Re: Survey Problem-QBASIC answer


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Posted by John Nichols on September 10, 2003 at 17:35:22 from 165.91.196.181 user Mcneacail.

In Reply to: Re: Survey Problem-QBASIC answer posted by John Nichols on September 10, 2003 at 14:03:41:

So the Swallows have to measure the island. The only problem that AR does not solve is the base measurement on the first triangle on the first day. He never actually has them measuring this length, or even having the gear to measure the length. This is a clear error in his writing. I had to use a set of scales and TW original map to work out a distance. AR as he copied did not have to work out the distance.

If Titty "theoretically" did what I did she would use the distance from the camp to the kraal as the base line. It is 0.6 miles. She could have gotton that with a ruler from TW original plan.

Whatever compass or theodolite one uses there are errors, inlcuding parallex (you look at it wrong), someone moves the pole, etc. The errors on a theodolite can range from seconds (60 seconds in a degree) to 1.25 degrees on a compass set for nautical use.

Irrespective of the lengths and angles we make mistakes. Now the advantages of using naval bearings is there is no mistake as to what is the meaning of the bearing. The Walker children in getting two people to look at the angle are being careful and double checking. And the compass is not like a theodolote where it is set up and then two people can take turns agreeing, here each one has to hold it and focus and look and hold. Plus there is no telescope built into the compass to more accuratley look at the pole at a distance.

Then once they had the measurement angles Titty could plot out the angles using the technique outlined in the other post. My guess is that AR in drawing the plan has copied an outline of the island and then fitted his own survey. He was either careless in transcribing the angles or he used a very course compass rose and averaged to much. If he used the compas rose drawn on the sketch then he would be lucky to get to an accuracy of 10 degrees. (I have the advantage of AUTOCAD, a large screen and transparancy sheets so I can compare mine to AR's.

Next question - what sort of compass would John and Titty have used as AR obviously had a model in mind when he wrote the story.

I HOPE THIS IS enough explanation.




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