Used Theodolite Buying Guide

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Used Theodolite Buying Guide

Most people have walked or driven down a road and seen a surveyor with a tripod looking off into the distance, particularly at an undeveloped piece of land. The surveyor was most likely using a theodolite. Anyone in a field-related business knows how important it is to be able to take angle measurements of the earth with a transit or theodolite. Someone on a budget, or who prefers the look and performance of older analog models, may prefer to shop for a used theodolite. A student who is learning to use a theodolite for the first time may not even fully understand what this instrument does or how it works and benefit from the cost savings that come with a used model.

In order to make a good purchase of a used theodolite, one should fully understand what they are and how they compare to transits. A bit of history on theodolites as well as a breakdown of the parts are helpful information as well. Finally, a buyer should know what to check when considering purchasing used equipment such as a theodolite.


Theodolites Defined

In short, the theodolite is a field instrument for measuring horizontal and vertical angles in relation to the earth. Theodolites are used by

  • Surveyors
  • Structural engineers
  • Civil engineers
  • Aerospace engineers
  • Builders
  • Cartographers
  • Photographers
  • Architects
  • Geologists
  • Sportsmen
  • First responders
  • The armed forces

Essentially, anyone whose profession, field of study, or hobby requires an assessment of the angles, elevations, and planes of the earth requires a theodolite.


History of the Theodolite

Although its origins are unclear, the theodolite appears to date back to the 16th century. Attributions of the invention to both Englishman Leonard Digges and German Joshua Habermel can be found in different sources. The modern theodolite appears to have been designed in 1787 by Jesse Ramsden. Technological advancements have continued to improve the instrument, including electronic functioning, which was added relatively recently.

The theodolite has played a tremendous part in civil engineering projects and advancements around the world. Many highways, railroads, and waterways were cleared with the aid of the theodolite. Even as early as the 17th century, the now little-known Canal du Midi in France was built by Pierre-Paul Riquet with the aid of a theodolite. The theodolite was used to mark the best route for channeling water to the canal by finding the flattest land. Quite a marvelous feat for its time, this canal connected the Atlantic Ocean to the Mediterranean, although it is today too small for anything but minor traffic.


Parts of a Theodolite

When purchasing a used theodolite, is is important to check all of the parts and be sure they are in proper working order. The following table lists a majority of theodolite parts and describes their function.
 

Theodolite Part

Use

Alidade

For using the telescope on a horizontal plane

Alidade level

A spirit level for positioning the alidade

Horizontal clamp

For locking the alidade

Horizontal tangent screw

To make slow movements in a horizontal plane

Vertical clamp

To measure vertical angles

Vertical tangent screw

To make slow movements in a vertical plane

Optical sight

To make visual observations

Optical plummet

To determine centering of the plumb bob

Plate level

For adjusting the vertical axis

Leveling screws

To adjust the plate level on a horizontal plane

Leveling head

To support the theodolite

Illumination mirror

To reflect light onto the measuring circles


The terminology used to refer to individual parts may vary slightly depending on the manufacturer's literature or other reference.


Theodolites Vs. Transits

Some people may hear surveyors and other users refer to transits instead of theodolites. The transit is actually a subtype of the theodolite that developed much later than the theodolite. Originally, the transit was a theodolite with a moving telescope mounted to it. The telescope on a transit theodolite rotates 360 degrees in the vertical plane. (A non-transit theodolite features limited telescope rotation.) The purpose of the telescope was for double-checking to minimize errors. The term "transit" is now used in reference to simple versions of theodolites. Most modern digital theodolites are exact and do not need this additional check.


Types of Theodolites

Within the theodolite category, there are several subtypes: repeating, directional, and electronic digital theodolites. Phototheodolites and total stations are combination instruments. All of these types are briefly described below.

Repeating Theodolites

A repeating theodolite allows the user to accrue several horizontal angle measurements in a running total. The final measurement is divided by the number of measurements to obtain an average. This process enhances accuracy.

Directional Theodolites

A directional theodolite does not measure angles per se; rather, it measures the directions of fixed points. The angle is then determined by subtracting the difference between two points.

Digital Theodolites

A digital theodolite is highly accurate and, perhaps surprisingly, often less expensive than an analog design. With a digital version, measurements can be obtained faster, and errors are reduced.

Phototheodolites

A phototheodolite includes a camera in the mechanism and is used especially for cartography and other similar applications.

Total Stations

A total station is not a true theodolite but a combination instrument consisting of a theodolite and a distance measure. A user can obtain a slope distance and then reduce this to horizontal and vertical angles.


Theodolite Calibration

Because a theodolite is supposed to be a finely tuned instrument, purchasing a used one is not without risks. The buyer should consult with the seller in detail as to the condition of the item in question. There are a couple of factors involved in checking the quality of and calibrating a theodolite.

First, a buyer should find out if there is any known existing damage to the instrument. Obvious external damage shows up in photos as cracks, gaps, rust, missing pieces, and so forth. Less obvious damage includes water damage, sand and dirt in the mechanisms, and internal breakage from a drop or fall.

Additionally, a buyer should ask when the last time the theodolite was used. It may be possible for a potential buyer to test out the used theodolite before committing to a purchase. If the buyer is purchasing equipment online or from a distance, the seller may be able to supply sample readings to indicate the instrument's level of function. A true antique may also have been assessed and appraised by an independent third party.


Where to Find Theodolites

A theodolite is a specialized piece of equipment that is not available at any mainstream store. Retailers that sell theodolites include brick-and-mortar and online suppliers of surveying equipment and geological equipment; however, these establishments may not carry used equipment. You may need to do a bit of hunting in order to find a reliable used theodolite. You can check advertisements in the back of industry-related publications. Occasionally, you may find a theodolite at a yard sale, antiques store, or auction. Theodolites are also sold online through classified advertisements and Internet auctions.

How to Buy Theodolites on eBay

You can find both new theodolites and used theodolites on eBay. You may also find refurbished theodolites for sale. Most theodolites are listed in the business and industrial department. To find them, browse by all categories and then navigate through successive links until you arrive at transits and theodolites.

The keyword search function on eBay is another way to go about your search for theodolites. To make use of it, simply return to the homepage and type a word or phrase such as "theodolite" or "digital theodolites" into the Search field, and then press Enter or click Search. You will be redirected to a page that displays all listings tagged with related terms.

Further navigation is possible by selecting various category filters on any landing page. You may choose, for example, to show only those listings that come with free shipping. Most importantly, you can specify the condition of the item, for example, new, refurbished, or used. You might also wish to enter a price range when searching for a used theodolite. Remember, if you have any questions about the function or quality of the instrument, you can always contact the seller directly before placing a bid or making a purchase.


Conclusion

The theodolite is a useful instrument for many people who work in field industries. A theodolite can also be helpful to outdoor sportsmen such as hunters and golfers. Invented hundreds of years ago and used in several famous engineering projects, the theodolite has now been updated for the digital age. However, some people prefer to work with older analog versions, and many of these are still in good condition and available for sale through various channels.

A theodolite must be both accurate and precise in its measurements. When such an instrument fails to deliver, it is essential to replace it with one that functions reliably. When buying a used theodolite, the consumer must verify that the instrument is in perfectly good working order; otherwise, the purchase is useless. Thankfully, there are many previously owned theodolites in excellent condition that are available for sale. These items include repeating theodolites, directional theodolites, digital theodolites, phototheodolites, and total stations. Also, a buyer should know whether he or she is looking for a transit or non-transit theodolite. Armed with this information, a buyer can make an informed purchase of a used theodolite and get accurate measurements for his or her next project.

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