P2-9-2 
Features 
GPS 
Auto-tracking 
Electro Optical 
Distance Meter 
Auto Level 
Laser Level 
Electronic Level 
Survey Range 
RTK-approx. 10km 
STA-20"30km 
approx. 2.5km 
within 100m in 
direct survey 
approx. 150m in 
direct survey 
approx. 120m in 
direct survey 
Accuracy (H) 
RTK- 3"4cm 
STA- l~2cm 
2 — 3cm(500m) 
5~10cm(l "2.5km) 
5mm( 1km) 
lOmm(lkm) 
2"3mm(lkm) 
Recovery/ 
Repetition 
good 
good 
normal 
normal 
very good 
Night Survey 
yes 
yes 
no 
yes,if auto 
no 
Effect of Climate 
small 
large 
little 
little 
little 
Staffing 
(Number) 
Fixed St.: at setup only 
Rovenat moving & 
collecting (1) 
Main Unit: at setup 
only 
Target: at all times(l) 
Main Unit: at all 
times 
Leveling Rod:at 
all times(2) 
Main Unit: at setup 
only 
Leveling Rod:at all 
times(l) 
Main Unit: at all 
times 
Leveling Rod:at 
all times(2) 
Efficiency 
good 
normal 
bad 
normal 
normal 
Price 
expensive 
expemsive 
cheap 
moderate 
moderate 
Table 2 Comparison of the Features of GPS and Laser Level 
^EQUIPMENT 
ITEM 
GPS 
LASER LEVEL 
REMARKS 
Position 
o 
X 
Height 
A 3 "5cm 
O 2 ~-5mm 
Measurement Range 
O 10km 
A 200m 
Distance from control station 
Data Stability 
A 
o 
tances compared to the electronic distance meter. 
Thus, we decided to combine the GPS advantage of provid 
ing 3-dimensional positioning from distant control points with 
the optic equipment that provides a high accuracy of several 
millimeters over short distances. 
Since the laser level had the highest possibility for automa 
tion among other optic equipment, we developed our highly 
precise subsidence measurement system with the combina 
tion of RTK-GPS and laser level. Table 2 compares the fea 
tures of GPS and laser level. 
3. Outline of the Developed System 
As described earlier, this system is a hybrid method sys 
tem combining the advantages of GPS and revolving laser 
level and using a computer. 
The following sections outlines the major components of 
this system, RTK-GPS and laser level, then describes the 
outline and principles of the combined system. 
RTK-GPS features real time 3D positioning, and consists 
of a GPS control station that transmits correction informa 
tion and the GPS rover stations that receives the data trans 
mitted from the control station to determine its position, as 
shown in Figure 1. When the distance between the control 
station and rover station is within 5 — 10 km, the system 
provides a horizontal accuracy of ± 2 " 3 cm and height 
accuracy of ± 3 " 4 cm. 
Figure 2 shows the chronicle data of height measurements 
in RTK-GPS fixed point observation. The measurements 
show a fluctuation of approximately ± 10 cm. When an 
accuracy of ± 1 ~~ 2 cm is called for using RTK-GPS, it 
will require adjustment over a long period of time. Figure 
3 shows the results of observing the relationship between 
the adjustment time and accuracy. These results indicated 
that an accuracy of within ± 10 mm can be achieved in 
approximately 4 hours of adjustment. 
3.1 Outline of RTK-GPS