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.

DIFFERENCE BETWEEN SURVEYING AND LEVELLING:

LECTURE NOTES : WHAT IS SURVEYING?

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Surveying is the branch of applied mathematics which deals with measuring

and recording of the size and shape of any portion of the earth’s surface, and

the delineation of the same in a map or plan.

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It deals with the setting out of works such as roads, railways, waterworks,

drainage schemes etc and the calculation of areas and volumes.

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It may be taken to include levelling i.e. determination of the relative heights

of different points on the earth surface.

TYPES OF SURVEYING

1. Trigonometrical surveying : - This is for the preparation of maps of large

extents of territory

2. Land surveying: - this is ranging from the land division system and extensive

topographical surveys and work for boundary commission to small ones such

as farms and estates.

3. Hydrographical surveying: - ranging from coast surveys to plans for harbour works.

4. Engineering location surveying: - For the construction of highways, railways

and various public works.

5. Preliminary or parliamentary surveys: - This is in connection with a projected

scheme such as the construction of a railway or a waterworks.

6. Exploratory surveying: - This is for geological, engineering and mining

enterprises including archaeological expeditions.

7. Environmental survey: - his is survey carried out for forest and water

parameters.

DIFFERENCE BETWEEN SURVEYING AND LEVELLING:

Surveying consists in making measurements in the horizontal plane while

Levelling is making measurements in the vertical plane.

In surveying, the measurements consist in fixing position of points in the

horizontal plane; 2 points fix a straight line while 3 or more straight lines

determine the plan of a plane figure.

WHAT IS VISUAL SURVEY?

This is also called reconnaissance survey.

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It is the preliminary inspection of an area to be surveyed.

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It is a see-for-yourself walk-over of the ground to be used for a fish pond or a

fish farm. It is first done with a view to visualise the work to be done.

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It is the venture taken to note and identify all the parameters to be measured

or surveyed.

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It is a rough sketch of the field or fields in which all positions and stations

are made in the field book.

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It is preliminary work done whereby the routes of the main chain lines are

noted.

WHAT DO YOU DO DURING VISUAL SURVEY?

(i)

The purposes of the survey should be noted.

This includes (a) is it for pond construction?

(b) Is it for damming? (c) Is it for irrigation purposes; (d) Is it for Hydro-electric power (HEP)? The purpose will determine the extent of the reconnaissance survey.

(ii)

The water parameters to be measured should be noted as from the beginning. Such parameters include : (a) Water level; (b) Geological attributes;

(c) Soil conditions (texture, structure and permeability); (d) Water pH, hardness,

alkalinity, chloride, phosphate, ammonia, sulphide, sulphite, dissolved oxygen

etc

SURVEY IN POND CONSTRUCTION

Through topographic survey, the proposed site is inspected and professionally

approved. The water, soil and other parameters are determined. Using levelling

instrument, the topography of the site is ascertained and the area to be dug/

excavated or filled is pegged out with the use of lines. 

LESSON NOTE ON SOME TERMS USED IN LEVELLING OPERATIONS

SOME TERMS USED IN LEVELLING OPERATIONS

1. Bench mark (BM): it is a fixed point on the earth’s surface whose level above ordance

datum is known.

2. Ordnance Datum (OD): it is the mean sea

level to which all other levels are related.

3. Back sight (BS): is the first sight taken after the level has been set up. A sight taken to a point whose height is known or can be calculated.

4. Foresight (FS): The last sight taken. A sight taken to a point whose height is required to carry on the line of level.

5. Intermediate Sight (IS): it is any other sight taken.

6. Reduced Level (RL): calculated level of a point above or below the datum.

7. Height of Instrument (HI): The height of the line of collimation above the datum.

8. Line of Collimation (LM): It is an imaginary horizontal line drawn between two points.

9. Rise and fall: The difference is height or is level between two is referred to as a rise or fall.

10. Change Point (CP): the point at which both a foresight and then a back sight are taken.

LEVELING INSTRUMENTS

These include:

. A Level e.g. theodolite, transit dumpy level(automatic level),e.t.c.

. A staff

. Devices for angle measurements e.g. graphometer, magnetic compass, prismatic compass, orientation compass

. Chain or tape

. Pegs, arrows and ranging poles