36
Figure 9 Observed linear trend in the difference of mile
chaînage values
Figure 10 Comparison of the mile chainage measured by the
odometer in moving and Stop&Go mode.
4.3 Comparison between GPS and odometer
From the GPS co-ordinates a mile chainage was computed in
correspondence of every reference point. Different procedures
for extracting the mile chainage from the 3D co-ordinates were
carefully evaluated. The results achievable with different
approaches strongly depend on GPS data acquisition interval,
the speed of the vehicle, the regularity of the trajectory and of
course, the accuracy of the GPS co-ordinates. In particular, the
procedure for computing the mile chainage by accumulating the
length of the differential vectors between two subsequent
instantaneous positions of the vehicle was firstly applied. The
results show that only with accurate data (dual frequency Post-
Processed) the mile chainage estimates can be considered
reliable for positioning also along very irregular paths. In
general, when dealing with meter accuracy data the cumulative
procedure results in the accumulation of the errors, too. A
simple approach to reduce the accumulated error is to use the
minimum number of GPS positions necessary to describe the
variation of trajectory from a straight line. To satisfy more
stringent requirements accurate and dense data can be adopted
in a smoothing procedure with the aim of limiting the errors in
the mile chainage.
In this work in consideration of the regularity of the speed of
the vehicle and the highway path, a simple approach, based on
selecting no more than ten GPS positions every kilometre was
adopted for mile chainage extracting.
In the following the mile chainage measured by the odometer
working in Stop&Go mode are taken as reference values.
Figure 11 shows the results obtained from the comparison of
the Stop&Go GPS mile chainages and the reference values.
Figure 11 Comparison between the mile chainage measured in
Stop&Go mode by GPS receivers and the reference values.
The systematic effect of about 3 meters can be explained by an
overestimation of the GPS mile chainage due to residual
cumulative errors. In order to obtain better results from the dual
frequency GPS data, a dedicated fitting procedure has to be
implemented, eventually including also the integration of data
from other sensors.
In order to analyse the quality of positioning in a normal
operational environment the results from data collected in
dynamic mode are compared. The comparison between the mile
chainage measured in RTCM and Post-Processed mode by Pro
XR/XRS GPS receiver and the odometric mile chainage (Figure
12) shows the same situation discussed before.
Figure 12 Comparison between the mile chainage measured in
Stop&Go mode by GPS Pro XR/XRS receiver and the
odometric mile chainage.
4.5 A configuration of a GPS based Positioning system for
road survey
The experience gained during the field tests has been utilised
for designing an optimal hardware and software configuration
for a GPS-based positioning system, shown in Figure 13. The
proposed configuration refers to the receivers and softwares
adopted in the experiments (Trimble) and includes both Real-
Time and Post-Processing positional capabilities.
