Lesson Note On Cross Staff Survey

Cross Staff Survey
 

This type of survey is undertaken to locate boundaries of a field or a field or plat and determinations of its area.

Principle:

The principle of the survey is to divide the given area in to no of right angled triangles and trapezoids and to calculate and plot the areas of triangles and trapezoids.

Instruments/Material Required:

Two Chains, arrows taps, ranging rods, cross-staff and drawing material

Procedure:

Two chains are usually provided one for measuring distance along the chain line and other for measuring the offsets. The cross staff is used to set out the perpendicular directions for offsets. In this survey, the base line runs through the center of the area, so that the offsets are left or right side of base line are fairly equal. To check accuracy length of the boundary lines may also be measured. After the field work is over, the survey is plotted to a suitable scale.

Calculations of Field Area:

By this method of survey, the field as divided in to right angled triangles and trapezoids are calculated as under:

1. Area of right -angled triangle = ½ base x Height.
2. Area of trapezoid= sum of parallel sides/2 x Height.

Add the areas of all the triangles & trapezoids and sum is equal to the total of a field.

The computations for area should be written in a tabular from as given below.

TABLE

Serial no.	Figure	Chain age in m	base in m	Offset in m	Mean offset in m	Areas in m2 A=col. 4.xcol.6
1	2	3	4	5	6	7
				Total		m2

Lesson Note On Principle of Leveling:

Principle of Leveling:

1. Simple leveling:

It is the simplest operation in leveling when it is required to find the difference in elevation between two points both of which are visible from a single position of the level. Suppose A and b are two such point and level is set up at 0, approximately mid way between. A and B but not necessary on the line joining them, after finding the reading on point A and point B, let the respective reading on A and B be 2.340 and 3.315 difference between them is 3.315-2.340=0.795 m.

2. Differential leveling:

This method is used in order to find out the difference in elevation between two points.

1. If they too apart.
2. If the difference in elevation between them is too great.

In such cases it is necessary to set up the level in several positions and to work in a series of stages. The method of simple leveling is employed each of the successive stages. The process is also known as compound continues leveling.

Methods of Determination of the Reduced Level of Point from the Staff Reading

1. Collimation Method:

It consist of finding the elevation of the plane of collimation ( H.I.)  for every set up of the instrument, and then obtaining the reduced level of point with reference to the respective plane of collimation.

1. Elevation of plane of collimation for the first set of the level determined by adding back side to R.L. of B.M.
2. The R.L. of intermediate point and first change point are then obtained by starching the staff reading taken on respective point (IS & FS) from the elation of the plane collimation. [H.I.]
3. When the instrument is shifted to the second position a new plane collimation is set up. The elevation of this plane is obtained by adding B.S. taken on the C.P. From the second position of the level to the R.L.  C.P. The R.L. of successive point and second C.P. are found by subtract these staff reading from the elevation of second plane of collimation Arithmetical check

                           Sum of  B.S. – sum of F.S. = last R.L. – First R.L.

2. Rise and Fall Method:

It consists of determining the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. The difference between there staff reading indicates a rise fall according to the staff reading at the point. The R.L is then found adding the rise to, or subtracting the fall from the reduced level of preceding point.

Arithmetic check

Sum of B.S. – sum of F. S. = sum of rise – sum of fall = last R. L. – first R.L.

Booking the staff readings:

The following points may be kept in mind entering the readings in a level field book.

1. The reading should be entering in the respective columns and in order their observation.
2. The first page is always a back side and the last one is ways a foresight.
3.  It a page finished with an IS reading, the reading is entered in the IS and FS columns on that page and brought forward to the next page.
4. The FS and BS of any change point are entered in the same horizontal line.
5. The RL of the line of the collimation is entered in the same horizontal line.
6. Bench marks and change points should be clearly described in the remark column.

Specimen pages of level field book: Collimation system

Station	Distance (cm)	Reading		RL of plane of collimation [HI]		Reduced level			Remarks
		BS	IS	FS
A								B.M.
B
C
Arithmetic check	Sum of BS-Sum of FS=  Last RL-1st -RI

Specimen pages of level field book: Rise & Fall System

Station	Distance(m)	Reading			Rise	Fall	Reduce level	Remarks
		BS	IS	FS				B.M
A
B
C
	Check :	BS-FS=Rise-fall=last RL -1st-Rl

Classification of leveling

Classification of leveling

1. Different leveling:

It is the operation of leveling to determine the elevations of points. Some distance a part or to establish bench marks.

2. Check leveling:

It is the operation of running levels for the purpose of checking the series of levels, which have been previously fixed. At the end of each day’s work, a line of level is run, returning to the starting point of that day with a view to check the work done on that day.

3. Profile leveling:

It is the operation in which the object is to determine the elevation of points at known distance apart along a given line, and thus to obtain the accurate out line of the surface of the ground. It is called the longitudinal leveling or sectioning.

4. Cross sectioning:

It is the operation of leveling to determine the surface undulation or outline of the ground transverse to the given line and on either side of it.

5. Reciprocal leveling:

It is then method of leveling in which the difference in elevation between two points, accurately determined by two sets of observation when it is not possible to set up the level midway between the two points.

6. Barometric leveling:

It is the method of leveling in which the altitudes of points are determined by means of a barometer, which measures atmospheric pressure.

7. Hypsometry:

It is the method of leveling in which the heights of mountains are found by observing the temperature at which water boils.

8. Trigonometric leveling:

It is then process of leveling in which the elevations of points are computed from the vertical, angles and horizontal distance measured in the field.

Steps in Leveling:

When the level is set up and correctly leveled, the lines of collimation will be horizontal. When the telescope is rotated about its vertical axis, it will revolve in a horizontal plane known as the plane of collimation. Therefore all staff readings taken with the telescope will be vertical measurements made downwards from this plane. There are two essentials steps in leveling.

1. To find the elevation or R.L. of the plane of collimation (H.I) of the level by taking a back sight on a bench mark.
2. To find the levitation of R.L. of any other point by taking a reading on the staff held at the point.

Height of Instrument (H. I.) = R.L. of the plane of collimation
                                         = R.L. of B.M. + B.S.
                R. L. of point    = H.I.-F.S.
                                         = H. I. – I.S.

It is the necessary that after every back side. [However many intermediate sight may be], there must be a foresight. Leveling should always commence from a permanent common bench mark and end on a permanent bench mark.

Lesson Note On Introduction to Runoff

Introduction to Runoff

Over the land surface, for the generation of runoff, the primary source of water is Rainfall. A part of rainfall that intercepted by the vegetation, buildings and other objects and prevented to reach them on grand surface is called as interception. Part of rainfall stored in the surface depressions which in due course of time gets infiltrate or evaporated is referred as depression storage [ Initial detention).

When these entire loses are satisfied then excess rainfall moves over land surface is known as overland flow and draining the same into channel or stream is termed as “Runoff”.

Definition:

Runoff:

Runoff is that portion of the rainfall or irrigation water [or any other flow]. Applied which leaves a field either as surface or as subsurface flow.

When rainfall intensity reaching the soil surface is less than the infiltration capacity, all the water is absorbed in to the soil.  As rain continues soil becomes saturated and infiltration capacity is reduced, shallow depression begins to fill with water, then the over flow starts.

Surface detention/ Detention storage:

The amount of water on the land surface in transit to words stream channels is called detention storage/surface detention.

Surface Runoff:

The runoff which travels over the ground surface to the channels of watershed

Subsurface Runoff:

The portion of unfiltered water which penetrated to shallow depth travels laterally and is intercepted by channels.

Runoff Cycle:

It is that part of hydrological cycles which galls between the phase of precipitation and its subsequent discharge in the stream channels or direct return to the atmosphere through evaporation and evapotranspiration.

Conditions Associated With Runoff Cycle:

1. This refers to the end of day period and beginning of the intense and isolated storm.
2. It is the stage after beginning of rainfall causes the overland flow, base flow, and development of channel storage.
3. It refers to the condition approaching the end of all isolated intense storm.
4. This is the stage indicating after end of rainfall where rainfall causes the overland low, base plot and development of channel storage.

Types of Runoff:

1. Surface runoff
2. Sub-surface runoff
3. Base flow

a. Surface Runoff:

That portion of rainfall which enters the stream immediately after the rainfall. It occurs when all loses is satisfied and rainfall is still continued and rate of rainfall [intensity] in greater than infiltration rate.

b. Sub-Surface Runoff:

That part of rainfall which first leaches into the soil and moves laterally without joining the water table, to the stream, rivers or ocean is known as sub-surface runoff. It is usually referred is inter-flow.

c. Base flow:

It is delayed flow defined as that part of rainfall, which after falling on the ground the surface, infiltrated into the soil and meets to the water table and flow the streams, ocean etc. The movement of water in this is very slow. Therefore it is also referred a delayed runoff.

Total runoff = Surface runoff + Base flow (including subsurface runoff)

Lesson Note On Definitions of teams in Levelling

Definitions of teams in Levelling

Levelling (or Leveling) is a branch of surveying, the object of which is: i) to find the elevations of given points with respect to a given or assumed datum, and ii) to establish points at a given or assumed datum. The first operation is required to enable the works to be designed while the second operation is required in the setting out of all kinds of engineering works. Levelling deals with measurements in a vertical plane.

Level surface: A level surface is defined as a curved surface which at each point is perpendicular to the direction of gravity at the point. The surface of a still water is a truly level surface. Any surface parallel to the mean spheroidal surface of the earth is, therefore, a level surface.

Level line: A level line is a line lying in a level surface. It is, therefore, normal to the plumb line at all points.

Horizontal plane: Horizontal plane through a point is a plane tangential to the level surface at that point. It is, therefore, perpendicular to the plumb line through the point.

Horizontal line: It is a straight line tangential to the level line at a point. It is also perpendicular to the plumb line.

Vertical line: It is a line normal to the level line at a point. It is commonly considered to be the line defined by a plumb line.

Datum: Datum is any surface to which elevation are referred. The mean sea level affords a convenient datum world over, and elevations are commonly given as so much above or below sea level. It is often more convenient, however, to assume some other datum, specially, if only the relative elevation of points are required.

Elevation: The elevation of a point on or near the surface of the earth is its vertical distance above or below an arbitrarily assumed level surface or datum. The difference in elevation between two points is the vertical distance between the two level surface in which the two points lie.

Vertical angle: Vertical angle is an angle between two intersecting lines in a vertical plane. Generally, one of these lines is horizontal.

Mean sea level: It is the average height of the sea for all stages of the tides. At any particular place it is derived by averaging the hourly tide heights over a long period of 19 years.

Bench Mark: It is a relatively permanent point of reference whose elevation with respect to some assumed datum is known. It is used either as a starting point for levelling or as a point upon which to close as a check.

Methods of levelling

Three principle methods are used for determining differences in elevation, namely, barometric levelling, trigonometric levelling and spirit levelling.

Barometric levelling

Barometric levelling makes use of the phenomenon that difference in elevation between two points is proportional to the difference in atmospheric pressures at these points. A barometer, therefore, may be used and the readings observed at different points would yield a measure of the relative elevation of those points.

At a given point, the atmospheric pressure doesn’t remain constant in the course of the day, even in the course of an hour. The method is, therefore, relatively inaccurate and is little used in surveying work except on reconnaissance or exploratory survey.

Trigonometric Levelling (Indirect Levelling)

Trigonometric or Indirect levelling is the process of levelling in which the elevations of points are computed from the vertical angles and horizontal distances measured in the field, just as the length of any side in any triangle can be computed from proper trigonometric relations. In a modified form called stadia levelling, commonly used in mapping, both the difference in elevation and the horizontal distance between the points are directly computed from the measured vertical angles and staff readings.

Spirit Levelling (Direct Levelling)

It is that branch of levelling in which the vertical distances with respect to a horizontal line (perpendicular to the direction of gravity) may be used to determine the relative difference in elevation between two adjacent points. A horizontal plane of sight tangent to level surface at any point is readily established by means of a spirit level or a level vial. In spirit levelling, a spirit level and a sighting device (telescope) are combined and vertical distances are measured by observing on graduated rods placed on the points. The method is also known as direct levelling. It is the most precise method of determining elevations and the one most commonly used by engineers.

Levelling Instruments

The instruments commonly used in direct levelling are:

1. A level
2. A levelling staff

Lesson Note On Study of Contour

Study of Contour

The purpose of topographic survey is to get the necessary data to produce a topographic map of the earth’s surface. This map will include contour lines, location of natural features, such as streams, gullies, and ditches and man-made features like bridges, culverts, roads, fences, etc. which are needful for detailed planning. The best practical method of presenting topography is by means of contour maps.

Contour or contour lines:

“A contour is an imaginary line of constant elevation on the surface of the ground”. Contours are represented on the map by contour lines. The contour and contour lines are often used inter-changeably.

Contour interval:

“The vertical distance between any two successive contours on a given map is called the contour interval”. Contour intervals usually vary from 25 to 250 cm in engineering work. In rough country, the vertical distance between contours is kept greater while in flat areas 25 to 50 cm contour intervals are used.

Characteristic of Contour Lines:

1] All points on a contour line have same elevation

2] Contour line close to each other on s plan view; represent very steep ground. Contour lines for apart indicate relatively flat land

3] On uniform slopes the contour lines are spaced uniformly .along plane surfaces these lines are straight and parallel to one another.

4] Contour lines Crosse ridge lines or valley lines at right angles valley contour are convex towards the stream.

5] Contour lines can not and anywhere, but close on themselves. Either within or outside the limits of map they can not merge or cross one another.

6] A series of closed contour on the map indicate a depression or a summit, depending whether the successes contour have lower or higher values inside

7] At ride line the contour lines form carves of U shape .At Valley lines   they farm sharp curves of shape

USES OF CONTOUR:

1] Information regarding character of a tract of a country (such as flat undulating, Mountainous, etc) is abstained.

2] In agricultural work, contours maps are useful as guide lines in planning land improvement project .the tile drainage system can be conveniently planned whit contour maps

3] Cost estimates can be made with the help of the contour maps.

4. Maps which show both topography and land use capability classification are important in conversation of farm land.

5. The most economical and suitable site for engineering works such as reservoir, canal, road, waterways, .etc. can be selected.

6. Quantities of earthwork and runoff from watershed can be computed.

7. Contours may be used to determine area of the catchments and the capacity of the reservoir.

8. A suitable route of a given gradient can be marked on the map.

9. The possible route of communication between different places can be determined from contour map.

Survey for Contour Map [Grid Survey Method]

The area is divided into a series of square. The size of these squares depends upon the nature and extent of the ground. Generally, they have sides verifying from 5 to 20m or 5 to 30m. The corner of the squares are numbered serially, as 1, 2, 3 … A temporary bench mark is set up near the site. The elevations of the ground at the corners of the squares are taken and contour lines are drawn by interpolation, “the process of spacing the contour proportionately between the plotted points is turned interpolation”. For precise work, the proportional spacing of the corresponding contour between any two points is calculated and measured.