Coordinate Calculations
Calculations involving rectangular co-ordinates are:
a) Calculation of Whole Circle Bearing (WCB) and
distance between two co-ordinated points.
i)
Calculate the difference in easting( E) and difference in northings ( N)
between the two points.
ii)
Determine the Reduced Bearing from:
E
Reduced bearing= tan ——
N
Iii) Convert the reduced bearing to a Whole Circle Bearing.
1st
quadrant
same
2nd
quadrant
180 – rb
3rd
quadrant
180 + rb
4th
quadrant
360 – rb
The correct quadrant can be found by inspection of the relative positions of
the co-ordinated points.
iii)Calculate the horizontal distance from:
E
N
Distance = ————– and ————–
Sin WCB Cos
WCB
Using both these formulae separately gives a check on
the calculation of the WCB
A further formulae could be used to determine
distance but this does no check WCB calculation.
Distance = E + N
Example calculation
Given: Stn A
179.724
mE
414.132 mN
Stn
B
142.171
mE
372.916 mN
A
179.724
414.132
B
142.171
372.916
————
———–
E 37.553
N
41.216
Reduced bearing = tan –1 37.553
41.216
= 42.2015
WCB= 180 + 42 20 15
= 222 20 15
Distance = 37.553
= -55.758 m
Sin 222 20 15
= 41,216 =
-55.758 m
cos 222 20 15
The two distances should be the same and of course, the negative sign is
ignored.
a) Calculation of rectangular co-ordinates of a
point (B) knowing the co-ordinates of another point (A), the WCB from that
point to B (A to B) and the horizontal distance between them.
The procedure is as follows:
i)
Calculate the difference in eastings and northings using the following
formulae:
E = distance x sin WCB
N = distance x cos WCB
ii)
Add E and N to the eastings and northings respectively of the known
point.
Example- calculation
Given: Co-ords
A
137.629
mE
473.126 mN
WCB A to B
136 27 19
Distance A to
B
53.249
E = 53.249 x sin 136 27 19
= 36.684 m
N = 53.249 x cos 136 27 19
= 38.597 m
Co-ords
A
137.629
473.126
Difference
36.684
-38.597
————
————–
Co-ords B
174.313E
434.529N
————–
————–
2. Traverse calculation
The commonest form of traverse is the ring traverse, which starts and finishes
at the same point, and is calculated as follows:
a) The following data is required to compute the traverse.
i) All internal horizontal angles
ii) All horizontal distances
iii) The WCB of one leg.
iv) The rectangular co-ordinates of one point.
i)and (ii) measured in the field
(iii) Usually an assumed bearing but could be magnetic, grid or
true.
(vi) Usually an assumed value unless the traverse
is to be tied to the Ordnance survey horizontal control or similar.
b) Distribute angular misclose equally to each measured angle.
c) Calculate the forward bearing of each line, working anticlockwise around the
traverse, using:
Forward bearing = back bearing of previous line + included angle
Ensure you finish back on the original bearing as a check against error.
d) Calculate E and N for each leg using the
forward bearing and measured distance.
e) Add up all E’s and N’s to check for large errors in
observation or calculation. The sums should be close to zero.
f)Starting at the origin station, compute the co-ordinates of each
station around the traverse (working anticlockwise) finishing back at the
origin. Any variation in the co-ordinates of this station is the eastings
and northings misclose.
g)Distribute the misclosures using either the Bowditch or Transit methods.
Bowditch
correction = Distance from
origin x E (or N) misclose
Traverse length
Transit
correction (for eastings) = E from origin x easting misclose
E
Note that
all E are treated as positive when calculating corrections.
h) Calculate corrected co-ordinates.
An example traverse calculation is tabulated in Figure 4.10.
3 Co-ordinate setting out
Co-ordinated points can be set out from a horizontal control by either:
a) Using bearing and distance.
b) By intersecting theodolite rays
The following notes detail the general approach.
a) Bearing and distance method
i) Two control stations are required. Calculate the WCB and
distance from each station to the point to be set out and the WCB between the
two stations.
ii) Set a theodolite over one control station and sight the other
station (RO) with the computed WCB between the two stations.
iii) Release the theodolite upper plate and rotate the instrument until
the computed WCB to the point is obtained.
iv) Mark the direction with a nail in a peg at approximately he
correct distance away.
v)
Change face and repeat to check for instrument error.
vi)
Measure the distance to the peg at the trial position applying whatever
corrections are deemed necessary (see paragraph-4). Using a pocket tape,
position a new peg at the correct position from the trial position.
vii)
Check by using a WCB or distance from another station.
a) Intersection
by theodolite
i)
Calculate WCB and distance as in (a) (i)
ii)
Two theodolites are required one set up over each control station.
iii)
Each theodolite is sighted on to the opposite station with the computed bearing
on the lower plate.
iv)
Each theodolite is then turned until the required WCB to the point is
found. Establish a peg on line and change face to check.
v)
The required point is at the intersection of the two theoidolite lines
vi)
The set out position must be checked by either a distance measured from a
station or a WCB from a third station.
vii)
This method should only be used if the resultant triangle formed by the two
control stations and the set out point is well onditioned.
(b) Other checks
Once all the points are set out from the control, it is essential that they are
checked relative to each other. Some slight adjustment to peg position
may be required.