COASTLINE LENGTHS AND AREAS OF ISLANDS IN THE CROATIAN PART OF THE ADRIATIC SEA DETERMINED FROM THE TOPOGRAPHIC MAPS AT THE SCALE OF 1 : 25 000 TEA

In this paper, modern definition of island established by the IHO has been accepted, and classification of islands, islets, rocks and rocks awash has been proposed according to their areas. The coastline of the Croatian part of the Adriatic Sea was digitized from topographic maps produced at the scale of 1 : 25 000 (TM 25). Topographic maps used for digitization are more precise than the maps that were used in earlier works and consequently the data on the number of islands and their coastline lengths and areas are more precise. Polygons of islands were closed in GIS package AutoCAD Map 2000, and each was given its name. From the obtained database and classification of islands, islets and rocks, in the coastal sea area of the Republic of Croatia 79 islands, 525 islets, and 642 rocks and rocks awash, or a total of 1246 have been recorded. Furthermore, it has been established that on TM 25 the island of Cres has the largest area (405.70 km), although in literature so far (including atlases) the island of Krk was most often cited as the largest island in the Adriatic Sea. The island of Pag has the longest coastline length of 302.47 km.


Introduction
Croatian part of the Adriatic Sea comprises the coastal sea area from the Bay of Piran (the Dragonja river mouth) in the northwest, to the middle of outer part of the Bay of Kotor in the southeast, excluding the 23,975 km long coastline section near Neum which belongs to the Republic of Bosnia and Herzegovina (RIĐANOVIĆ, BIĆANIĆ, 1993).According to Klemenčić (1992), the land area (without islands) of the Republic of Croatia is 56,609.59km 2 and the coastal sea area (with islands) about 33,200 km 2 .In comparison with the area of the east Adriatic territorial waters, the island area covers a very large area, being divided in several island groups: Western-Istrian islands, Kvarner islands, North-Dalmatian islands, Middle-Dalmatian islands and South-Dalmatian islands (STRAŽIČIĆ, 1987).
Internal waters of the Republic of Croatia comprise ports and bays on the coastline of land and islands, and sea parts between the low water line on the coastline and baseline.Baseline is the line between mean low water along the coast of land and islands, straight baseline closing the entrances to ports and bays, and straight baseline which connects the defined points along the coast of land and islands (POMORSKI ZAKONIK, NN 17/94).Croatian territorial sea spreads from the baseline of internal waters towards the continental shelf boundary up to a distance of 12 M (Fig. 1).
In literature, different data are mentioned about the number of islands, islets, rocks and rocks awash in the Croatian part of the Adriatic Sea.The problem is in the fact that terminology is not standardized and there are no general criteria for island classification, so that the limits between islands, islets, rocks and rocks awash are not clearly defined (STRAŽIČIĆ, 1987).According to "Hydrographic Dictionary", island is a piece of land completely surrounded by water, islet is a small island, rock is the natural occurring material that forms the firm, hard, and solid masses of the ocean floor, rock awash is a rock at chart datum (IHO, 1994).
The purpose of this paper is to present the results of calculations of coastline lengths and areas of islands, islets and rocks in the Croatian part of the Adriatic Sea, according to the categorization suggested by Duplančić Leder et al. (2000 a, b).

Short hystorical review
Many authors based their information about the number of islands in the Adriatic Sea on the paper by Austro-Hungarian naval officer Sobieczky (1911), who offered in his work detailed information on indentation of the eastern coast of the Adriatic Sea.The data were the result of the hydrographic survey campaign carried out in 1873.Hydrographic survey sheets were on scales 1 : 28,800 and 1 : 14,400.In his paper, the islands having area below 0.3 km and the islands inside the isobath of 5 m were not counted.Sobieczky (1911) counted 71 islands, 641 islets, 409 rocks or rocks awash, which is a total of 1121 on the eastern coast of the Adriatic Sea.
According to Rubić (1952), island is a piece of land surrounded by water with the coastline length of over 10 km.Islet has the coastline length between 1.5 and 10 km and rock below 1.5 km.According to this definition, Rubić (1952) counted 69 islands, 558 islets, and 413 rocks, or a total of 1040 in the eastern part of the Adriatic Sea.

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Fig. 1.Map of the maritime territorial boundaries and limits at the Adriatic Sea Sl. 1. Karta morskih teritorijalnih granica Jadranskog mora Irić (1955), in the publication of the Hydrographic Institute, systematized earlier results, listed all the islands, islets, rocks and rocks awash, and quoted their dimensions in kilometres and nautical miles.According to Irić, in the Croatian part of the eastern Adriatic there are 66 inhabited islands, 652 uninhabited ones, 389 rocks, and 78 rocks awash, or 1185 in total.In the Montenegrin part of the coastline there are no inhabited islands, 7 uninhabited ones, 37 rocks, and 4 rocks awash, or a total of 48 islands, rocks and rocks awash.
The results of Sobieczky were systematized again by Stražičić (1987), who compared his results with the results of other authors, and counted 60 islands, 653 islets, 438 rocks or rocks awash, or a total of 1151 in the eastern part of the Adriatic Sea.
In 1997 Croatian Ministry of Development and Reconstruction stated the information, not mentioning the authors, according to which in the Croatian part of the Adriatic Sea there are 718 islands, 389 rocks (head above sea level), and 78 rocks awash (head below sea level), or a total of 1185 islands, rocks and rocks awash.
United Nations Convention on the Law of the Sea, Part IV, Article 46 defines the eastern coast of the Adriatic Sea as archipelagic coast (URL 1).This area is a unique location in the Mediterranean area, even in the Earth.Because of that fact it is necessary to categorize islands of this area with special caution.On that basis, Duplančić Leder et al. (2000) proposed the categorisation of islands, islets, rocks and rocks awash as a geographic notion as follows: -Island is a piece of land completely surrounded by the sea, with the area larger than 1 km 2 .
-Islet is a piece of land with the area between 0.01 and 1 km 2 .
-Rock or rock awash is a piece of land with the area below 0.01 km 2 .The difference between rock and rock awash is that rock is always visible above sea level while rock awash is sometimes submerged below sea level.

Methods
The first step was digitization of the coastline of land and islands covering the Croatian part of the Adriatic Sea on topographic maps at the scale of 1 : 25 000.Topographic maps (edition 1974) were produced by the Military Geographic Institute (VGI) in Belgrade.Transformation of coordinates from the local digitization system in the map projection (5 th and 6 th zone of Gauss Krüger's projection) was done in ArcInfo 8 software package using Helmert transformation.For this specific purpose, a programme in AutoCAD Map VBA module was made, to project graphic entities of AutoCAD image directly from one projection to another.For easier reference and analysis, the data were then transformed into Gauss Krüger's conformal transverse cylindrical projection, with the central meridian 16° 30' E (FRANČULA, 1981).In order to reduce linear distortions, the linear scale 0.9997 was introduced along the central meridian (TUNJIĆ, LAPAINE, 1998).Maximum linear distortion along the bounding meridian is 40 cm per kilometre; the value of area distortion ranges from -0,06% along the central meridian to 0,08% along the bounding meridian (LAPAINE ET AL., 1993).
Polygons of islands were made in GIS software AutoCAD Map 2000, and the base for designing topology was prepared using Map Cleanup.Each polygon was given its name (Fig. 2), and on this basis the polygon topology was made.Polygon areas P on (Bessel) ellipsoid were calculated using the methods and formulas published by Štemberger (1986, 1992) and Lapaine et al. (1993): where x i and y i are coordinates of the closed polygon points (x 1 =xn, y 1 =yn), p i is the factor of correction for each addend because of the projection.
For Gauss Krüger's projection, with linear scale factor along the central meridian 0,9997, local linear scale in the arbitrarily point is: where R = 6,377,000 m is the mean radius of the ellipsoid in the observed point, so that the local area scale for conformal projection is: Area accuracy estimation m p can be calculated as: where m T is the mean error of each particular point, while coordinates of points are supposed to be uncorrelated and their mean error of abscissa and ordinate to be equal (LAPAINE ET AL., 1994).
Coastline length d can be calculated according formula (5) as the sum of distance d i between two adjacent points (LAPAINE, 2004): Mean square error m d of coastline length d: Area accuracy estimation m p and mean square error of coastline length m d will be object of future study, and will be calculated using formulas (4) and ( 6).
Because of extensive calculations, a programme in AutoCAD Map VBA module was made, which calculates coastline lengths and area with corrections directly from the polygon on AutoCAD image of the Adriatic Sea.Calculated values are entered into the database which is connected with the graphic representation, providing the base for a GIS project of the coastal area within AutoCAD Map (Fig. 2).

Results
From the obtained database the number of islands was determined.Categorization and classification of islands, islets, rocks and rocks awash in the Croatian part of the Adriatic Sea was taken from Duplančić Leder et al., (2000a, b) according to their areas.In the Croatian part of the Adriatic Sea 79 islands (Tab.2), 525 islets (Tab.3), and 642 rocks and rocks awash were determined, or 1246 in total (Tab.1).
Table 2 shows coastline lengths and areas of islands in the Croatian part of the Adriatic Sea, while Table 3 gives coastline lengths and areas of islets in the Croatian part of the Adriatic Sea.
Total area of the insular area which belongs to the Republic of Croatia is 3259 km 2 , while a total length of the insular coastline is 4398 km.The value of 3259 km 2 for a total area of insular area is close to the value given by other authors [3177 km 2 (RUBIĆ, 1952)] while the value of 4398 km for a total length of the insular coastline is considerably different [4013 km (IRIĆ, 1955); 4058 km (RIĐANOVIĆ, BIĆANIĆ, 1993)].
This fact can probably be explained by different precision of the used maps and methods of geodetic calculation.
Area of islands covers 3195.71km 2 , while area of islets covers 62.41 km 2 , totalling 3258.12 km 2 .Area of rocks and rocks awash covers 1.44 km 2 , which together with the area of islands and islets represents the area of islands, islets and rocks of 3259.57km 2 .
Coastline length of islands is 3573 km, while coastline length of islets is 717 km, totalling 4398 km.Coastline length of rocks and rocks awash is 107 km, which together with the coastline length of islands and islets represents the coastline length of islands, islets and rocks of 4398 km.
From the obtained results it follows that the largest islands in the Adriatic Sea are Cres with an area of 405.70 km 2 , and Krk with an area of 405.22 km 2 .It should be pointed out that in earlier literature, including atlases, Krk was usually cited as the largest island.Only some authors pointed at incorrect calculations of the areas of these two islands (e.g.FRANČULA, 1994;FRANČULA, 2001).Krivičić (1993) hypothesized it might be possible that the two neighbouring islands are identical in area, which would make them unique in the world.The smallest island is Smokvica Vela (Kornati) with an area 1.04 km 2 .
The island with the longest coastline of 302.47 km is Pag, being the fifth according to area value.The island with the smallest coastline length of 5.8 km is Vele Orjule.The biggest islet is Badija with an area of 0.97 km 2 , while the smallest one is Galicija covering 0.01 km 2 .

Conclusions
Eastern coast of the Adriatic Sea is one of the best indented coasts and belongs to the so-called archipelagic coast.The number of islands in Croatian part of the Adriatic Sea, as determined in this paper, is greater than it was published earlier in literature: -According to the proposed classification of islands (Duplančić Leder et al., 2000a), in the coastal sea area of the Republic of Croatia 79 islands, 525 islets, and 642 rocks and rocks awash have been recorded, or a total of 1246 on topographic maps produced at the scale of 1 : 25 000 (TM25).
-Total area of the insular area which belongs to the Republic of Croatia is 3259 km 2 , while a total length of the insular coastline is 4398 km.
-The largest island in the Adriatic Sea determined from the topographic maps produced at the scale of 1 : 25 000 (TM25) is Cres with an area of 405.70 km 2 and the smallest island is Smokvica Vela (Kornati) with an area 1.04 km 2 .
-The island with the longest coastline of 302.47 km on TM25 is Pag, while the smallest coastline length of 5.89 km belongs to Vele Orjule.
-The biggest islet on TM25 is Badija with an area of 0.97 km 2 , while the smallest one is Galicija covering 0.01 km 2 .
This paper represents the beginning of a systematic analysis of the Croatian insular area, which should encourage scientists and economists, providing them with guidelines for carrying on their studies in this field.The authors propose that determining of coastline lengths and areas of islands, islets and rocks according to the set criteria should be done from larger scale charts, e.g. 1 : 5000, which is an extensive and demanding task.
Tab. 1 Number, coastline length and areas of islands, islets, rocks and rocks awash in the Croatian part of the Adriatic Sea Tab. 1. Broj, duljina obaalne crte i površina otoka, otočića, grebena i hridi u hrvatskom dijelu Jadranskog mora Coastline lengths and areas of islets in Croatian part of the Adriatic Sea Tab. 2. Duljina obalne crte i površina otočića u hrvatskom dijelu Jadranskog mora