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Richard M. Pisko wrote:
> On Sat, 31 Jan 2009 03:24:50 -0700, Nicolàs de Hilster  
>  wrote:
>
>   
>> I also attached a scan of two pictures from that job, the quality is not
>> optimal, but they give a good idea.
>>     
>
> Yes, I can see the tower alignment of one set, and the two directions  
> involved.  Unfortunately, my computer appears to be a victim of a recent  
> change in Google Earth, which now requires DirectX 9+ to operate; and  
> there seems to be no available updated driver for my graphics card.  I'm  
> not sure if the terminology is correct, but I know I have a problem.
>
>   
I attached the area as a screen dump. The inset shows the same area at a 
smaller scale.
>> 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....
>>     
>
> Yeah, one of those things that sticks in the memory,  You were told the  
> suspected cause of a problem . . . so that's where you started.   "It's  
> not what you don't know that gives the most trouble; it's what you do know  
> . . . that isn't so."
>
> I now am warned there is a potential problem with the circle charts being  
> used with (or constructed from) the wrong base points, if the A2 sized  
> chart is not easily compared to the larger area chart.  Did someone using  
> a station pointer catch the error, or was it obvious the dredge was not in  
> the channel?  500 meters off is a lot and should show up easily.  Maybe I  
> can use the Library's computer to see and print out that part of the river  
> by using your .kmz file.
>   
The area around it was not completely featureless, so they soon found 
out. As you can see 500 metres is quite a distance in respect to the 
peninsula. At first they thought they were 700 metres off, but that was 
only guessed. In the end we knew it was about 500 metres, the distance 
between the masts (you can see them below the line with the '470m' above 
it).
> I have sentimental feelings about the T2; it was the first accurate  
> theodolite I used.   What a change from the old open faced 20 second  
> vernier transits that were our instruments in school!  I bought a T2 of  
> the correct early 1960s vintage from Ebay, and it's still smooth and  
> accurate after the usual minor adjustments.   I have to say it's more  
> fussy to set up than the T1A, which I think is about the fastest optical  
> theodolite to get into action of any of that era; but of course the T2 is  
> much more precise.  The city did not have a Distomat at that time, and I  
> haven't seen a working model for sale at any reasonable price, so I have  
> decided I really don't need one for my collection.  :-)
>
>   
I was lucky to use an automatic (that is the vertical circle had a 
compensator) T2 at school. In my collection I have two older T2's, one 
of which is on my web site. Sadly enough the distomath is lacking on 
both instruments as well. I still use one of the T2's for sun shots when 
I need a reference azimuth in the field (for gyro calibrations).

Nicolàs


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