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GPS Conversions - Guidance from UK Hydrographic Dept.



There are many problems associated with locating wrecks using a GPS receiver. A few thoughts from the Wrecks' Officer about our records and GPS might be in order  - to dispel confusion (or create more). Firstly let us consider the way that our records are structured.

From 1 January 1999 the listed position for the wreck or obstruction is given in degrees, minutes and decimals of a minute. From that date too, for new information, the positions listed in the 'Surveying Details' text field is given in the same format. Prior to that the positions given in Wrecks' section printouts have been given in degrees, minutes and seconds both in the listed positions and in the 'Surveying Details'. Entries in the 'Surveying Details' prior to January 1999 will remain in this format. A note is inserted in the records to indicate from where the change of format applies. In an ideal world all entries in the 'Surveying Details' text would all be amended, however with some 58000 records to consider this is, realistically, not going to happen. Considerable effort is being put into to editing the text of each record to take out all of the irrelevant material and make the remainder easier to read. Completing this task will take a number of years.

Many navigation receivers will give positions in degrees, minutes and decimals of a minute or as degrees, minutes and seconds. My own Garmin hand held will do this. Take care which you use! It is easy to convert seconds to decimal minutes - simply divide the seconds by 60. If we supply data a table giving the decimal equivalents of seconds is included. In older descriptive text we run the degrees, minutes and seconds together without spaces. Thus 51 45 24N becomes 514524N. In current and future text 51 45.40N will be written as 5145.40N. Why? Simply to avoid typing all the spaces!

In passing it is worth considering what both 0.01 of a minute and a second (approx 0.017 minutes) represent on the Earth's surface. One hundredth of a minute is some 18.5 metres in latitude and about 12 metres in longitude in the South of England or some 9 metres in longitude in the North of Scotland. A second is 1/60 of a nautical mile and thus it is about 30 metres in latitude and some 20 metres in longitude in the South of England or some 15 metres in longitude in the North of Scotland. OK so far? It might be time for tea before considering the joys of datums.

The idea of a datum arises because the dry part of the world is lumpy. When surveyors use a theodolite to take measurements of angles and various instruments to measure distances, they have to have some way of calculating the results. For small areas (for instance when setting out a supermarket building) it is fine to assume that the earth is flat. We have all done some trigonometry at school and the formulae are relatively simple. (Does anyone remember the sine and cosine rules)? Now on a bigger scale, say over the United Kingdom, things get a bit more complicated. Clearly the lumpy surface of the real world is too complicated to use for calculation and the Earth is not flat so another shaped surface has to be used. Take a sphere, roughly the size of the earth. Squash it along the North/South direction and wiggle it a bit to get the best fit over the UK. Bravo! Keeping it simple, you have now Ordnance Survey 1936 datum (we call it OGB in our listings). Calculations on this curved surface are complicated but possible. (The squashed sphere is known as Airey's Spheroid - he was the Astronomer Royal in the latter part of the last century). Over the greater part of Europe another spheroid is used, the International Spheroid, and the best fit over the whole of Europe from the West of Ireland to the Ural Mountains is known as European Datum.

OK so far? Put the tea aside and try something a little stronger. Charts of our coastal area have always been on Ordnance datum as this was convenient and prior to the advent of satellite navigation systems it did not matter. However GPS is a global system and the squashed sphere that is a good fit in the UK is not the best fit to the real, lumpy surface over the whole world. So a revised, squashed sphere was invented. It is known as the World Geodetic System 1984, or WGS84 for short (we call it WGD in our listings). Logical to call it WGS when it fits the world. The trouble is that this squashed sphere does not match up with the Ordnance Survey one in the UK, so there is a difference in apparent position for the same point on the Earth's surface when you go from one to the other. This has caused some problems to those not familiar with the idea of applying shifts to the position displayed on the GPS received. Consequently UK coastal charts are being reissued referred to WGS84, thus no correction to position will be necessary prior to plotting. In the fullness of time all the charts from all nations in Northern European waters will doubtless be so amended. Around the UK our data will generally give both the OGB position for the object and the WGD position. You will see that there are simply two positions for the same item at the same point on the Earth's surface! Where the surveyed position for the object has been established on WGD the position will be the accurate, surveyed position. Where this is not the case the WGD position will be calculated from the OGB position by mathematical means or, where this is not possible, by the use of the shifts shown in the title block of the chart. The method of establishing the WGD position will be shown in the listing. Modern GPS receivers can generally be set to show either position. You simply set the receiver datum to Ordnance Survey and use the OGB position or set the receiver to WGS84 and use the WGD position and Bob's your uncle, ideally you can find the wreck. Note that charts that have been amended and reissued as noted above will state that they are referred to a WGS84 compatible datum, while some wreck records will state that they are now referred to ETRS89 (European Terrestrial Reference System). This is an academic difference and for all practical purposes the WGS84 and ETRS89 positions are identical.

So you think that you are going to find the wreck? Ah, it is not yet that simple. Remember that there are errors in the GPS system. Some small ones are inherent with cheaper units but for a long period the accuracy of the system was deliberately degraded by the USA. The quoted accuracy of the system was then 100 metres for 95% of the time. So for over an hour a day it could well have been be outside 100 metres. Recently the degradation was removed and raw GPS positions are certainly good to better than 15 metres. Differential GPS allows virtually all errors to be removed, both for the older degraded positions and for the current undegraded system. Positions should be good to a few metres. Do remember that this position is for the receiving antenna, not for the echo sounder transducers!

Still not found the wreck? In addition the wreck had to be positioned too, usually by the survey vessel sounding over the top. However the survey ship had positioning errors as well and the size of these errors, despite the best efforts of the survey team and always assuming that no gross mistake was made, will depend on the navigation system employed. For modern work the wreck position will be good to probably a dozen metres, but for old work it could be dozens of metres. If Decca was used many years ago (remember it was the latest thing in the early 1950s) possibly tens of dozens of metres are possible. Many records are based on positions reported by the sinking vessel, members of the public, fishermen etc. Often there is no way of knowing how accurate the report is (or was) and they have to be accepted at face value. Perhaps you have found the wreck now?

Reach for the smelling salts, for the wreck may not be there at all! Vast, and steadily increasing, torrents of data flow into the Hydrographic Office and an increasing amount of this data causes chart corrections (the well known Notices to Mariners weekly booklet) to be issued. Clearly not all amendments to the chart carry the same urgency or importance and due to the sheer volume of potential corrections many amendments are held over to the next New Edition of the chart. A wreck could have been disproved or lifted, or the depth over it may have been amended, but the chart (even if corrected to date) will not show this. The latest data is available from the Wrecks' Section.

Best of luck with the wreck searching! We are always interested in new information. In many areas the survey information is scarce or incomplete. Even an accurate GPS position for a wreck previously only positioned by Decca is immensely valuable. Details of a new wreck could save the loss of a net or a nasty accident. A copy of a suitable format to render information is included. We will keep information in confidence for a period of 5 years if requested, but if we must chart the hazard if it is significant to navigation.


N McEACHAN
Lieutenant Commander, Royal Navy
Wrecks' Officer



Conversion Tables
SECONDS - DECIMALS OF MINUTES

0.5 .01 15.5 .26 30.5 .51 45.5 .76
        1 .02 16 .27 31 .    52 46 .77
 1.5 .03 16.5 .28 31.5 .53 46.5 .78
     2 .03 17 .28     32 .   53 47 .78
2.5 .04 17.5 .29 32.5 .54 47.5 .79
3 .05 18 .30 33 .55 48 .80
3.5 .06 18.5 .31 33.5 .56 48.5 .81
4 .07 19 .32 34 .57 49 .82
4.5 .08 19.5 .33 34.5 .58 49.5 .83
5 .08 20 .33 35 .58 50 .83
5.5 .09 20.5 .34 35.5 .59 50.5 .84
6 .10 21 .35 36 .60 51 .85
6.5 .11 21.5 .36 36.5 .61 51.5 .86
7 .12 22 .37 37 .62 52 .87
7.5 .13 22.5 .38 37.5 .63 52.5 .88
8 .13 23 .38 38 .63 53 .88
8.5 .14 23.5 .39 38.5 .64 53.5 .89
9 .15 24 .40 39 .65 54 .90
9.5 .16 24.5 .41 39.5 .66 54.5 .91
10 .17 25 .42 40 .67 55 .92
10.5 .18 25.5 .43 40.5 .68 55.5 .93
11 .18 26 .43 41 .68 56 .93
11.5 .19 26.5 .44 41.5 .69 56.5 .94
12 .20 27 .45 42 .70 57 .95
12.5 .21 27.5 .46 42.5 .71 57.5 .96
13 .22 28 .47 43 .72 58 .97
13.5 .23 28.5 .48 43.5 .73 58.5 .98
14 .23 29 .48 44 .73 59 .98
14.5 .24 29.5 .49 44.5 .74 59.5 .99
15 .25 30 .50 45 .75 60 1.00
When converting decimal minutes to seconds,
if two values are shown, use the higher value

 

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copyright Mark Ellyatt
email@inspired-training.com