Lesson Note On Differential Leveling

Differential Leveling

Overview

Differential leveling is the process of measuring vertical distances from a known elevation point to determine elevations of unknown points. The most common methods to determine elevation are through the use of 1) a compensator type, automatic (engineering level) and level rod(s), and 2) an electronic digital barcode leveling instrument with barcode rod. A thorough knowledge of leveling principles and proper application of methods and equipment will prevent costly delays and generate the needed results and accuracy.

Preferred methods of obtaining elevations (in order of preference/accuracy) are:

·         Differential Leveling - control bench marks, cross-sections, point elevations

·         Trigonometric Leveling - for Digital Terrain Model (DTM), 3D Model

·         Indirect leveling (and location) by measuring horizontal distances and vertical angles

·         Three-dimensional coordinates - both horizontal position and elevation are computed by processing the measurements

·         GPS survey - given the appropriate equipment, procedure and data collection.

Equipment

·         The Rod

o    Rods are, in essence, a tape supported vertically, and used to measure the vertical distance (difference of elevation) between a horizontal line of sight and a required point above or below it. The most common types are the Philadelphia rod- a 2 piece rod usually 13' in length, the Frisco rod- a 3 piece rod 12'-15' in length, the Lenker rod- a moveable face, direct reading rod, and the fiberglass-rod- a multi-section, extension rod from 8' - 45' in length. Electronic digital levels use a barcode marked rod. Precision leveling, known as First Order leveling, to extend or establish vertical control over long distances, requires use of invar scale rods.

o    All rods should be checked and maintained to ensure consistent readings. Cleaning and adjustment or repair should be done as needed.

·         The Level

o    The compensator type, automatic (engineer) level is made by various manufacturers, and is a precision, self-leveling instrument, equipped with a built-in compensator that automatically maintains a horizontal line of sight and has a telescope with approximately 30-power magnification. The level mounts on a standard surveying tripod, or a fixed-leg tripod for more precise leveling.

o    The electronic digital level is also a precision, self-leveling instrument with additional advantages. Advanced capabilities include automatic measurement of height and distance by reading a barcode rod, high precision by employing image processing technology, data display and data recording either internally or to a data collector, installed software for elevation stake-out or other leveling procedures. The digital level offers greater productivity and simplicity in all applications of leveling work.

o    All leveling equipment must be checked regularly and properly maintained to ensure that it remains in proper working condition and that systematic errors are eliminated to produce the expected results. This includes daily checks, periodic routine maintenance and yearly cleaning and adjustment by a qualified repair shop.

o    Daily - keep the instrument clean by wiping occasionally, especially when used in dusty or wet environment. Operate and adjust the motions. With a compensator type, automatic level, observe the compensator while adjusting level screws to make sure that motion is smooth. If erratic compensator movement is detected, have a qualified repair shop service the instrument.

Instrument Check

On a regular basis and before beginning a leveling project, perform a peg test for collimation error to make sure that readings are consistent. If not, have the instrument serviced.

Anytime an instrument is dropped or severely bumped, or suspected of the same, it should be checked immediately. If questionable, have the instrument serviced before continuing use.

Leveling Methods

·         2nd Order for primary elevation control, extending or establishing bench marks on a project.

·         3rd Order intermediate or temporary bench marks, turning points on a project; aerial photo mapping control; major structures.

·         General data collected for a DTM, topographic mapping, cross sections, or other purpose.

Leveling Tolerance

Datum

The NAVD 88 vertical datum shall be used on all TxDOT projects unless specified by TxDOT prior to the beginning of the survey.

Three-Wire Leveling

Three-wire leveling is the process of reading and recording a rod reading for each of the three horizontal cross-hairs on each shot, then averaging the readings for agreement with the center reading. This method is most accurate as it gives immediate results and a check/confirmation of rod reading. If a difference is detected a check can be done on the spot, before moving on.

An electronic digital level can perform and record this process automatically, after sight of the rod is made. Automated data collection eliminates transposition errors and data is downloaded into a computer for complete analysis.

This is the preferred method for setting or checking control bench marks. Three-wire leveling will be used for all orders of leveling except general.

Bench Marks (BM) and Temporary Bench Marks (TBM)

The primary purpose of running a level loop is to establish points of known elevation or bench marks. They are solid, well protected points that can be relied upon to remain undisturbed and unchanged. They should be positioned so as to be usable from a wide area and away from the construction as much a possible so as to remain undisturbed. Consideration should be given to construction activities such as utility re-location proposed within the ROW. It is preferred that bench marks are located in a public ROW, to allow for continued accessibility.

The exact location of the bench mark should be carefully selected, then sketched and recorded in notes, a field book or a data collector.

The numbering or identification system should be consistent with district numbering convention, if any and should be identified with enough detail for another crew to locate the bench mark easily. Bench marks set should always be turned through as part of a closed level loop.

Examples of good bench marks are an iron rod driven to refusal or a TxDOT cap set vertically in a concrete monument (similar to a Type II ROW Monument), a ROW monument, a point or corner of a stable concrete structure, or occasionally a spike set in a large tree or utility pole. It is usually advisable to use a variety of types of bench mark monuments as utility relocation or construction activity may remove objects from a ROW.

The bench marks should not include objects subject to tampering or removal.

Two or more bench marks should be used from the specified reference datum (NAVD 88 unless directed by TxDOT). These bench marks should be identified, confirmed undisturbed, and elevations proved by running a complete level loop between the two, returning to the starting bench mark.

Distance between bench marks should be confirmed with the TxDOT project manager or the district survey coordinator before beginning a project.

Turning Points/Temporary Bench Marks (TP/TBM)

Turning points may be points set either before or during the course of the survey, or natural or man-made points in the area. They must be solid, well defined (or marked) and permanent enough to remain intact until the level loop is finished. Points with a small, sharply defined top are preferred to large flat surfaces.

Turning points should be marked when used so as to insure that the rod is in the exact same place when the backsight and foresight are taken. They are also marked because turning points that are part of a closed level loop are points of known elevation that may have value during future surveys in the area.

Temporary bench marks can be turning points that remain or additional intermediate bench marks placed for added convenience.

Temporary bench marks set in trees or power poles should consist of a spike (railroad spike, boat spike, or large nail spike) set horizontally approximately 1 to 2 ft. above ground elevation, also free of above obstructions to the level rod.

General Considerations / Objectives

During the course of running a level loop, choose/set turning points and bench marks to accomplish the required objective and accuracy.

Balance shot distances – The rodman and instrument man must work as a team to balance the backsight and foresight distances. This can be accomplished by use of a digital level, estimating distance by pacing, three-wire stadia difference or, when available, by observing stationing marked on the project. Balanced backsights and foresights, essential in precise leveling, will help eliminate errors caused by refraction, the curvature of the earth and an instrument that is out of adjustment and are an especially necessary procedure when establishing control bench marks.

Maximum sight distance – care should always be given to observe recommended or required distance of sight, depending on the purpose of leveling. See the NGSspecifications and Table 4.3 TSPS Manual of Practice Chart for Tolerances for Conditions, found in this chapter, or the manufacturer’s recommendations.

Control points and bench marks should always be set to the highest level of accuracy suitable for the project or a higher level if it can be justified.

Lesson Note On Surveying Field Work

Surveying Field Work

Field work in surveying consists of making and recording measurements. The operations are as follows:

1. Measuring distances and angles to:
   • establish points and lines of reference for locating details such as boundary lines, roads, buildings, fences, rivers, bridges, and other existing features
   • stake out or locate roads, buildings, utilities, and other construction projects
   • establish lines parallel or at right angles to other lines, measure inaccessible distances as across rivers, extend straight lines beyond obstacles such as buildings and do any work that may require use of geometric or trigonometric principles.
2. Measuring differences in elevations and determining elevations to:
   • establish permanent points of known elevation (bench marks)
   • determine elevations of terrain along a selected line or area for plotting profiles and computing grade lines
   • stake out grades, cuts, and fills for construction projects.
3. Making topographic surveys wherein horizontal and vertical measurements are combined.
4. Recording field notes to provide a permanent record of the field work.

Planning

Planning is probably the most important part of the performance of a control survey utilizing GPS survey measurement techniques. Proper planning will give one added confidence that quality data will be collected. Regardless of the level of the survey, the items listed below should be addressed before the field data collection process begins.

Lesson Note On Factors Affecting Field Work

Factors Affecting Field Work

The surveyor in the field must constantly be alert to the different conditions he or she encounters and the requirements of the survey. The weather, terrain, personnel, purpose, and accuracy of the survey, systematic procedures, and the expected rate of progress are some of the factors that will affect the work.

Physical factors such as terrain and weather will affect each field survey in varying degrees. Measurements using telescopes can be stopped by fog, mist, or dust. Swamps and flood plains under high water can impede taping surveys. Lengths of light-wave distance measurements are reduced in bright sunlight. Generally, reconnaissance will predetermine the conditions and alert the survey party to the best method to use and the rate of progress to be expected.

The status of training of the personnel is another factor that affects field work. Experience in handling the survey instruments and equipment can shorten survey time without introducing errors, which would require resurvey. The personnel factor is a variable that will affect the rate of progress.

The purpose of the survey will determine the needed accuracy, which, in turn, will influence the selection of instruments and procedures. For instance, comparatively rough procedures can be used in measuring for earth-moving, but grade and alignment of a highway must be much more precise, and require more accurate measurements. Each increase in precision also increases the time required to make the measurement, since greater care and more observations must be taken.

Each survey measurement will be in error to the extent that no measurement is ever exact. Besides errors, survey measurements are susceptible to mistakes or blunders. These arise from misunderstanding the problem, poor judgment, confusion on the part of the surveyor, or simply from an oversight. By working out a systematic procedure, the surveyor will often detect a mistake when some operation seems out of place.

Survey speed is not the result of hurrying; it is the result of saving time through the following:

• the skill of the surveyor in handling his field equipment
• the intelligent planning and preparation of the work
• the process of making only those measurements that are consistent with the accuracy requirements.

Lesson Note On Field Notes

Field Notes

The field notes of the surveyor must contain a complete record of all measurements made during the survey with sketches and narration, where necessary, to clarify the notes. The best field survey is of little value if the notes are not complete and clear. They are the only record that is left after the field party leaves the survey site.

All field notes should be lettered legibly. Numerals and decimal points should be legible and permit only one interpretation. Notes must be kept in the regular field notebook and not on scraps of paper for later transcription. The field notebook is a permanently bound book (not loose-leaf) for recording measurements made in the field.

Field note recording takes three general forms: tabulations, sketches, and descriptions. Two, or even all three forms, are combined when necessary to make a complete record.

Tabulation — Measurements may be recorded manually in a field book or they may be recorded electronically through a data collector. Electronic data collection has the advantage of eliminating reading and recording errors.

Sketches — Sketches add much to clarify electronic data collection files and should be used as a supplemental record of the survey. They may be drawn to an approximate scale, or important details may be exaggerated for clarity. Measurements may be placed directly onto the sketch or keyed in some way to the tabular data. A very important requirement of a sketch is legibility. It should be drawn clearly and large enough to be understandable.

Descriptions — Tabulations with or without added sketches can also be supplemented with descriptions. The description may only be one or two words to clarify the recorded measurements, or it may be quite lengthy in order to cover and record pertinent details of the survey.

Note: Erasures are not permitted in field notebooks.

Individual numbers or lines recorded incorrectly shall be lined out and the correct values added. Pages that are to be rejected are crossed out neatly and referenced to the substituted page. This procedure is mandatory since the field notebook is the book of record and it is often used as legal evidence.

LESSON NOTE ON HORIZON : Introduction

The horizon is the line that separates the Earth from the sky. Some of the best places to see the unobstructed horizon are beaches, where the ocean meets the sky in an uninterrupted line. Here, Venus and the Moon rise together over the Atlantic Ocean horizon on the coast of Jacksonville, Florida.

The Horizon Line is the place where the ground and the sky appear to meet.It is also your eye level.

The horizon is the line that separates the Earth from thesky. 

There two main types of horizons—Earth-sky horizons and celestial horizons. Both Earth-sky and celestial horizons have different sub-types of horizons.

The local horizon, geographic horizon, and sea-level horizonare all Earth-sky horizons. The astronomical horizon and true horizon are celestial horizons. 

Earth-Sky Horizons

The local horizon, also called the geometrical horizon, is the visible boundary between the Earth and sky. The localhorizon may include trees, buildings, and mountains.

The geographic horizon is the apparent boundary between the Earth and sky. Mountains, trees, and other elevated features are not considered part of the geographic horizon.

The sea-level horizon is the geographic horizon at sea level. One of the best places to see the sea-level horizon is abeach. The ocean and the sky provide a clean, flat line where the Earth seems to meet the sky. If you're standing on the beach looking out at the sea, the part of the sea that "touches" the horizon is called the offing.

Celestial Horizons

Celestial horizons are used by astronomers. They aremeasurements of the position of the Earth relative to the rest of the sky.

The astronomical horizon is the imaginary horizontal planealways at a 90-degree angle from the observer's zenith (the point directly above the observer). Astronomical horizons are great circles that surround the observer.

The true horizon is the imaginary plane that passes through the center of the Earth, perpendicular to its radius. From orbit, the true horizon is spherical, following the shape of the Earth.

LESSON NOTE ON TYPES OF HORIZON LINE

THREE TYPES OF HORIZON LINE

1.     ASTRONOMICAL HORIZON : In astronomy the horizon is the horizontal plane through (the eyes of) the observer. It is the fundamental plane of the horizontal coordinate system, the locus of points that have an altitude of zero degrees.

2.     VISIBLE HORIZON: Horizon, in astronomy, boundary where the sky seems to meet the ground or sea. (In astronomy it is defined as the intersection on the celestial sphere of a plane perpendicular to a plumb line.) The higher the observer, the lower and more distant is his visible horizon.

3.     TRUE HORIZON : 1. The boundary of a horizontal plane passing through a point of vision.
2. In photogrammetry, the boundary of a horizontal plane passing through the perspective center of a lens system.

4.     The 'EVENT HORIZON' is the boundary defining the region of space around a black hole from which nothing (not even light) can escape. In other words, the escape velocity for an object within the event horizon exceeds the speed of light.

If the Earth is assumed to be a sphere with no atmosphere, then the distance to the horizon can easily be calculated. (Note: The Earth's radius of curvature actually varies by 1% between the Equator and the Poles, so this formula isn't absolutely exact even assuming no refraction.^[

LESSON NOTE ON Importance of the Horizon Line

Importance of the Horizon

The concept of the horizon is important to different types of work, including aviation, navigation, and art. 

Pilots use the horizon to keep aircraft level while in the air. Using a method called "attitude flying," they can control their aircraft by determining the relationship between the aircraft's nose, or front end, and the horizon. Pilots can change their altitude or flight pattern by changing thehorizon to be composed of mostly sky (increasing their altitude) or mostly ground (lowering their altitude).

Before the introduction of modern tools such as global positioning system (GPS) devices, sailors depended on a clear view of the horizon to navigate the ocean. The sun's position to the horizon told sailors what time of day it was and what direction they were sailing. 

At night, sailors could use celestial navigation, or the appearance of certain stars or planets relative to thehorizon. As the Earth turns, stars and constellations rise and set on the horizon, just like the sun. Different constellations appear at different times of the year, or are only visible from certain places. The rising of the constellation of the Southern Cross, for instance, signaled that sailors were in the Southern Hemisphere.

Artists use the concept of the horizon to create perspectivein paintings and drawings. When depicted with shapes getting smaller and less detailed closer to it, a horizon can create the illusion of depth on a flat canvas. 

The horizon was also important for early methods ofcommunication. Before the radio and telegraph were invented, people couldn't communicate with anyone farther away than the local horizon.