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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
Figure 9 Satellite availability along the surveyed track 
In these areas the final accuracy can decrease and for this 
reason some ad hoc and a posteriori survey based on different 
approaches such as the traditional one , has to be performed. In 
these area these surveys can be used not only for checking but 
also to improve the final precision of the MMS survey. These 
surveys, if referred to the same reference system of the 
GPS/RTK survey, can be used to define, locally, the reference 
frame. This determination can be used to adjust the survey 
performed by the MMS. 
Therefore, the following working phases have been identified: 
1 - Surveying of some points using GPS and total station on 
points identified on images, at the beginning and at the end of 
the track, were the navigation is relied just to the IMU sensor. 
2 -Transformation parameter determination between the two 
datasets (of the MMS and of the local survey) for the two areas 
using least square approach. 
3 - Application of transformation parameter to the MMS survey 
from the beginning to the end of the considered track. 
For the point 2 it is necessary to choose the transformation 
typology, for example a 6 parameter transformation (roto- 
translation) can be adopted. 
For the step 3 it is necessary to evaluate the strategy (time 
dependent or space dependent) that permits to estimate the 
appropriate transformation parameters for any areas along the 
considered track. 
This part of the work is still in progress and it is fundamental in 
this study to take into account also the drift characteristics of 
the used sensor. 
5. CONCLUSION AND REMARKS 
MMS constitute a useful tool for acquire high detailed survey 
combining different sensors for positioning and survey. This 
methodology was applied for the preliminary survey for the 
project of the first lag of the Bologna underground. The 
complete track start from low density area (good for satellite 
navigation) and across the city centre, where the street width is 
reduced and the presence of high buildings create obstacles for 
the sky visibility. In these conditions the positioning system 
relies only on IMU sensor which cannot maintain a good 
trajectory for long distances. 
Some considerations have been made on the repeatability of the 
survey. Two independent - forward and backward - surveys on 
the same areas have been compared and the maximum value of 
the discrepancies was contained in about ±20 cm. The 
discrepancies in height are quite the same as in north and east 
component. 
To check the accuracy of the results obtained by the MMS 
systems, some independent topographical surveys have been 
performed, by using mainly GPS-RTK technique. In these four 
areas, points well recognizable in the images have been chosen 
and surveyed, to compare the absolute positions and the 
photogrammetric measurements. 
The mean value of the r.m.s has been of 10 cm in the north and 
east component, and 5-6 cm in the height. 
These results have been obtained in areas with good sky 
visibility. 
In order to reduce this problem, data acquired by ‘external’ 
surveys can be used not only for a quality check, but also to 
determine transformation parameters along the track. These 
latter parameters can be used to correct the MMS trajectory 
between two lacks of GPS reception . These problems are the 
subjects of the follow-up study. 
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