Preliminary evaluation of photogrammetric-Remote Sensing approach in monitoring shoreline erosion

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Preliminary evaluation of photogrammetric-Remote Sensing approach in monitoring shoreline erosion

Sources of information
There are essentially two main sources of information: Those information obtained from field investigations conducted as part of this study and those from previous related studies and past records. Aerial photographs acquired in 1966, 1968, 1975 and 1980 were used to complement the 1986 and 1988 satellite imagery for analysing the changes particularly the shoreline. The 1966, 1968 1975 aerial photographs were taken at scale of 1:40,000 and 1:25,000 for 1980. The Thematic Mapper data of Landsat-5 acquired July 1988, and multispectral SPOT-1 data of 1986were used to provide the latest thematic information of the study site. Information obtained from field investigation were used as control to evaluate the movement of shoreline analysed by the proposed photogrammetric remote sensing approach.

Ancillary data that were obtained from current field surveys or past records including hydrographic charts. And resurvey of cadastral lots aligning the shoreline of the test site. Early small scale hydrographic surveys completed in 1937 and 1972 for the study area were used to quantify shoreline and seabed changes. The latest hydrographic survey of the site was carried out in 1988 as part of this study.

Geomertic Correction and Data merging
The aerial photographs and the satellite imagery of the area of interest were initially registered to a common geodetic base prior extraction of shoreline information. The Rectified Skew Orthomorphic (RSO) coordinate system adopted by Directorate of Surveying and Mapping, Malaysia (DSMM) were used for this purpose.

For the aerial photographs the geometric correction is performed by creating a corresponding model of the terrain from the overlapping photographs by means of stereoplotter. A WILD-AG1 stereoplotter linked to a microcomputer is used in and exterior orientation of the aerial photograph is performed emprically at six model points configured as in conventional photogrammetric approach; two at upper left and right corner, two at left and right of the principal line, and two being at lower left and right corner of the overlapped area.

The absolute orientation of the model is then performed by properly scaling and levelling it to three known ground control points priorly plottted geodetic base at scale of 1:10,000 . the three ground control points were among the nine control points used later for registering the satellite imagery to this geometric corrected model. A difference of 0.5 metre planimetry and 1.0 metre for heights is tolerable at all control points in the absolute orientation. The shoreline is then extracted and plotted. The procedure involved in the interpretation and capturing of the shoreline from this model is explained in the following section.

On the other hand, the subscene of the satellite digital data were processed in the Diplx ARIES -III , digital image analysis system. The satellite subscene data is registered to the same 1: 10,000 geodetic base by image-to-image basis. Second degree polynomial transformation function given in equation (1) were best found .to suit the transformation of the systematically corrected satellite data to Rectified othomorphic projection of Malaysia. Hashim et. al. (1988) also addressed that, second degree polynomial function show a more stable residuals pattern in the transforming level l B SOPT-1 data to RSO projection. The cubic convolution resampling scheme to a pixel size of 5 metres is then then followed ,The summary of the image-to-image registration in merging the two data set is tabulated in table l.

X= A ₁+A₂ x² +A₃ x +A₄ Y² +A₅ Y+A₆ xy
Y= B ₁+B₂ x² +B₃ x +B₄ Y² +B₅ Y+B₆ xy
Where As' , Bs are the transformation coefficients ,
X,Y are the ground control points ,and
X,Y are the image coordinates .
In merging both data sets, the shoreline extracted from the analysis by interpreting the appropriate clues of the desired shoreline is firstly converted to a raster file .the rasterization of the photogrammetrically captured shorelines is carried out by a simple vector-raster conversion as illustrared in the next section.

Table.1. Summary of the image-image registration involved in merging the data captured by photogrammetric means to the satellite subscene.

GCP	SLAVE POINTS		MASTER POINTS		RESIDUALS		Rem.
GCP	Line	Pixel	North	East	N	E	Rem.
1	149.0	114.0	592773.0	569506.0	0.2	-0.3
2	336.0	507.0	592149.0	570684.0	-0.6	-0.6
3	324.0	810.0	591106.0	571606.0	-0.1	0.6
4	619.0	199.0	591383.0	569783.0	1.4	1.1
5	786.0	370.0	590897.0	570299.0	-0.1	-0.9
6	624.0	59.0	591358.0	569366.0	-0.9	-0.2
7	76.0	281 .0	573000.0	570000.0	0.7	0.2	C
8	82.0	617.0	593000.0	571000.0	0.1	.0 .4
9	41.20	26 .0	592000.0	570000.0	0.1	0.1
10	418.0	612.0	592000.0	571000.0	-1.2	0.4	C
11	425.0	942.0	592000.0	572000.0	-0.2	0.4
12	761.0	941.0	591000.0	572000.0	4.5	-2.3	C
13	755.0	606.0	591000.0	571000.0	2.6	-0.1	C
14	748.0	270.0	591000.0	570000.0	-3.7	-0.4	C
Standard errors of pixel estimate = 0.53 m Standard errors of line estimate = 0.52 m Standard errors of pixel estimate of check points = 1.03 m Standard errors of pixel estimate of check points = 2.07 m;