A2.2NP1 Environmental Practical 1. TOPIC 1 TECHNIQUES IN BASIC SURVEYING. Basic ideas. Surveying  the creation of a scale representation of the ground surface  is a basic activity in many areas of environmental management. A survey will be one of of two types:
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The basic principle of resection
Known position
Known position
Measuredangle
Unknown position
Measuredangle
Measuredangle
Known position
Similar triangles have corresponding angles equalbut are of differing size
Conguent triangles are identical
Any closed polygon can be subdivided into contiguous triangles
These should be chosen to make as many of the triangles asclose to equilateral as possible
1. A reconnaisance survey will establish the dimensions of the area, relative levels, significant features, accessibility, obstacles etc
2. Establish an accurate baseline by measurement from existing survey points, natural features, buildings etc. If none are available then the baseline must be fixed by absolute methods.
3. Establish as required any further controlpoints by triangulation or trilateration from the baseline.
4. Incorporate detail by tacheometry, traversing, tape & offset or whatever other method is appropriate.
5. The intermediate stations should where appropriate be crosschecked with the control points by resection and all traverses should be closed at a control point.
6. Inaccessible detail should be incorporated by triangulation or plane tabling from the ends of the baseline.
7. If a topographic survey is being undertaken, levelling traverses should be carried out around the survey stations and the baseline tied to the local benchmark by a closed traverse.
8. The use of a theodolite or total station will enable both the position and the elevation of stations to be found simultaneously by combined tacheometry and triangulation or by trilateration
How to establish relative plan positions
correction factor = xy  yz
= xy(1  cosa)
x
h
a
y
z
How to destermine relative elevations
Unlike chain surveys, levelling surveys account directly for slope and incorporate this data into the whole measurement exercise
AIMS:
Relies upon:
The “rise and fall” method
The absolute (datum) level of point A is 100.522m
The change of level is 2.312m  2.533m = 0.221m
The reduced level of point B is 100.301m
A
B
2.312 m
2.533m
IP 1
Datum line: 100.522m
(from OS Benchmark)
Backsight
Interm.
Foresight
Rise
Fall
R.L.
Distance
Remarks

Point A
2.312
100.522
2.533
Point B
0.221
100.301
1.2
At the next stage, B becomes the backsight and C is the new foresight
The new change of level is 1.674m  1.631m = + 0.043m
The absolute level of point C is 100.344m
B
C
1.674 m
1.631m
IP 2
Backsight
Interm.
Foresight
Rise
Fall
R.L.
Distance
Remarks

Point A
2.312
100.522
Point B
1.674
2.533
 0.221
100.301
1.631
+ 0.043
100.344
Point C
1.2
Backsight
Interm.
Foresight
Rise
Fall
R.L.
Distance
Remarks



Point A
2.312
100.522

Point B
1.674
2.533
 0.221
100.301

2.504
1.631
+ 0.043
100.344
Point C

0.956
+ 1.548
101.892
3.010

2.413
2.016
+ 0.994
102.886

2.718
0.305
102.581

11.913
9.854
2.585
 0.526
102.581
9.854
 0.526
 100.522
2.059
2.059
2.059
CHECKS
OK
The intermediate sight is taken at the base of the channel between B and C
The new change of level is 1.674m  2.988m = 1.314m
The absolute level of the intermediate point C is 98.987m
B
C
Intermediate sight
1.674m
2.988m
IP 2
Backsight
Interm.
Foresight
Rise
Fall
R.L.
Distance
Remarks

Point A
2.312
100.522
Point B
1.674
2.533
 0.221
100.301
2.988
1.314
98.987
channel
1.631
+ 0.043
100.344
Point C
Next FS
1.2