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Richard M. Pisko wrote:
> On Wed, 28 Jan 2009 00:46:20 -0700, Nicolàs de Hilster  
>  wrote:
>
>   
>> For what I understood Cook charted New Zealand using a plane table from
>> a ship and still did quite a proper job. He must have used similar
>> techniques to get his positions right.
>>     
>
> I liked that recent CBC-TV hour on Cook, at least the part that included  
> the very good survey of Newfoundland.  It did show him doing the work from  
> the beach with a simple vane alidade, but I believe reading angles with a  
> sextant and plotting them with a station pointer would let him do good  
> work from the deck of an anchored ship.
>
> The charts he made of the coastline of NL were shown on the program (I  
> hope they were the real ones) and a little "photoshopping" was done to  
> superimpose Cook's on the older charts, and then on the most modern  
> version.  He did what looked to be excellent work.  I wonder if he had a  
> way to keep the sextant level, so as to avoid taking the "slanted" angle,  
> while still being able to align his chosen reference stations.
>
> Were your shore stations in Nigeria set up to be more or less at the level  
> of your chart room on the ship?  It wouldn't make very much difference, I  
> suppose, but I do wonder if the effort was thought worth-while.
>
>   
The shore marks in Nigeria were a bunch of high voltage line masts. 
Attached you will find a kmz file that will direct Google Earth to the 
area involved once double clicked. If you look closely around the 
peninsula you will see the masts standing in the water around it on the 
north and east side. The masts are standing on concrete bases, a few 
metres above the water. Even though the masts were quite wide it was 
still possible to get a reasonable position as they were very 
symmetrical. It was just a matter of trying to superimpose the masts 
onto each other. Aiming the sextant more or less to the horizon would 
give the horizontal angles.

I also attached a scan of two pictures from that job, the quality is not 
optimal, but they give a good idea.

There is a nice anecdote to this job. I was sent there as they found out 
the cutter suction dredge was positioned some 500 metres in error using 
the circle charts. All the survey work of the masts was done by local 
Nigerian surveyors (and done well as we will soon find out). The quality 
of the local surveyors' work was however questioned and I was asked to 
come over and sort out the problem. Using conventional traversing with a 
Wild theodolite (T2) and a distomath (there was ample space on the 
concrete bases of the masts to use them as observation platforms) I 
surveyed all the masts, starting at the south east corner and following 
the masts until they reached land again in the west. On both sides of 
the power line there were land based trigonometry points that I used to 
connect my traverse to. After some serious maths (done of course with a 
pc, not too much sweat involved) I found out that the coordinates of the 
masts were quite well established, so the local surveyors had done their 
job properly. I then went back into the field with them to find out why 
there was still this 500 metres error. I asked them to take me to the 
start of the series of masts and started counting them and compare the 
numbers they used. As can be seen on Google Earth there are four masts 
east of the peninsula, which were indeed numbered one to four. They had 
however continued counting the northern series with number five at the 
corner again, giving this mast two numbers. The skipper on board counted 
the corner mast only once, so was one mast too far to the west, a much 
simpler cause of the problem than I initially imagined. In the end it 
took me some three weeks to solve a counting error that could have been 
checked on the fingers of one hand....



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