surveying is concerned with the fixing of position whether it be control point or point of topographic detail and ,as such requires some form of refernce system.
the physical surface of the earth , on which the actual survey measurments are carried out, is not mathematically definable. it cannot therefore be used as a refernce datum on which to compute position .
alternatively, consider a level survace at all point normal to the direction of gravity , indeed the points surveyed on the physcal surface of the earthe are frequently reduced initially to their equivalent position on the geoid by projection along their gravity vectors
the reduced level or elevation of a point is its height above or below the geoid as measured in the direction of its gravity vector ,or plumb line , and is most cmmonly referred t as its hight above or below mean sea level MSL this assumes that the geoid passes through local MSL ,
which is acceptable for most practical purpose ,however due to variations in the mass distribution within the earth ,the geoid which although very smoth is still an irregular surface and so cannot be used to locate position mathematically ,
the simplest mathematically definable figure which fits the shape of geoid best is an ellipsoid formed by rotating an ellipse about its minor axis ,
where this shape is used by a country as the surface for its mapping system , its is termed the reference ellipsoid ,
the majority of engineering surveys are carried out in areas of limited extend ,in which case the reference surface may be taken as a tangent plane to the geoid and the principles of plane surveying applied ,
in other words the curvature of the earth is ignored and all points on the physical surface are orthogonally projected onto a flat plane as illustrated in ,
for areas less then 10 km square the assumption of a flat earth is perfectly acceptable when one considers that in a triangle of approximately 200 km2 ,
the difference between the sum of the spherical angles and the plane angles would be i second of arc , or that the difference in length of an arc of approximately 20 km on the earth surface and its equivalent chord length is a mere 8 mm ,
the above assumption of a flat earth , while acceptable for some positional application , are not acceptable for finding elevation , as the geoid deviates from the tangent plane by about 80 mm at 1 km or 8m at 10 km from the point of contact , elevation are therefore referred to the geoid ,at least theoretically but usually to MSL practically
an examination of figure 1,2 clearly shows the basic surveying measurements needed to locate points A,B and C, Assuming the direction of B from A is known then the measured slope distance AB and the vertical angle to B from A will be needed to fix the position of B relative to A,
the vertical angle to B from A is needed to reduce the slope distance AB to its equivalent horizontal distance AB for the purpose of plotting , whilst similar measurements will fix c relative to A it also reqires the horizontal angle at A measured from B to C (B,A,c) to fix C relative to B,
the vertical distance defining the relative elevation of the three points may also be obtained from the slope distance and vertical angle or by direct leveling relative to a specific reference datum , the five measurements mentioned above comprise the basis of plane surveying and are illustrated in ,i,e AB is the slop distance AA the horizontal distance AB the vertical distance BAA the vertical angle , and A,AC the horizontal angle (0)
it can be seen from the above that the only measurement needed in plane surveying are angle and distance , nevertheless , the full impact of modern technology has been brought to bear in the acquisition and processing of this simple data angles may now be resolved with singles -second accuracy using optical and electronic theodolites : electromagnetic distance measuring (EDM) equipment can obtain distance up
to several kilometres with millimetre precision , depending on the distance measured ; lasers and north- seeking gyroscopes are virtually standard equipment for tunnel surveys; orbiting satellites are being used for position fixing offshore as well as on; continued improvement in aerial and terrestrial photogrammetric and scanning equipment makes mass data capture technology an invaluable surveying tool; finally , data automatic plotting of field data
