Diseño de un autotransformador con tercer devanado en delta mediante

There are a number of data sources and techniques to assess coastal change, represented by cliff-lines and shorelines in this research. The advantages and limitations o f these methods have been discussed by many researchers (e.g., Dolan et al. 1980, Dolan et al. 1983, Hooke and Kain 1982, Anders and Byrnes 1991, Byrnes et al. 1991, Moore 2000). In this section, the use of aerial photographs and topographical maps in the present research is justified. The techniques, which were developed especially for photographs with no camera information, are also illustrated.

2.2.1 Materials o f detecting coastline change

In the past shoreline changes have been determined from general patterns detected by visually comparing old photographs to highly accurate engineering surveys, based on analysis of rectified aerial photographs or satellite images, analysis of maps, and/or repeated field survey (beach profiles, cliff-line surveys, etc.). Documentary sources like travel diaries and newspapers sometimes also convey coastline change information which can be used to supplement more formal analyses. The techniques used specifically to measure cliff recession rate include: (1) the comparison o f positions o f the shoreline on different editions of maps and/or of aerial photographs, (2) the comparison of old and recent photographs, (3) direct measurement from pegs or nails driven into cliff face, (4) surveying, (5) calculating volume of talus deposits, (6) microerosion meters, and even (7) dated inscriptions o f graffiti carved on cliff faces (Sunamura 1983, 1992).

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The methods chosen depend on both the research objectives and data availability within the study area. Since the change o f a cliff-line is not as active as of a shoreline, the ideal data for analysing the spatial variability of cliff retreat is that which possesses characteristics of quantifiability, long-span record, and accuracy. The advantages and disadvantages of these methods are summarised as follows.

Documentary data, such as travel diaries and newspaper reports, are not quantifiable. These sources are 'fragmentary and frequently unintelligible', and require much search and interpretative work to be useful (Homan 1938, cited from Hooke and Kain 1982). Old photographs, usually not deliberately produced for illustrating coastline position, sometimes can provide pictorial information o f coastline change. The main disadvantage o f them is that they have almost always oblique views, and the exact scale is usually difficult to determine (Dolan et al. 1983), but they can provide a little auxiliary information in coastline change when other data are not available.

Topographical maps

In comparison with other media, topographical maps have the advantages o f long-period records, and relative cost- and time-savings, especially when extensive coastal areas are included, and have been used to quantitatively determine the historical shoreline position in many studies (e.g., Dolan et al, 1979; Galgano and Leatherman, 1991). In the USA, NOS (National Ocean Service) topographic maps, which have been surveyed and mapped specifically for coast areas, have been produced since the 1830s (Byrnes et al, 1991). The usefulness o f maps for quantifying shoreline change, however, depends on their accuracy standards and scale (Anders and Byrnes, 1991). That is if the map is compiled especially for the coast area, and if the scale is large enough and the features of interest are not undetectable due to generalisation.

Aerial photographs

Aerial photography was the first method o f remote sensing and even today in the age of the satellite and electronic scanner, the aerial photograph probably still remains the most widely used type o f remotely sensed data. Absolute advantages o f the aerial photograph are its time freezing ability and very high spectral and spatial resolution. It is often the most economical means of measuring shoreline position (Anders and Byrnes 1991). Usually cliff-lines can be traced more easily and accurately. In addition, the characteristics of availability, synoptic viewpoint, and three-dimensional perspective are also reasons which make it so popular.

» There are limitations when aerial photographs are applied to the analysis of beach changes (Stafford and Langfelder 1971): (1) the photographs record beach conditions and locations existing at a specific time; (2) only horizontal changes in beach location and areas of change can be determined and not volumes of materials eroded or accreted; (3) inherent errors that exist in the photographic image include scale variations between photographs caused by altitude variation of the photographing aircraft, scale variation within a photograph caused by camera tilt at the instant of exposure, and relief distortions caused by

elevation differences within the terrain depicted on the aerial photographs. Many studies have drawn attention to these issues. Relief displacement, however, is usually ignored when dealing with the sedimentary coast (e.g., Leatherman 1983), mainly due to the limited change on terrain’s relief. Certain specific problems may occur when applying aerial photographs to assess long-term erosion rates. In the Tertiary Mudstone area along the Oregon coast, a block slowly slides toward the ocean when wave erosion cuts back its seaward edge at just about the same rate, and the sea cliff actually remains approximately stationary in position (Komar and Shih 1993).

Satellite images

Nearly continuous monitoring of the coasts worldwide is now possible through the use of digital satellite data (Morton 1991). The earliest data set only extended back to the launch of the first satellite, LANDS AT, in 1972. Among those more readily available commercial images, the highest spatial resolution is about 10 m of SPOT, however, which was launched in 1986. The major limitation in the use o f digital satellite data for accurate analysis o f coastline change is their horizontal resolution o f a pixel (Leatherman 1993). Above all, these images are also subject to the same sea level discrepancies as aerial photographs (Morton 1991). Unless the coastline change is significant enough (i.e., much larger than the resolution of satellite images adopted) over the study period, the result will not be satisfactory.

Field surveying

Repeating topographic surveys at ground-control stations is a standard field method for quantifying shoreline change (Eliot and Clarke 1989, cited from Morton 1991). The data collection process of field survey, however, is time-consuming and usually expensive, and it can provide little help when information on past long-term shoreline change is required although the accuracy can be up to the order of 0.01 m or less (Dolan et a l 1983). Moreover, accuracy o f measurement is meaningless when it greatly exceeds the resolution of shoreline definition. Defining the cliff-line is not so problematic; on the contrary, short-term cliff-line change is sometimes not significant enough to be detected, unless the cliff forming materials

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is extremely weak, e.g., unconsolidated glacial deposits. Since the extent o f the current study area is large (> 150 km in length), field survey becomes impractical for a personal project as such.

According to previous research, the magnitude of retreat distance over several decades may be up to 100 m at certain sites along the Huatung coast (Hsu and Shen 1995). The width of 100 m in the real world is 4 mm on 1:25,000 maps, and is only 2 mm on 1:50,000 maps. The scale o f the pictorial materials applied here must be no smaller than 1:50,000. On the

other hand, the scale of 1:20,000 of aerial photographs is usually considered as the usable upper limit for shoreline change research by coastal researchers (Tanner 1978, Byrnes, et a l,

1991, Crowell et a l, 1991, Thieler and Danforth 1994). Therefore, sequential topographical maps and aerial photographs with “proper” scales seem to be the most appropriate for quantifying the coastal change along the Huatung coast.

For quantifying coastal change along the Huatung coast, great efforts have been made to collect as many pictorial materials as possible. Maps and aerial photographs that cover the Huatung coast have been carefully examined (Shen 1998) and are briefly summarised as follows.

The first historical map of the whole Taiwan Island was completed in 1625 (Tsao 1979). All such type maps published before the end o f 19th century did not utilise modem mapping techniques and thus only roughly illustrate the coastal configuration, topography, rivers and the place names of major settlements (Hsia 1992). The scale and the accuracy o f coastal configuration of these maps are too small and too rough for the research purposes.

Topographical maps for the Huatung coast with contemporary surveying and mapping techniques were not produced until 1895. Several medium- and large-scale topographical maps have been made since then (Table 2.1). These maps are of various quality. Careful assessment of the suitability of each set of maps is necessary especially when the amount o f change is not extremely remarkable. Among all maps, those published in 1979 (1:5,000) and 1986 (1:1,000) by Agricultural and Forestry Aerial Survey Institute, R.O.C. are of the best quality. These two sets are essentially orthogonal enlargements of the original aerial photographs, on which the image when the photographs were taken is kept. Therefore, the cliff-line is relatively easily identifiable, while the shoreline is not delineated. On other medium-scale maps, the shoreline has been delineated by mapmakers. The definition of the shoreline is vague. The 'waterline' seems to be the operational definition according to the statement in the unpublished operational manual (Factory 401, Combined Service Force, Ministry o f National Defence, R.O.C.).

Some problems have prohibited the quantitative usage o f those medium-scale maps published before 1920. Such problems include (1) the exact project, coordinate system and sometimes the origin of coordinate system is unknown, (2) good-quality control points are difficult to define, and (3) the principle of the delineation of the shoreline is unknown. Above all, the accuracy o f three sets published before 1919 is less reliable, as can be judged even by eye (Shen 1998). Although these sets of maps are not appropriate for quantitative measurement, they do indicate the coastal features (sea cliffs, sandunes, terraces, and also the

Table 2.1; Major medium- and large-scale topographical maps available for Huatung coast

M a p s S c a le C o o r d in a te Y e a r

Taiwan topographical m aps 1 :50,000 longitude/latitude 1895*'

Taiw an Pau-Tu (topographical m ap) 1 :20,000 longitude/latitude 1904*2

Taiw an topographical m aps o f aboriginal area 1 :50,000 longitude/latitude 1 9 0 7 -1 9 1 6 * 3

Taiwan landuse m aps (cadastral) 1:5,000*'* local? 1919

Taiw an topographical maps 1 :50,000 longitude/latitude 1929*3

T aiw an topographical m aps 1:25,000 T M , 6 d egrees 1955**

Taiwan topographical maps 1:25,000 T M , 6 d egrees 1960*2

Photographic B a se M aps for Taiwan area 1:5,000 T M , 2 d egrees 1979*8

C oastal A rea Landuse M aps 1: 1,000 T M , 2 d egrees 1986*9

Taiw an topographical maps 1 :2 5 ,0 0 0 T M , 2 d egrees 1992 (1 9 7 9 )* '°

*1 Investigated in 1895 and published in 1898 for the Chengkun g area. *2 Published in 1904.

*3 The whole set was compiled during 1907-1916. *4 The maps are generalised from the original maps o f scale 1:1,200. *5 The whole set o f maps were completed in 19 2 4 -1 9 3 8 , and those o f the Huatung coast were published in 1929.

*6 The first edition o f 1:25,000 topographical maps conducted by field survey was made in 1951, however, it is the one revised in 1955 o f the same series was adopted in the current research.

*1 Based on the 1:25,000 topographical maps published in 1955, compiled in 1960 and published in 1967.

*8 and *9 Time noted is the year when the aerial photographs, on which maps based, were taken for the Huatung Coast. *10 Based on 1979 photographic base maps and revised with 1986 and 1991 aerial photographs; published in 1992.

Table 2.2: Aerial photographs used for detecting the coastal change o f Huatung coast

Y e a r 1 9 5 1 1 9 6 4 1 9 9 0

C am era type recon n aissan ce metric m etric

C am era inform ation not available partly available availab le

P hotograph type nearly vertical* nearly-vertical* nearly-vertical*

S ca le ca. 1 :1 5 ,0 0 0 ca. 1:17 ,0 0 0 ca. 1:17,000

* It means the tilt angle o f the photograph is no larger than 3 degrees, and is usually regarded as the vertical photograph.

Table 2.3: Pictorial materials used for detecting coastal change for various stretches and periods of Huatung coast

F e a tu r e A p p lie d s tr e tc h M a p / A e r ia l p h o to g r a p h

S horeline change Huatung coast T opographical m aps [1 9 2 9 ; 1:50,000]

T opographical m aps [1 9 9 2 (1 9 7 9 ); 1 :2 5 ,0 0 0 ] C liff-lin e change

Several stretches o f central and southern section o f Huatung coast

Photographic B a se M aps [1 9 7 9 ; 1:5,000] C oastal A rea Landuse M ap s [1986; 1:1,000] T opographical m aps [1 9 9 2 (1 9 7 9 ); 1:2 5 ,0 0 0 ]

C liff-lin e change C hilin and C hiaping stretch

alon g C hengkung co a st

A erial photographs [ 1 9 4 8 /5 1 ; ca. 1:1 5 ,0 0 0 ] Photographic B a se M aps [1 9 7 9 ; 1:5,000] Coastal A rea Landuse M ap s [19 8 7 ; 1 :1 ,0 0 0 ] A erial photographs [1 9 9 0 ; ca. 1:17,000]

B each w idth H T coast (northern)

H T coast (central and southern)

Photographic B a se M aps [1 9 7 9 ; 1:5,000] C oastal A rea Landuse M ap s [19 8 6 ; 1 :1,000] C hange o f beach

w idth C hengkung coast

T opographical m aps [1 9 2 9 ; 1:50,000] Photographic B a se M aps [1 9 7 9 ; 1:5,000] Coastal A rea Landuse M aps [1986; 1:1,000]

original coastal configuration) and provide Useful information to show the trend of coastline change in a qualitative way, especially the one published in 1904.

One thing should be noted is the topography of medium-scale maps published after 1979 were all generalised from larger-scale orthogonal maps. For example, the 1:25,000 maps which were published in 1992 were actually based on the 1979 orthogonal maps (1:5,000). They were also revised according to the maps o f 1986 (1:1,000) and aerial photographs o f 1991. These revisions were mainly on the man-made constructions and land covers but not the topography. The shoreline indicated on this map is thus its position of 1979 but not o f 1992.

In terms o f the aerial photographs, the first edition covering the whole study area was taken in 1948 ~ 1951 by the Reconnaissance Wing, the 7th Fleet, US Navy (Mr C.-Y. Chen, senior officer in Agricultural and Forestry Aerial Survey InsWXutQ, personal commun. 1992). It is believed that only the print copies o f these aerial photographs are available in Taiwan. Partly due to its very limited accessibility, it is the first time that this material has been used for the research of environmental change. The quality of this set of photographs is very good (with scale around 1:15,000) and it also fills in the large gap between 1929 and 1979 (derived from maps). Only a few other sets of aerial photographs were taken prior to 1974, and the one which covers the whole extent of Huatung coast was taken in 1964. After that, the coast was taken more frequently, sometimes, twice a year.

Considering the availability of the topographical maps, those photographs taken in 1948/51 and in 1990 (the up-to-date one when the current author started this project) have been adopted for quantifying the coastal change, and the other sets are for qualitative comparison (Table 2.2). A problem with these two sets of aerial photographs is the lack of the details o f camera information. Even the focal length is unknown for the old set, while it is 6 inches for the new set (Mr C.-Y. Chen, senior officer in Agricultural and Forestry Aerial Survey \ns\\X\xiQ, personal commun. 1992). This has made it impossible to adopt a standard method to carry out rectification. The way to solve this problem is illustrated in the next r

section.

All pictorial materials applied for detecting the coastline change of the Huatung coast are listed in Table 2.3. The large-scale maps published in 1979 and in 1986 are also used to define control points for the rectification of aerial photographs (see below). Considering the huge amount o f work involved in digitising and rectifying, medium-scale maps were used when dealing with the extensive coastline, while those large-scale maps and aerial photographs were only applied to the shorter stretch of the coastline. Thus, the temporal

coverage for different lengths of coastline varies. It covers a period of 5(X years (1929 ~ 1979) for detecting shoreline change over the whole Huatung coast, and a period o f 39 years (1951 ~ 1990) for detecting the change o f beach width and of cliff-line o f the Chengkung coast.

2.2.2 Photogrammetry and error analysis

There are three different types of materials used in the current research: aerial photographs without camera information, medium-scale maps with the shoreline indicated and large-scale photomaps without the shoreline noted. The use of these materials to analyse coastal change involves two general procedures: (1) compilation of cliff-line and shoreline position data in analogue or digital form, and (2) a comparative analysis o f these data to determine changes over a specified time interval (Byrnes et a l 1991). The transformation o f topographical maps is rather straightforward when the projection and the coordinate system are known. As to the aerial photographs, several commercial packages are available for rectifying, e.g., PCI and ERDAS. The prerequisites are the accessibility of hardware and software and the availability of diapositives and camera information of the aerial photographs o f interest. The lack of camera information of 1948/51 photographs has prohibited the use o f these packages and a procedure was developed to cope with this particular set o f materials in use. Two more steps are needed for aerial photographs than for topographical maps — scanning and removing relief displacement. The technique is explained step by step (Figure 2.2).

Pre-processing

Several pre-processing steps were completed before the procedures o f compilation. All aerial photographs (negatives) were scanned into digital form, using a Contex Full Scale Scanner (FSS-500), SUN SPARC 2 workstation, and CADlmage/Scan software (Ver 1.0) in August and September, 1993 in the Department o f Geography, National Taiwan Normal University. The resolution o f the images is 300 dpi in format TIFF (Tag Information File Format). Each side of a pixel represents a ground area o f about 1.27 m and 1.44 m in 1948/51 and in 1990 photographs, respectively. The TIFF image files were subsequently converted into HIPS format for further analysis using the HIPS package (Human Information Processing System), which is a command-line driven system, was originally written by the Human Information Processing Laboratory, Psychology Department, New York University but has been extended and updated over years by programmers in the Department of Photogrammetry & Surveying, UCL and elsewhere (P. Lewis, Lecturer in the Geography, \}CL, personal commun. 1993).

Digitising and operational definitions o f cliff-line and shoreline

1979, 1986 and 1992 (1979)

fM ^ tisatipn o f cliffUne,