Investigations

**Maintaining
Straight Lines**

Some people believe it is important to maintain
straight lines when using least squares to produce a cadastral model. The
fundamental goal of cadastral surveying is to mark the land as per the
intention of the original surveyor. If a technique to preserve straight lines
is not used, the least squares method tends to put small bends at each parcel
corner that is supposed to lie on a straight line. For this project a technique
that uses 180º angles to maintain straight lines was adopted. The technique
involves finding three nodes that are supposedly in a straight line and
entering a 180º angle using the angle input in Fixit 3. The weight given to
these angles is important in this technique. For this project a standard
deviation of 0.1 seconds of arc was given to the 180º angles. The smaller the
standard deviations the smaller the changes that are likely to be made to each
observation.

Figure
5.1.3 Difference between adjustment with and without straight-line constraints

**Maintaining
Arc Radii**

There are several curved boundaries in the
Werrington sample area. For each of these boundaries the chords bearings and
distances were entered into the Fixit 3 input. The bearings were entered as
plan bearings and the distances as horizontal distances. An extra constraint
was used to help maintain the arcs radii. This involved adding an extra node
for centre point of each arc. The radii of the arcs were added to the input
file as horizontal distances from the centres of the arcs to the points that
lie on the arcs. The standard deviations of the horizontal distances were given
a value of 0.1mm.

**Parallel
Lines**

Some people believe it is important to maintain
parallel street frontages when creating a cadastral model. To maintain the
parallel lines it was important to first maintain the straight lines of the
street frontages. This was done by using the straight line constraints
discussed previously in maintaining straight lines. Once the street frontages
had been straightened, different techniques to keep these lines parallel were
investigated.

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**Orientation**

For both the Hurstville and Werrington
datasets, the method of orientation by plan was used. Plan bearings were
entered for each line of each lot, with each plan having its own swing
parameter. This method of orientation worked well. Two other possible methods
of orientation are orientation by lot and orientation by point. To investigate
these different methods a subset of the Werrington dataset was used

**Cadastral
Reference Marks**

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Cadastral reference marks can play an important
role in coordinating land parcel corners. They can be used as the control in
the adjustment process. The Werrington dataset from Dean Watkins thesis had 37
reference marks that were measured in the field. The marks were measured using
GPS. In this project the reference marks were assumed to be free of error and were
held fixed in the adjustment. Below is an image showing the position of
cadastral reference marks with a red dot.

**Weighting
Observations**

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The weighting of observations can have an
effect on the final coordinates of an adjustment. Some features such straight
lines, parallel lines and arc radii, can be maintained by adding extra
observations and giving these observations a small standard deviation.

**Accuracy
of Coordinates**

Fixit 3 produces error ellipse which can be
used to illustrate the accuracy of the adjusted coordinates. Details of the
error ellipses are given in the Fixit 3 output files. The error ellipses
represent a 95% confidence interval. This means that 95% of the time the point
lies within this error ellipse.