data from TOPEXPoseidon, Jason-1 and Jason-2OSTM has shown a steady increase in global mean sea level. The illustration of sea level increasing in 1993-
2010 can be seen in Figure 4.6.
Figure 4.6. Monthly average of global mean sea level pattern based on data of tide
gauge and altimetry satellite
The average level of rising between over from 1993-2010 based of satellite- derived data was 0.28 mmyear. If this rising is compared with rising level from
tide gauge data in 1903-1920 and 1953-1970 100 and 50 years ago, the rising average nowadays is 4.8 times higher than the value in 100 years ago and 3.9
times higher than the value in 50 years ago. Even if we compared global mean sea level from tide gauge data in the same previous periods 100 and 50 years ago
with the tide gauge data in this period 1993-2010, the rising average nowadays is 5.2 times higher than the value in 100 years ago and 4.2 times higher than the
value in 50 years ago. The value of the change is not the same between tide gauge and satellite, but the pattern of chart Figure 4.6 represent the same meaning, the
linear increasing of sea level with uncertain value. The data of sea level rise in from tide gauge and satellite in specific time can be seen in table 4.2.
Table 4.2. Global rising average from tide gauge and satellite in specific time scale
Period Global
Rising Average from tide
gauge data
mmyear Period
Global Rising
Average from
Satellite data
mmyear 1903-1920
0.058796296 1993-2010
0.28226601 1953-1970
0.072222222 1993-2010
0.317241379
4.1.4. Eustatic And Isostatic Sea Level Change
As the effect of regional and global sea level rise, mean sea level in Java Sea also increases. Mean sea level data from BMKG Semarang as the measured of
tide gauge represent those rising. The data that used in is data from January 2004 to August 2010 even there is exist previous data from 1985 to 2005 from
PT. PELINDO Tanjung Mas, Semarang Appendix 5. Those data data from PT. PELINDO can not used in this research because there is abnormal trend of
sea level change from 1998 to 2005. Different with data from PT. PELINDO, data from BMKG BMKG’s tide
gauge located in the same area with PT. PELINDO’s tide gauge shows the increase of sea level change in Semarang City follows linear regression function
where Y=10.58X-21180.35 with R
2
=0.87. In 2005, sea level average 47.764 cm was increased around 10.904 cm from it level in 2004 and level sea in 2006
50.204 cm was respectively increased around 2.440 cm from it level in 2005. Monthly mean sea level from January 2004 to August 2010 also shows the
increasing with 65.64 cm of total increase level on those periods or with monthly average of rising was 0.94 cmmonth complete data from BMKG Semarang can
be seen in Appendix 4. As the increasing of regional sea level sea level in Indonesia area with rising
level 2.535 cmyear, isostatic mean sea level in 2005 was reached 39.394 cm. isostatic mean sea level in 2006 was reached 41.929 cm, isostatic mean sea level
in 2007 was reached 44.464 cm, isostatic mean sea level in 2008 was reached
44.464 cm, isostatic mean sea level in 2009 was reached 49.535 cm, and isostatic mean sea level in 2010 was reached 52.070 cm. The complete data about local
mean sea level from BMKG and real isostatic mean sea level as the result of
calculation can be seen in table 4.3. Table 4.3. Real sea level values in Semarang City
Year MSL
from BMKG cm
Calculated Isostatic
MSL increase
2.535 cmyear
2004 36.859
36.859 2005
47.764 39.394
2006 50.204
41.929 2007
59.292 44.464
2008 80.972
46.999 2009
89.610 49.535
2010 94.669
52.070
4.2. Land Subsidence
Land subsidence in Semarang city had been occurring in northern part of its city as the area that experiencing highest subsidence level. There are three factors
that causing land subsidence in Semarang City i.e.; geological structure of soil as natural factor, groundwater extraction, and buildingconstruction load that
accelerating land subsidence in Semarang City and become major factor of land
subsidence itself.
This research was not measured subsidence directly in the field or using active remote sensing technique but the value of subsidence can be estimate by
calculating the deviation between mean sea level from tide gauge data and real sea level value. Real sea level value can be calculated by adding rising value of
regional sea level to tide gauge data in the first year in this research, 2004 is the first year when data of MSL was recorded by tide gauge and this first year data
became the baseline to estimate real sea level value in next year. In short, MSL in 2004 as the result of tide gauge recording is considered as real mean sea level at
