... so STOP READING!... if you have somehow arrived at this page BEFORE seeing the pages that lead up to it!
You should at least have seen the Theolodite: The Tool of the Trade page before reading what follows. (When you've read what's there, use the link at the bottom of it to come back here. (Unless, even better, you first visit the Map Making Main Page which the link at the top of the Theolodite page will take you to. (If you haven't seen it previously.))
And now for the secret, if you HAVE seen the pages leading up to this one....
The secret of my success...
Here is the secret of my success in making a good theodolite! 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 theodolite 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 center 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 theodolite base! Easier isn't just attractive to the lazy... it also increases the chance that the job will be done well, and that the theodolite 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 theodolite 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 theodolite's scale. It has accurately spaced lines. Converting to degrees is a trivial spreadsheet exercise. In any case... I simply put my readings in a data file, 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 straight line. Put a narrow, vertical pole at one end. Put the theodolite in the middle. Go to the other end, plant another vertical pole, but place it carefully so that it, the theodolite and the first pole all line up. Go back to the theodolite. Read the angle between the poles. If your theodolite gives a reading of 200, then your units are just less than a degree. Multiply your readings by 180/200 to convert 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 got good results with a very simple pointer. Eventually, I gave my theolodite a Vernier scale, which lets me directly read to 1/10th of the units marked on the main scale. It wasn't hard, and was an additional fun challenge in itself. But by all means start with simple scale/ simple pointer.
Using a Vernier scale isn't a new idea... you will find it on many precision (mechanical) instruments. You can work it out! (The Wikipedia article mentioned above goes into it.)
On my PC just now, the photo there had weird interference lines. Hold your "ctrl" key down. Press "+" a few times... until the photo is larger, and clear. ("Ctrl+0" (zero) restores your browser to normal operation.)
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!)
Click here to go up to the preceding page about making the theodolite
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Page tested for compliance with INDUSTRY (not MS-only) standards, using the free, publicly accessible validator at validator.w3.org. Mostly passes. There were two "unknown attributes" in Google+ button code, and three in the Flattr. Sigh.