I am a retired Mechanical Engineer with a little time on my hands. I've always wanted to have a transit and I recently bought a Lietz BT20 Theodolite Transit. It has the trough compass which only functions to point north. I set it on north and then tried to swivel the transit to look south but it will only swivel about 50 degrees in one direction and about 93 degrees in the other before hitting some type of internal hard stop. it will not swivel 360 degrees without unlocking and turning the base which causes you to lose the north orientation. Is this a bug or a feature? I can work around it by pointing it north and swiveling the scope vertically 180 degrees so it is upside down and looking south. That dosen't seem like the right way to do this. I'm thinking that it should swivel 360 degrees. Any ideas as to why the transit won't swivel without unclamping the outer ring?
Hey Granpa,
The instrument should be free to spin 360 degrees.
There are clamps to stop various bits from turning - maybe one of these is not quite fully undone
It is called a Transit because that telescope can be transited (flopped 180).
Thats the most accurate way of establishing a continuing line and it can also be used to increase accuracy and compensate for some instrument errors with both Left Face/Right face measurement
Thanks for the info. Right now I can set it looking north, flip the scope to look south, rotate it ccw 90 degrees to look east, then flip the scope again to look west. It has approximately 142 degrees of rotation. That is with the lower base locked on north and the upper base unlocked. If I unlock the lower base it spins freely 360 degrees but I lose use of the compass and protractor. That is not what I expected but perhaps it is a standard feature. Is that typical of all theodolites?
No, something is broken. Both the top and bottom motions should allow a full spin through 360+ degrees without any stops when the corresponding locking knob is loosened.
It is intuitive to assume it should spin 360 degrees. I don't suppose it is possible to find an exploded view of the parts for this model.
Surveyors generally send their equipment back to their instrument dealer for repairs. When new, a user manual is included with transits and theodolites. Don't usually see a parts diagram or exploded view in those.
I think this is obsolete.
Obsolete? Not necessarily.
It has value just because it does not need batteries to work.
After the Apocalypse you could use it and the stars to work where you are and which way is north
But not suited to today's commercial surveying - no distance measurement, no electronic data out
These things are selling on ebay for $150. They probably cost ten times that much 30 years ago. They are still going to be just as accurate as they were back then. My first project is to run a fence line down a section line. All I need to do that is true south and I can get that from this unit. I'll not be trying to survey. When needed, it wil make an awesome tripod level.
This instrument should allow you to run a good straight line. But be careful what you use for a south reference. The compass will not necessarily point the same direction that it would have at the time the land parcel was defined and neither is likely to match an astronomical azimuth. Read about changing magnetic declination.
What Bill said.
The direction of a section line is determined by monuments. In many counties there are records of them, usually but not always kept by the County Surveyor. They were set by compass in the first place, perhaps 100 to 150 years ago, depending on what part of the country you're in. But the monuments control after that.
You need to find official monuments at both ends of the line you want to run. A compass bearing will not be accurate enough to run a fence line. The fence might end up 20 feet into the neighbor's property.
Here's a manual for the instrument. It does have an internal drawing (p. 5) showing the light path, but it's not easy to read. It won't necessarily help you figure out why it won't rotate a full 360 degrees. Starting on Page 12 there are some tests that will show you whether the instrument is properly adjusted. But actually making the adjustments takes some experience. You might be better off just allowing for the error.
thanks for the manual. A paper copy came with mine but it is in bad shape. Also, it is a TM-20A manual and does not have the internal drawing. It is helpful. I believe i see a small ball bearing that the upper base would use to spin when the lower base is locked. I am always hesitant to disassemble something like this because it could have preloaded bearings that would get out of sorts if disturbed. I've seen that happen with other things and the bearings tend to lock up. Which is what this one might be doing.
I've spent some time scratching my head over the section line. I have a friend who was the county engineer here. I'm going to discuss this with him.
