Global Positioning System Reference
In-Depth Information
the number of satellites in view, dilution of precision (DOP) and age of corrections over the
accuracy and stability of the NRTK GPS solution was discussed in some studies, e.g. Aponte
et al., 2009.
The performance of surveying with the network RTK approach in construction sites was
evaluated by two tests. The first test was executed for checking performance in determination
of planimetric coordinates and the second test for evaluation of performance in height
determination. The first test was carried out during construction of a large building in Dubai
for checking of positions of surveying marks of the footings, landscaping and the access
road of the building. 48 points were used for checking purposes including 18 points on the
boundary of the road, 19 points for the footings, and 11 points for the landscaping. These
points were set out using a calibrated total station of 1 second precision. Next, point
coordinates were computed from the working drawings and uploaded to a GPS controller,
where a single GPS dual-frequency receiver was independently used for positioning of the
test marks by utilizing data from the Dubai NRTK, known as Dubai Virtual Reference
System (DVRS). The network consists of five continuously operating reference stations with
baseline lengths ranging from 23.4 km to 90.8 km and uses the VRS algorithm. The position
of each test point was determined after 10 seconds of data collection, which were recorded
at 2 seconds interval. The shifts between the positions determined from the two methods,
namely: calibrated total station and NRTK using observations from GPS, were measured
and compared to represent the precision of the latter method if used in construction sites
instead of the former method. Figure 4
illustrates the differences between the two methods.
The statistics of coordinate differences between the two methods in easting and northing are
given in Table 1. The average norm of the spatial differences (
) was
generally less than 1.45 cm whereas the maximum difference was 3.45 cm. The small errors
can be explained by the presence of one of the network reference stations within a few
kilometres, which would be picked up by the system as the master reference station. Thus,
most orbital and atmospheric errors were cancelled and the remaining errors would mainly
be due to data noise and multipath. These results show that the GPS-RTK network approach
can be used in the setting out of medium-accuracy surveying marks.
3
2.5
2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
3
Error in Easting Coordinate (cm)
Fig. 4. Positioning differences between GPS Network RTK and the total station
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