position which as we have seen were generally not entirely accurate and in reading the KTB you generally can’t tell which are which except that the one around noon is generally a real navigational fix.

Some U-boat commanders did provide us with proof that their dead reckoning position were not accurate by identifying each real navigational fix and also logging the difference between it and their current dead reckoning position in their KTB. They were generally off by several nautical miles, sometimes more than 20 nautical miles. Again this is all the technology of the day permitted.

So while the numbers they (both the ships and the U-boats) had to work with were generally somewhat inaccurate, dead reckoning positions were the best they could do.

Now to complicate the issue, let’s look at what happened during an attack.

When a ship detected a U-boat it generally started running away as fast as possible and if they were armed and the U-boat was in range they opened fire on the U-boat to try to make it submerge so they could get away, and immediately sent out a distress call on its radio. Sometimes it included its position but there was no time to take a real navigational fix, they just used their last hastily updated dead reckoning position & sometimes, since the radio operator didn’t necessarily have an updated one he used the one he had (someone from the bridge was supposed to bring him one every few hours). At any rate it was better than nothing. The longer they ran the more inaccurate their position got though and to top it off the U-boat generally tried to silence the ship’s radio by gunfire or jam it with their own radio or both. U-boats did monitor the international distress frequency during attacks to determine if their target was transmitting on it and if so they intensified efforts to stop those transmissions.

All this assumes that the U-boat didn’t sink the ship before it could transmit a distress signal at all or if they did transmit one, someone picked it up. Many times their distress signal wasn’t picked up. Atmospheric conditions affect radio transmissions too.

Even if the ship couldn’t get out a distress signal that was picked up, sometimes the Captain or one of the ship’s other officers brought their location information with them when they abandoned ship and if they were rescued, that information ended up in the records, but it was generally just their dead reckoning position. Many times they weren’t rescued and that information was lost.

The U-boat did have a position though but it was generally a dead reckoning position too. They frequently took a navigational fix as soon after the attack as was safe, weather permitting though.

That said, the U-boat location might well be the only location information that survived the attack.

However if the ship was in a convoy at the time of the attack other ships in the convoy, specifically the convoy commodore, would have a position as would the convoy escort. There might also be aircraft involved that would have a position.

At any rate when multiple vessels and / or aircraft were involved it could be extremely difficult for a researcher to put it all together after the fact and find the documents in the archives. There was a British plane overhead at the time, well I think it was British, but which one and where would the records be if they survived the war? A lot of the enabling information was inaccurate at the time due to the fog of war or just didn’t make it into records that can be found in the archives by researchers no matter how hard they try. Consequently, the official sources and books often disagree but that is what we have to work with today. We can only go on “best evidence” from respectable sources.

Regardless of the source of the location information it was generally just a dead reckoning position though. It might be right on or not very close.

Now to add insult to injury, there is one more problem with the position the U-boat logged in its KTB. It generally wasn’t in latitude and longitude, it was just a grid reference on a secret German Naval map.

Most folks don’t have access to one of those maps today and the grid reference was generally only accurate to about a 6 nautical mile square on top of that. I say generally because a few U-boat commanders further refined the grid position logged in their KTBs.

The full grid consisted of 536 large squares that covered almost all sea areas of the world. A few of the squares aren’t square either and they aren’t necessarily the same size but a square was typical.

The grid location might look like this in the KTB: “DN 1369”.

Here is how it works: Each of the large squares on the Grid Map has a 2 character designation. In our example “DN”.

Each large square is broken down into 9 smaller squares that each have a 1 digit number and each of them into 9 smaller squares that each have a 1 digit number and each of them into 9 smaller squares that each have a 1 digit number and each of them into 9 smaller squares that each have a 1 digit number. The smallest square represents about 6 nautical square miles of ocean.

However, some U-boat commanders logged an extended version like DN 1369 upper-left or lower right etc. which is a location within the 6 square nautical miles.

But what is the latitude & longitude we can navigate to for DN 1369? Well if you don’t have a map that shows the correct location of all the squares and the latitude & longitude that goes with it, you have a problem.

However there is a free, online grid converter that will convert grid references to latitude & longitude and plot the location on a map. You get the latitude and longitude for the center of the square and for all 4 corners of the square.

It will also convert a latitude & longitude to a grid reference and plot the location on a map.

Of course all radio messages to & from a U-boat were encrypted with the Enigma machine but the grid references were so valuable to the enemy they were coded before they were encrypted. Thus DN 1369 might be written in the KTB, but what was sent out by the radio might be BL 3495 before it was encrypted by the Enigma machine. That way if the enemy was able to break into the Enigma encryption they were still left with BL 3495 as the location instead of the real one.

And just to screw around with those that might eventually get their hands on a KTB, sometimes the coded version of the grid reference (BL 3495 in our example) was used in the KTB instead of the real one (DN 1369).

So you see, there are a good many stumbling blocks that have to be overcome just to get what is generally not necessarily a completely accurate dead reckoning position.

I guess it depends on what you are going to do with the information.

If you want to know exactly where one of these wrecks is, you have to find it first. Catch 22.

The location information that survived WWII is just a place to start searching and it may be a long way from the wreck.

Hope this answers your question.


Ken Dunn

Subject Written By Posted
Accuracy of Coordinates Sergio 10/08/2020 03:37PM
Re: Accuracy of Coordinates Michael Lowrey 10/09/2020 12:27AM
Re: Accuracy of Coordinates Mark McShane 10/09/2020 05:12AM
Re: Accuracy of Coordinates Ken Dunn 10/09/2020 06:25PM
Re: Accuracy of Coordinates Urs Heßling 10/11/2020 12:01PM
Re: Accuracy of Coordinates Ken Dunn 10/11/2020 01:00PM
Re: Accuracy of Coordinates Urs Heßling 10/11/2020 02:09PM
Re: Accuracy of Coordinates Sergio 10/12/2020 09:45AM
Re: Accuracy of Coordinates Ken Dunn 10/12/2020 12:32PM
Re: Accuracy of Coordinates Sergio 10/12/2020 01:51PM