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TagsLens (Optics) Surveying Optics Applied And Interdisciplinary Physics Natural Philosophy
File Size9.2 MB
Total Pages34
Document Text Contents
Page 2

Co tents
Basic principles
Plane Table Surveying
Chain Surveying


Page 17

The telescope
The surveyors telescope show~ in Fig.4. provides a line of

collimation passing through the optical centres of the lenses
and the cross hairs. The e epiece magnifies both the real
image and the cross hairs in the same proportion as these are
both on a common plane. The eyepiece is usually the Ramsden
type, consisting of two pIano-convex le~ses mounted a short
distance apart in a self-contained case, wh i ch is threaded on
the outside to allow it to be screwed f'orwar-d or back in the
telescope body to bring the cross hairs and image into clear
focus. This arrangement of lenses reduces spherical aberration.
The object glass is usually a compound lens to reduce
chromatic aberration.


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Fig 4
Two forms of focussing mechanism are used to bring the

image onto the cross-hair plane as the distance of the object
from the telescope varies. These are:

(a) External focussing: The body of the telescope is made
of two concentric close fittinG barrels, the object
glass mounted in one and the eyepiece and cross-hairs
in the other. A rack and pinion mechanism operated by
the "focussing screw" advances or retracts one tube in
relation to the other, thus increasing or reducing the
distance between object glass and eyepiece/cross-hairs.
This method was common on older instruments and is more
accurate for tacheometry purposes, but has the
disadvantage that wear on the tubes may allow entry of
water, dust, etc., and impair the efficiency of the

Cb) Internal focussing: A double concave lens mounted on ()
frame is fitted inside a one piece telescope body
between the object and eyepiece lenses. The frame
pocition is adjustable by means of a rack and pinion,
as before, which slides the lens tov:ards or away from
the object glass.


Page 18

The concave len disperses the light rays from the
object glass to greater or lesser degree depending on
the position of the lens and thus allows focussing of
the image on the cross-hairs. The disadvantage of this
method is the loss of brilliance due to the extra lens.

Originally, spiders yeb was used, but these broke easily

and were difficult to replace. Today, very fine lines are
etched on a piece of very thin optical glass fastened to a
"reticule", forming an interchangeable capsule which fits
into a flanged metal ring called the "diaphragm", held in the
telescope barrel by four capstan headed screws - which should
only be touched when changing or making major adjustments to
the cross-hairs.

This term refers to relative motion between the object and
the cross-hairs when the eye is moved to and fro across the
eyepiece, and means that the image and cross-hairs are not on
the same plane. To eliminate parallax a piece of white paper
is held in front of the object lens and theeye-piece moved in
or out until the cross-hairs stand out clear and black. The
telescope is then focussed on the object and tested for
parallax again, the procedure being repeated if necessary
until the parallax is eliminated.
The spirit level

Cormnonly fastened to the ba.r-r-e I of the telescope on a
levellirig instrument, but usually on the top plate of a
Theodolite. The more sensitive spirit-levels are barrel
shaped curved glass tubes, with the less sensitive only a
portion of the surface is curved, when thee'termed non-
reversible levels. The glass tube is filled with ether or
alcohol with a sm2.11air space left to form a bubble. These
fluids are less viscous than water, ~nd have a much lower
freezing point, but a greater ey-pansion, so that a level left
in very hot sun may burst.

The top surface of the tube has graduations etched on it
which aid in centralising the bubble in the centre of its run.

The sensitiveness of the bubble is defined as the amount the
horizontal axis of the tube has to be tilted to cause the bubble
to move from one graduation mark to the next, e.g. 1 division
)0 sec. means a tilt of )0 sec. of arc above the horizontal
v.Ll L cause a "run" of one graduation.


Page 33

Trapezoidal rule

sed when there are any nu _ber of or-d i nat s or height s,
-ay be used for volumes if are~s of sections are substituted

for ordinate lengths.
Area = Interval (half sum of 1st and last ordinate + remaining


Prismoidal formula

Al and A2 areas of ends.

Am are of middle 08ction.
L over all distance between end sections.


Page 34


Plane Table Surveying is a ouick method of measuring areas
or sections of land. All of the work is carried out on ~ite
by the Surveyor. It is not very often used as a practical
method of surveying for reasons of general accuracy and
weather conditions in this country but it is very useful on
more drier climates. There are four main methods a) Radiation,
b) Intersection, c) Resection, d) Traversing.

To carry out a Plane Table Survey you will need a drawing

board on an adjustable tripod with rotating head. A small
level i.e. boat level with a compass to orientate the survey.
An Alidade sighting straight edge, plumbing fork and plumb
bob with stationers pin. Ranging poles, pegs and arrows with
steel measuring tape or chain. Some of these items are shown
in Fig.1.

To illustrate the method of Plane Table Surveying Fig.2.

shows a typical building site to be surveyed.
Set up the Table in the centre of the site and level board

ensuring that all aspects of the site can be seen. Locate all
other points i.e. change of boundary lines, positions of
.anholes, gates, poles, trees etc. Then using the Alidade

sight onto the fixed points measure the distance from the
centre peg marked on the ground. By using a suitable scale
draw in the radiated line on the paper fixed securely to the
board from the centre pin which should be directly plumbed in
over the measuring peg on the ground. When carrying out this
process the board should be clocked into position until
EiBhtings and measurements can be taken in sequence A,B,C etc.
for the whole of the site. If this procedure is carried out
carefully then a reproduction of the site will be recorded to
sc~le onto the board.


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