Detail Surveys
Detail surveys are a basic prerequisite for building and land development
In most cases, the purpose of detail surveys is to indicate features on, and adjacent to, a
property
There are a number of site features and levels required by architects and planners.
We can undertake topographic surveys using a range of technologies, from remote
access technologies, Total Stations and scanners (where there may be restricted
access) to GPS technology (large scale topographic surveys).
Features of general detail surveys include:
Title information
Contours
Spot levels
Site co-ordination and location of true north
Location of existing visible services (sewerage, power poles, telephone)
Outline of adjacent buildings
Window position and heights for adjacent buildings facing subject site.
Height and width of major vegetation
Location of ancillary structures (garden sheds)
Level benchmark to Datum where applicable, etc
Or any other feature of a site that may impact upon development potential.
Site detail extras may include accurate boundary definition, boundary marking and
services search.
Highly accurate internal surveys of buildings can also be provided to show room layouts
or design purposes.
Detailed Surveying
To locate topographical and man-made features relative to a survey framework, measurements
are made with the tape from the lines during the course of chaining. The control framework is
used to fix detail or engineering features. Distinction is made between soft and hard detail.
Hard detail implies well-defined points of reference such as buildings and walls, while soft detail
refers to the natural features such as riverbanks.
There are 2 basic ways in which details can be fixed
offsetting
tie-lines
An offset from a traverse line to a point is the perpendicular distance of a point to a traverse line
Whereas
A
Tie Line is a line drawn from the desired point of detail to the traverse line. Such
measurements should be as short as possible.
.
Chain /Traverse line
Detail
Off-sets
Traverse line
Farm
Note: Other means of checking the survey
include radiation, intersection and resection.
existing triangles, should be incorporated where
Sides Measured by tape
Tie lines
Additional
necessary
check
to
lines,
check
crossing
on
the
measurements taken. Intersection points of the lines are called stations.
Radiation
The simplest form of radial survey only requires a single survey assistant with a tape measure
and compass. One end of the tape is attached to a point in the middle of the site. The diver
simply swims around the site recording the distance and bearing of each feature from the
central point.
Plane Control
The general approach to any survey is to locate the general position of pre-marked control
points with respect to the local, natural or engineering features (details). The most satisfactory
method used is the co-ordinate system but bearings may also be used.
Co-ordinate System
In the simplest surveys covering small areas, the relative positions of the control points are
calculated in a co-ordinate system, rather than directly plotted by scale and a protractor. This is
the most satisfactory method because:
-
it enables errors to be assessed or adjusted
-
each station is plotted independently from precise calculations
-
it is independent of angle measuring instruments
In plane surveying a system of plane rectangular Cartesian co-ordinates is used to define the
positions of points in plan. The axes of such a system have to be defined and it is usual in
practice to adopt north and east directions for this purpose. The N-S axis is the principle
directions or reference meridian to which bearings are related. This can be chosen from one of
the following:
-
the true meridian or true north
-
magnetic north
-
natural grid north
-
An arbitrary direction e.g., one selected like which is the convenient direction.
The positions of points may also be plotted as:
-
grid co-ordinates (eastings and northings)
-
polar co-ordinates (distances (d) from origin & angle of set from a set line)
-
geographical co-ordinate system (latitude & longitude)
Bearing System
The use of bearings is another method of describing the direction of a line. Bearing of a line is
defined as the smallest angle, which that line makes with the reference line. The approach
used in plane surveying is such that angles do not exceed 90. In this manner, bearings are
measured relative to the north or south ends of a meridian and are placed in one of the
quadrants so that they have NE or NW.
N
Accordingly
B
60
132
A
284
C
-
AB N60 E,
-
AC S48 E,
-
AD N76W,
Control Survey
Two different methods of control survey will be considered namely Traversing and
Triangulation. As implied previously in chain surveying, the relative positions of a framework of
reference points and features are determined to act as a base for detail surveying. Control
frame works can be formed using a variety of observing techniques and the positions and
orientation fixed with respect to a known reference.
Compass Traversing
In this method a series of control points each one being inter-visible with its adjacent station will
be chosen to fulfil the demands of a survey. The lines joining these stations are called traverse
lines. The survey then consists of the measurements of angles between successive lines (or
bearings of each line) and lengths of each line. Given the co-ordinates of the first station and the
bearing of the first line, the co-ordinates of all successive points can be calculated.
If the figure formed by the lines closes in at a station (starts and finishes at a point with known
co-ordinates), forming a polygon then a closed traverse has been obtained. The two forms of
closed traverse being a closed-loop traverse and a closed line traverse. A traverse starting at
say station A and ending at station T that had not been co-ordinated previously is called
unclosed traverse.
See figures below
B
C
K
M
A
R
D
R
D
Closed line traverse
G
Closed loop traverse
E
T
R
S
Q
P
Unclosed traverse
Each type has its own particular use but the closed traverse is the most satisfactory figure since it’s
the easiest to which correction of errors can be applied. The unclosed traverse survey can be carried
out when the survey is comparatively long and narrow such as for a pipeline. A closed traverse
survey may be used for a framework of surveys for housing or factory sites, determination of
perimeter of a forested area. They may also be used when setting out tunnels, which are being
driven under, built up areas. A precise control survey is often used for original mapping and for
setting out of linear engineering works such as roads. A theodolite measures the angles.
Choice of station
Stations should be chosen aiming at
1. Good visibility between stations and bearing in mind the detail that has to be picked
2. Stations should be chosen according to the nature of the place
3. Stations should be chosen so that there is no displacement of stations overtime in case there is
a time lag between the start and end of the survey.
4. Stations should be able to serve for control of levelling, contouring operations and detailed
survey.
Errors
Depending on the degree of accuracy, the measurements should be corrected for errors due to
sag, temperature, slope and any other defects.
Example
C
B
D
A
F
E
Closed loop traverse
LINE
ANGLE
A
B
C
D
E
AB
BC
CD
DE
EF
MEAN INCLUDED (<)
60
126
97
65
67
DISTANCE (M)
100
25
40
35
56
Angles are observed clockwise.
D
B
A
C
Unclosed traverse
O
OBSERVED HORIZONTAL CLOCKWISE ANGLES
O - A - B 203
A - B - C 225
B - C - D 124
HORIZONTAL DISTANCES
AB 200
BC 235
CD 210
0
