BMKG Semarang. BMKG’s data is not corrected yet because tide gauge is located in unstable soil which sinking linearly. To obtain real sea level,
BMKG’s data must be corrected with average of regional sea level change. The period of data from BMKG and regional sea level change from University of
Colorado must be the same to obtained the correct result. Real sea level can be calculated as given:
1 where
is real sea level, SL is recent sea level from BMKG and Sreg is average of regional sea level change.
3.4.2. Estimating Land Subsidence
This research estimating land subsidence level by calculating the deviation from sea level which was recorded using tide gauge by BMKG in Semarang
City and isostatic sea level. Isostatic sea level is obtained by add up yearly sea level value in first
recorded year with yearly change of regional sea level. By this calculation, the isostatic sea level in next year can be calculated. Land subsidence level can be
calculated then by calculating the average of deviation in last year of data recorded by BMKG and isostatic sea level in same year.
2 where
is yearly land subsidence level, SL
ly
is sea level in last year data recorded from BMKG and SL
r
is real sea level value in the same year with SL
ly
and n is time period year.
3.4.3. Digital Terrain Model
Digital terrain model in this research is not measure directly when field survey was conducted. To represent terrain pattern, the combination of land
subsidence level and data of terrestrial survey from Public Work Agency of Semarang city was used. When recent terrain model has been drowned, the
submergence area can be drawn easily based on height pattern of the research location. The general step of terrain model mapping can be seen in figure 3.3
Figure 3.3. Flowchart for terrain model of Semarang City coastal area
3.4.4. Landuse and Infrastructure Mapping
Landuse and infrastructure is the properties that related to human activity. It change along the time whether it getting better or worse. In Semarang City, it
seems to be worse because the fact nowadays some of infrastructure has been covered by water permanently and can’t be used anymore as it function. One
factors that causing its worse is flooding. Analyzing the change as the effect of flooding, landuse and infrastructure map is needed and it will be easily to
analyzed if the format of map is in digital format. To create digital landuse and infrastructure map, there are two sources that
can be used i.e. printed rupabumi map of Semarang City that has been released by BAKOSURTANAL in 2002 and digital satellite images that derived from
ALOS-AVNIR in 2009 with 10 m of spatial resolution. The format of both data source is different, one is digital and the other one is in analog format. To
convert analog map to be digital, it must be scanned, registered and digitized where the result of digitizing is used as the baseline for correcting satellite image
geometrically. Once it corrected, it can be used to updating digital map as digitizing result from 2002’s printed map. The steps of landuse and
infrastructure mapping in this research can be seen in Figure 3.4. 2008 Terrestrial Survey Result
Semarang City Public Work Office
Subsidence Point Survey
InterpolateGridding InterpolateGridding
Subsided area Digital Elevation Modeling
DEM
Semarang City Terain Model
Figure 3.4. Flowchart for landuse and infrastructure Mapping
3.4.5. Ground Truth
Ground truth or field survey in this research in intended to refers of a process in which form on a satellite image, result of data processing and
analysis is compared to what is in the reality at the present time in order to verify the contents of the form on the image and result of analysis. Field survey
also intends to collect some other information, which will use in this research, which cannot be derived from other sources and must be taken directly in the
research location. Data that collected from ground majority truth will be used as reference for
infrastructure updating. Ground truth also utilized to comparing the result of projection and previous occurrence of inundation. Preliminary result of data
processing was consulted to the expert as part of the research. All of that has been conducted so that the final result will not be so differ from it reality.
2009 ALOS-AVNIR Images Printed Rupabumi 1:25.000
Geometric Correction Scanning
Registration On Screen Digitizing
Creating Color Composite Image
Landuse Infrastructure Mapping Field Survey
Updated Landuse Infrastructure Map
