Forest Engineering & Surveying – An Introduction

Forest Engineering & Surveying

Forest Engineering is a unique branch of study wherein scientific, social, economic, and practical knowledge are applied so as to invent, design, build, maintain, and improve the physical structures, equipments , machines, materials, and processes systems concerning to Forest. This discipline of engineering is broad, and covers a range of more specialized other fields of engineering, each with a deeper  emphasis on particular areas of applied technology, science, and types of application.

Survey, scope and types of surveying,

Webster’s Dictionary defines the term the term “Survey” as "to determine and delineate the form, extent, position, etc., of a tract of land, by taking linear and/or angular measurements and by applying the principles of geometry and trigonometry." Thus,  Surveying can be defined as the art of making such observations and measurements—linear and or  angular—as will determine the relative positions of structures on the  surface of earth , an representing the same on a plan or a map, drawn to a fixed scale.

The drawing is called a Play, if the scale is large enough to show even minor details. When  the scale is small and the drawing translates over  a large area, showing a few details only, it is called a Map

In short, Surveying, may be regarded as the science and art of map making. The process of surveying includes both field work of taking measurements and office work of calculations and drawings . The drawing usually shows horizontal distances; Vertical distances may also be approximately represented, by means of hachure, contours, etc.

 Leveling involves the determination of elevations and relative heights of various points or objects on the surface of earth , above a fixed reference point called datum.

Leveling also deals with measurements in a vertical plane. Levelling and Surveying are generally considered as distinct operations. However,  Leveling is only a branch of Surveying-- the latter including the former.

Objects and Scope of Forest Surveys:

Knowledge of theory and practice of map reading and surveying are an important part of a forester's training. He/ She should be able to prepare maps and interpret them so as to deliver to his role in management, protection, administration of a forest estate. A forester is often required to  map out fire-burnt areas ;  to  demarcate or check forest boundaries ; detect and 'rectify encroachments and illicit possessions, lay out felling coupes prepare plantation maps and stock maps ;; align extraction roads and paths, and prepare plans of areas to be cleared, or fenced or planted ; for a variety of other purposes connected with his duties.

Following are the types of surveying that are mostly used by a Forester:

( i) Chain Survey

(ii) Chain and Compass Survey

( iii) Plane- -table Survey

(iv) Levelling.

 

CLASSIFICATIONS (BRANCHES/DIVISIONS/) OF SURVEYING

• Primary classification : Surveying may be broadly categorized into :

Plane Surveying, it is assumed here that the surface of the earth is plane, and not curved as it actually is ; that is curvature of the earth is ignored. As, Upto about 250 sq. km area, the spherically of the earth introduces only a negligible error, such that the surface of the earth can be assumed as a plane in  ordinary surveys. Thus, the line joining any two points on the earth's surface is considered  as straight and not curved, and the angle between any two straight lines on the surface as plane and not spherical. It is used for ordinary surveys where high degree of precision is not requisite such as forest surveys,

In Geodetic Surveying, the curvature of the earth's surface is taken into consideration. A line connecting any two points on the surface is considered to be an arc and not a straight line, and all angles of triangles as spherical and not plane. It, thus, involves know-how of spherical geometry and trigonometry. Geodesy or Geodetic Surveying is used where area involved is large and great accuracy and  precision desired. In Geodetic Surveying, refined methods of observation and computations to determine the absolute position of a series of points on the earth's surface that  as controls for all other surveys. Such surveys, called Geodetic or Great Trigonometrically Surveys, are usually of a national character and carried out through Survey of India Department, Govt of India.

 

1. b) Secondary classification of Surveying, Surveys may further be classified and sub-classified in a variety of ways :—
2. Classification based on the nature of the Field of Surveys

(a) .Land Surveys (b) Marine or Navigation Surveys (c) Astronomical Surveys

Land Surveys are further classified as;--

(i) Topographical Surveys, carried out for delineating the natural physical features of the earth's surface, such as hills, valleys, ridges, rivers etc

(ii) Cadastral Surveys are those in which additional details such as boundaries- of paths,  fields, and other such artificial details are also determined. These are field-to- field surveys conducted by Govt. in a Revenue in a village, for recording existing rents ,settlement of land revenue, and fixing fair rents. The scales usually adopted are 1/4000, 1/2000 and 1/1000 corresponding to old scales of 16".1 mile for villages. 32 inches or 64 inches to a mile for towns.

(iii) City Surveys for laying drainage channels, sewers,  roads, streets, , water pipes,

(iv) Engineering Surveys for designing and construction of engineering projects, such as canals, railways, roads, etc

 

I I. Classification based on  Methods Employed in Surveying :

(I) Traverse Surveys       (ii) Triangulation Surveys

III. Classification based on Instruments Used:

(I) Chain Surveys

(ii) Compass Surveys

(iii) Plane-table Surveys

(iv) Theodolite Surveys

(v) Photographic Surveys

(vi) Technometric Surveys.

Iv. Classification based on Objects of Surveys

(i) Geological & Soil Surveys

(ii) Mineral Surveys

(iii) Archaeological Surveys

(iv) Military Surveys.

Two Great Principles of executing a Survey —

First principle is to work from the whole to the  part and not from part to the whole.

The soul of this principle is to establish a system of control points with high level of accuracy, by traverse or triangulation. However, among them the job may be done by less accurate and, as such, less expensive methods.

For example, in triangulation survey, the main triangles should be of as large size as possible and hence as few as possible, and surveyed with greatest accuracy. These will be further sub-divided into smaller minor or secondary triangles, That will be generally measured with less rigid methods , and thus by using  less elaborate instruments.

The idea of working in this way is to avoid the accumulation of error, which could   have occurred if  we proceeded from part to the whole, and also while  localizing minute errors, if any. This principle also applies to similar operations such as leveling. Thus, in contouring on a large scale it is advised  to establish a system of bench marks with accurate precise level. The actual survey of the contours or form lines can be done by using an Indian Clinometers or  Abney Level  or a similar instrument of less accuracy. In the method of traverse survey by chain angles, minor errors get magnified as work progresses. This method is opposed to the above principle and should be moved to only in exceptional situations.

 Second principle is to work out the dimensions of main lines and positions of the new points by at least two independent processes. The points may be fixed by -linear, angular or both measurements. In chain survey, measurements and plotting of the main lines and station points are checked by use of check or tie lines.

Methods of Locating a Point

Relative positions of points are ascertained by measurements from at minimum two points of reference whose positions are correctly known. Let A & B be the two known points on the ground and the plan, and let C be the point that is to be located w.r.t. A & B on paper. - This can be done by any of the following methods as explained in the figure below

 

 

Step 1: Measure AD or BD and perpendicular CD : Fig

Step 2: Measure AC and BC : Fig. 1.1 (b).

Step 3: Measure angle BAC and distance AC ; or angle CBA and distance BC :

Step  4: Measure angle CBA and distance AC or angle CAB and distance BC :

Step 5: Measure angles CBA and CAB : Fig. 1.1 (e).

Applications of survey:

The surveying methods can be used for the design and construction of:

1. Farm road
2. 2. Irrigation and drainage channels
3. 3. Planning of farm building and service structures
4. 4. Soil conservation structures
5. 5. Preparation of field map
6. 6. Laying of railways and bridges culverts etc.