Map Making- The Scale and Pointer
Here is the secret of my success in making a good transit! It was many years, during which I pursued many dead ends, before I had the following two 'Eureka!' moments...
First Brainwave: The base of the transit is three dimensional. Instead of putting the reference scale on the top of it, put it on the edge!
The base must be cut as more than half of a circle. The scope swings around the centre of the circle. The moving part extends over the edge of the base. Along the edge of the base, a scale is marked. This can simply be uniformly spaced vertical lines. (It helps, of course if every tenth line is darker than the others.)
The approach just described is much easier than trying to mark many, many, accurately spaced radiating lines on the top of the transit base! Easier isn't just attractive to the lazy... it also increases the chance that the job will be done well, and that the transit will therefore measure well.
It helps to put numbers on the top of the base, marking 10 units, 20 units, 30 units, etc.
Second Brainwave: The transit does not have to read in degrees. I think a full circle with mine is 1058 units. It is very easy to multiply my readings by 360/1058 to turn them into degrees, if I want to use a protractor marked in degrees. The big benefit of accepting non-degree units is that you can use good quality graph paper for making the transit's scale. It has accurately spaced lines. Converting to degrees is a trivial spreadhseet exercise. In any case... I simply put my readings in a datafile, and a program I have prints for me a 'circular protractor' with the bearings to the various items of interest marked along the edge. I still have to draw them on my map! (The program that draws the map from the data comes next!)
You will probably want to know how many of your units equal one degree. Find a place where you can see a long way in a strainght line. Put a narrow, vertical pole at one end. Put the transit in the middle. Go to the other end, plant another vertical pole, but place it carefully so that it, the transit and the first pole all line up. Go back to the transit. Read the angle between the poles. If your transit gives a reading of 200, then your units are just less than a degree. Multiply your readings by 180/200 to convret your readings to degrees. (You have measures a straight line, and a straight line is an 'angle' (even if a weird one) of 180 degrees, isn't it?!)
Pointer: I'm getting good results with a very simple pointer. With the help of a photocopier, I could create a vernier 'pointer'. All I need to do is fix a small set of vertical lines to the swinging part of the transit, in place of my simple pointer. It would be half as high as the thickness of the transit base. If, on that, there are 9 lines in the distance required for 10 on the scale fixed to the base, I will have a system which lets me directly read to 1/10th of the units marked on the main scale. At present, I simply estimate the tenths. Making the piece with the slightly wider lines is easy if you have a photocopier with enlarge/ reduce facilities.
(I can't easily describe how you read things from a vernier pointer, but the idea isn't new... you will find it on many precision instruments. You can work it out! (The left hand end of the moving bit points to the reading as my simple pointer would. Where a line on the moving bit lines up with a line on the fixed bit determines the tenths of units of the reading. See... I said I couldn't explain it!)
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