Accuracy control. Considering that the variable of time represents an important factor to
monitor the dynamic behaviour of structures or buildings, an accuracy control of the external
reference frame definition based on high precision topography is assessed. Due to the complexity
of the mediaeval wall, two reference systems are established independently for the outer and inner
side of the walls. A total of seven control points are surveyed from the outer side, while 10 control
points are surveyed from the inner side of the wall. Particularly, control points are measured using
high precision topographic equipment, a Leica TCA2003 total station [15], and applying multiple
intersection techniques, taking angular and distances measurements. The horizontal angles are
observed by directional method, reading the horizontal circle in both the backsight and foresight
directions. Multiple observations of the angle are made, with the circle being advanced prior to
each reading to compensate for the systematic errors. Each angle is measured six times (three
direct and three reverse). The final angle is taken as the average of all measured values. Likewise, a
total of six distances (three direct and three reverse) are observed for each control point. The final
distance is taken as the mean of all measured values.