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.
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.
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
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.
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.