At this point I hope that you are aware of your
- Track Gauge
- Maximum Gradient
- Minimum Radius
With these figures you now have the basis to form a design
in your garden so you need a site plan.
One of the best tools for surveying is the eye. Carrying out
a good visual survey is the most useful method of surveying - it can also be
misleading. Without a visual horizon it is easy to get levels wrong. A large
flat lawn can appear absolutely level and even walking over it could feel level
but it could quite easily be on a slope which would make even big engines
Producing a Survey
Professional engineers nowadays will do this by establishing
at least two known points with co-ordinates. Co-ordinates are denoted in three
fields - Eastings, Northings and height. Then using a total station (an
electronic instrument that measures angles and distances) they would take
co-ordinate readings of identifiable objects such as building corners, road
edgings, fence posts, boundary lines and spot levels.
From these coordinates the engineer is then able to draw on
a computer a scale plan, complete with contour lines and points with heights.
For our purpose I am assuming that the majority of us do not own or have access
to a total station or theodolite and therefore need to carry out a survey using
a more traditional method.
For our purposes and resources this is the simplest method
of land surveying. Traditionally a measuring chain would be used. The two types
of chain would be an engineers chain at 100' long with 12" links and a Gunters
chain 66' long with 7.92" links. Incidentally, ten square gunters chains equals
A 30 or 50m tape measure will not cost too much and is a lot
easier to use than an old chain. Other than the equipment the method used to
carry out a land survey has not changed in centuries.
The basic principle is based around a triangle. Once the
lengths of the three sides of a triangle are known it can be reproduced on paper
at any desired scale using the simple instruments of a scale rule and a pair of
compasses. For mapping out a garden you would position a triangle with one side
as nearly parallel as you can get to a boundary or building. By measuring the
triangle and the short offset distances from the lines the garden can easily be
plotted. For most gardens more than one triangle would have to be
The offset distance is the distance from one of the lines to
any given object. This is measured at a right angle to the line using a tape
measure. The right angle is judged by eye and therefore should only be used for
shorter distances usually the length of a pocket tape measure is enough.
you do need to measure a further distance a right angle can by found by using a
3 4 5 triangle.
|A triangle with sides of 3 4 5 will have a
right angle on the intersection between the lines 3 and 4. Therefore if you
measured 800mm along the off set line, 600mm along the measure line then hold a
measure out from this point with 1000mm length were the two intersect will give
you a right angle.
For our purposes the easiest and clearest method
for recording the readings is to draw a quick sketch of the whole area to be
surveyed. The triangles used can be roughly drawn and the lengths of their sides
written on. The offset readings can also be written on complete with a sketch or
note of where it is taken from.
Although it will look nice a complete and
detailed sketch of the garden, it is not really necessary. Only the objects
which may affect the proposed railway need to be accurately plotted. Items such
as trees and paths which are some way away from any potential construction can
be drawn with a bit of guess work and free hand sketching. If there is the
slightest chance you may extend into these areas then take that little extra
time while you have the equipment.
In drawing your plan you first need to choose a
scale which will fit on the page. Once you have done this start plotting the
main triangles using a pair of compasses and a scale rules. By using the
measurements from your sketch you should start to see the outline of your garden
start to take shape.
||Further details can then be drawn in by free
hand. If, for example you took measurements of a path at 1m intervals, you can
draw in the path to some degree of accuracy by simply drawing a free hand line
which dissects your plotted points.|
Once all the details are completed rub out all
the triangle and offset lines and ink over all the details which you want to
keep. At this stage it is worth making several photocopies which you can then
work from. Using the scale ruler and a pair of compasses any proposed route can
be simply plotted on to the plan.
|This only produces a dimensional plan and does
not take into account of levels. If you have a garden with a steep slope
measurements should be taken by holding the tape measure level. Where there are
large steps a pole can be used to mark the end of one tape and the start of the
As in the above diagram, if you measured the
direct line from A-B you would not get the horizontal distance of 50m but
instead 50.249m - an error of 249mm enough to throw out the best laid plans.
Ground levels are also a large influencing factor on design and a proper
level survey needs to be carried out.
Next Stage - Level Survey