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Site Survey


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Building Index>>> Part 7

Introduction

At this point I hope that you are aware of your requirements

  • Track Gauge
  • Maximum Gradient
  • Minimum Radius
  • Clearances
  • Track Formation

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 struggle.

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.

Chain Surveying

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 one acre.

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 used.

After 10 years - still a temporary sketch

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.
If 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.

Drawing Your Plan

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 next.

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




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