Theodilite vs. Transit 

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wizardgmb
Posts: 4

Joined: 11/19/2009
Topic  Theodilite vs. Transit       Flag »  Reply »
I'm retired and try to keep busy by teaching myself new skills. I've always had an interest in surveying and was pretty good at geometry and trigonometry back when I used them regularly. We recently bought our "retirement" home which is on a relatively large piece of property. I've decided this is as good a time as any to learn a little about and experiment with surveying. The property was recently surveyed so I'm just going to plot items of interest on the land and the topography. I've ordered a couple of books and been doing some online reading to get started.

I'd like to buy a used optical theodilite/transit off ebay to work with but the terms have me a bit perplexed. I understand that a theodilite has inherently greater accuracy than a transit. However, it appears from what I've seen that some transits don't have a compass built into the base to reference magnetic north. Does this mean that transits are intended to reference lines established during a survey with the higher accuracy theodilite? That would suggest transits are relegated to the building trades and surveying relies only upon the theodilite.

Is my interpretation correct? Do I need a built in compass to puter around my property?

Thanks for letting a very rank amateur post an inquiry!

Regards,
George
  Friday, November 20, 2009 at 11:08:16 AM
k-bob
Posts: 15
Location: MI USA

Joined: 2/6/2009
Reply  Re: Theodilite vs. Transit Flag »  Reply »
George - great that you're interested in our beloved profession!

The terms transit & theodolite essentially describe the same general instrument. Theodolites actually predate transits, being used in Europe as far back as the early 1700's, while the first transit was an american invention, circa 1830. The primary difference was / is in the capability of a transit to have its telescope revolve (transit) a full 180 degrees about its horizontal axis. Early theodolites telescopes could not do this -although pretty much any modern theodolite has this capability today. Try to ensure that what you're potentially buying is not just a 'contractor grade' transit or theodolite and you should have more than adequate accuracy for your needs.

On the question of a compass, know that the use of compasses to establish lines was prohibited / discontinued (on public land surveys) beginning in 1894. Bearings were established by astronomical observations which would certainly be above & beyond your interest. If you're just interested in making a general site plan of your property, just set the transit / theodolite in a position where you can reference at least two discernible points - like property corners, house corners, etc. - and when you tie in your items of interest, they will be relative to actual physical features of your property. You really wouldnt need to establish bearings or azimuths (i.e. direction) to accomplish what you appear to be seeking.

Hope this helps & good luck!
  Monday, November 23, 2009 at 4:48:51 PM
wizardgmb
Posts: 4

Joined: 11/19/2009
Reply  Re: Theodilite vs. Transit Flag »  Reply »
Thank you for your reply.

There is a new wrinkle in my plans since I first posted my inquiry. Unfortunately, the neighborhood kids pulled up most of the rebar corner marking points so carefully placed by the survey team. Their ATV trails surround my property on three sides and I assume they were afraid of damaging their tires. I'm hoping to avoid paying for another survey so I can satisfy my interests or engendering ill will with new neighbors by accusing their kids of vandalism.

I'm more curious about the compass and the alternatives now since all of the lot lines on the professional survey reference degrees, minutes and seconds from magnetic north.So far I've found one out of the six markers and it's at the T junction of two neighboring properties, on our longest line. It's out of sight of the buildings and telephone poles which are the only fixed points on the survey. Unless I can find another marker that the kids missed under the newly fallen leaves, the only way to work from that point seems to be following the bearing relative to magnetic north shown on the drawing. I guess I could work out from the buildings but they are at least six hundred feet from the lot lines.

If the transit mounted compass isn't reliable by today's standards, is there any way to reference an optical survey to magnetic north without using electronics such as lasers and GP's? It seems to me that even taking sightings from the geodetic survey marker 3/4 of a mile away would require some reference to magnetic north or another similar marker within sighting distance.

I guess it's time for me to dig into the first of the books that arrived yesterday. I know less than I think I do and that's just enough to be dangerous. :)

Regards,
George
  Wednesday, November 25, 2009 at 1:32:46 PM
k-bob
Posts: 15
Location: MI USA

Joined: 2/6/2009
Reply  Re: Theodilite vs. Transit Flag »  Reply »
George-
I cannot in good conscience furnish advice towards a layperson such as yourself trying to 'resurvey' his property when the existing corners have been pulled or are known to be missing.

I'm sorry to hear that the corners were destroyed. The neighbors should be told because its not just your corners that were removed, they are their's as well and as such, they should have an interest in the preservation of the corners. Also, its illegal to remove or destroy boundary markers. I don't envy your position, unfortunately it happens alot.

Surveyors establish line typically by reference to an established line (section line, plat line, road alignment, etc) with a known bearing or azimuth. These bearings should be in reference to astronomic north, not magnetic north as you keep stating. It is on occasion that directional line be actually established or verified by astronomic observations (solar or polaris observation) by a survey crew. Again, the use of a compass is simply not used but in the rarest of instances, at least for demarcation of boundary lines.

Good luck
  Thursday, November 26, 2009 at 11:39:43 AM

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