Use a transit

In the landscape construction industry, as in many other fields, professionals use a transit to measure horizontal and vertical angles and to prolong straight lines. Transits also can determine differences in elevation.

A transit essentially is a telescope that revolves around horizontal and vertical axes, with mechanisms to measure angles. The readings you obtain allow you to map a site and determine the position of its features with great precision.

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Why do we use a transit? You might use a transit to determine property boundaries, locate existing natural and man-made features, locate proposed improvements or carry out other jobs where angle measurements are necessary. By using geometry or trigonometry and a known horizontal measurement, you can even use a transit to measure the height of structures or trees.

Transits have several key features: * A telescope with horizontal and vertical cross-hairs.

* Horizontal circles (a fixed circle scaled in 360 degrees and a rotating circle) with either a vernier or an electronic device for measuring horizontal angles. A vernier is a short auxiliary scale inscribed on the rotating circle to measure fractions of degrees on the stationary circle. The rotating circle is equipped with a locking screw and a fine adjustment screw to allow precise sighting on a point.

* Vertical circles, equipped like the horizontal circles, for measuring vertical angles. Measurements for vertical angles by non-electronic and electronic transits vary, as I'll explain below.

* Two or more leveling bubbles with three or four leveling screws.

A transit may be read in the direct position or rotated on the vertical axis to be read in the reverse, or plunged, position. When prolonging a line, it is more accurate to plunge the telescope on the vertical circle than to turn a 180-degree angle with the horizontal circle.

Different types There are many types of transits-you should read the manual that applies to the specific transit you are using or obtain instruction from a person who is experienced with that instrument.

* A non-electronic transit is equipped with a 360-degree horizontal circle graduated into degrees (and fractions of degrees) with a vernier to measure angular readings even more precisely. Most are capable of measuring only to angular minutes, but some can measure seconds. On the vertical circle, the vernier is graduated so that the reading is 0 degrees with the telescope in a horizontal position. Rotating the telescope up or down increases the reading to a maximum of 90 degrees in the vertical position. To accurately read the vernier, you will need to use a magnifying glass.

* Electronic transits have direct-reading horizontal and vertical angular displays, reading as precisely as 10 or 20 angular seconds. The horizontal electronic display may show angles either to the right or to the left, which may be changed by a push button. The vertical display is a zenith angle, which means that with the telescope pointed straight up, the display reads 0 degrees (the opposite of the case with non-electronic transits). With the telescope in a horizontal position, the display reads 90 degrees in the direct position or 270 degrees in the reverse (plunged) position.

* A special type of transit, called a theodolite, can be used to measure angles to a fraction of a second.

Setting up To accurately measure angles, you must start with two points on the ground at least 150 feet apart, with a known distance and a known or assumed direction. These points may be land-corner, street-centerline or other survey monuments or two or more temporary points set in the ground.

1. First, place the tripod on the ground near one of the known points. This will be your Instrument Point (IP). Spread the tripod legs about 3 feet apart so that the tripod is stable. Adjust the tripod legs so that the upper plate is horizontal and near chin level.

2. Next, remove the transit from the storage box and firmly affix it to the tripod. Keep one hand holding the transit until this step is complete. A dropped or damaged transit may cost hundreds of dollars to repair and result in inaccurate surveys, which could be even more costly.

3. Carefully pick up the tripod and transit and place it over the Instrument Point (IP) using a plumb bob or the optical plummet. Level the transit by adjusting the tripod legs and reset it over the point so that the center of the instrument is directly over the IP. Push the tripod leg points into the ground. Never set up a transit with the legs on a hard surface unless you provide a means to keep the legs from slipping. Re-level and adjust the transit on the tripod so that it is directly over the IP.

4. Level the transit with the leveling screws by checking through 360 degrees of rotation. Start with the plate bubble parallel to the two leveling screws. To adjust, simultaneously turn one screw clockwise and the other counter-clockwise until level. Rotate the telescope 90 degrees and re-level the instrument. Then repeat in 90-degree increments until the plate bubble remains in the same position throughout the entire circle. Check the level bubble before each reading. Sun shining on a transit will heat and expand the sunny side, requiring a re-leveling of the instrument.

5. Sight the other known point, called the Backsight Point (BS), with the horizontal angle set at 0. On electronic transits, there is a button labeled "0SET" to be pushed when it is sighted on the BS. Most older non-electronic transits have two horizontal motions. Use the upper motion to set the horizontal reading to 0, then use the lower motion to sight on the BS.

If the distance between the points is not already determined, you must measure it.

You are now ready to begin using your transit. Using the two points (the IP and the BS), with a known horizontal distance between them (which establishes what's known as the baseline), you can map existing features with only one helper by reading horizontal angles to each feature from both points. You also can use this method to determine heights (elevation) by reading horizontal and vertical angles from both points.

The math (geometry and trigonometry) required to make the necessary calculations for surveying is beyond the scope of this article.

However, you easily can perform simple measurements of elevation changes with a leveling rod (see illustration, below) as long as you carefully set up and level the transit as I described. A level is similar to a transit, but you can only use it for leveling.

Prolonging straight lines, a useful and frequently needed ability in landscape construction, also is relatively simple.

Rules to survey by Follow these tips to ensure that your work is accurate and that you do not have to repeat a survey:

* When sighting the BS, use as a target a vertical pointed pole or plumb bob placed directly over the BS point. Sight on the pole or plumb bob string with the horizontal cross-hair as close to the point as possible to ensure that the vertical cross-hair reflects the true angular position of the point. Unlock the horizontal and vertical circle screws, sight your next point and record the horizontal (and vertical) angle in your field book.

* Reading one angle with the telescope in both direct and reversed (plunged) positions will compensate for minor instrument errors and minor setup errors. If the difference between the two is great, correct the instrument or remedy the setup error and repeat. Average the two readings to arrive at a more accurate value.

* Watch for errors in recording the values in your field book. Transposition of numbers and reading errors are the most common. To ensure greater accuracy: read, record, check reading, check recording.

Newer equipment Over the years, other equipment has begun to do the job of transits and tape surveys: electronic distance measurement (EDM) equipment in the 1950s, total stations in the 1980s and global positioning systems (GPS) in the 1990s.

* EDM is an instrument that measures distances with a laser.

* A total station is a combination electronic transit and EDM. It can use an electronic field book to record angular readings and distances. This equipment is especially useful on location surveys for mapping existing features and for construction staking. You can rent this equipment for a nominal cost from firms listed under "Surveying Instruments" in your Yellow Pages.

* GPS equipment relies on orbiting satellites to determine locations on the ground. At the present time, GPS is most useful in large areas in conjunction with aerial surveys, or for mapping in inaccessible areas.

Dean Connett is a surveying consultant and a retired visiting professor in the civil engineering department at the University of Nebraska (Lincoln, Neb.).

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