XXII International Cartographic Conference (ICC) A Coruña, Spain, 11-16 July 2005 Hosted by: The International Cartographic Association (ICA-ACI) ISBN: 0-958-46093-0 Produced by Global Congresos

REMOTE SENSING DATA APPLIED TO REALISTIC TERRAIN DEPICTION ON TOURIST MAPS.

Jacek Drachal

Instytut Geodezji i Kartografii, ul. Modzelewskiego 27, 02 - 679 Warszawa, Poland, e-mail: jacek@igik.edu.pl

The great variety of available tourist maps is due to consumer interest and cartographers' ability to create new map designs. A very important part of the map message is to invite the user to visit the place mapped by creating a convincing image of the terrain. Realistic view cartography is a good example of the recently developed possibilities of tourist mapping. Remote sensing data available now create rich sources of information for 'real-look' terrain, the image which can not be drafted with a pen or brush. From a cartographic perspective not all the information on the terrain view is equally important. This paper demonstrates the possibility of producing a realistic image of the terrain using only part of image information. After decomposing the image into layers the image information content is reduced and simplified. This procedure enables us to control the image graphic elements and to design the optimal image. INTRODUCTION We live in times of constant changes resulting from the abundance of ideas for things that never existed and for improvement of things that already are. In tourist cartography new ideas are also realized both in content of the map as in its graphic form. In such circumstances coming back to the photographic map to watch it over again should not surprise anyone. There are at least two encouraging occasions to do so. At last we have possibility to care about photographic images seriously having proper computers and since recently we have also satellite images of the Earth surface in natural colors and of ground resolution equal to that of aerial photographs. Both satellite and aerial images supply the right material for creating true images of the terrain. Their unique value besides the already established possibility of making various kinds of measurements lie in that they provide us with a very instructive view of the Earth from a distance. There are two other functions of a map which are associated with such "view ", navigation and visualisation. Particularly interesting here seems the potential for visualisation which indirectly shapes our imagination about the world, our atlas of mental maps. Photographic similarity to the real view activates our mind and makes it more prone to keep this kind of image longer. Photographic images of the Earth make us also more sensitive to the real landscape ready even to contemplate it and watching this kind of map we are unintentionally turned from map users to observers of the view. Surely this is only a hypothesis now but after some time when we learn how to benefit from remote sensing data in new map designs we shall awaken the interest in both cartography and natural landscape among many new disciples. PROBLEM WITH PHOTOMAPS The raw image either aerial or satellite is not at all as good for visualisation as the map, especially at smaller scales when objects at human scale are not easily to recognize. It takes time to find out basic objects and to remember their locations. On the other hand there are parts of the image which show things impossible to find on maps because they are too difficult to draw or they are omitted according to the generalisation rules. These unique natural representations are worthy enough to work on photographic maps. The most exciting are the photographic representations of three types of locations. The first include complicated situation on the border between land and water of the sea, lake or the river which is usually too much simplified on maps. The second are mountainous areas but also dunes depicted with convincing stereographic effect. The third are places with subtle color variations of rocks or vegetation. Cartographic rules of generalisation treat all three specialties without respect and simply omit them so perhaps this is the reason that photographic images seems so curious.

The weak points of images taken from a distance are dual in character. Some result from the nature of the situation and so images do not show things which can not be seen as being obstructed by trees or other objects, things which are too small or too poorly distinct or in the shadow. Other weak points are related to the registration technique. The atmosphere and the optical glass suppress saturation of colors and suppress the high frequency component of the image making it less sharp. The center projection makes outer parts of the image less perceivable than the center part. Satellite images are advantageous in this case but they are also more expensive in popular usage. The weaknesses make the image more complicated than the map and usually there is not too much to see at first glance. Rather it takes time and observation to grasp what means what. Obviously it is not a visualisation in traditional cartographic terms. In the image there is contained quite much of information about the landscape but apparently not the right data are accentuated so the image looks confusing. In aerial photographs visual image variables such as color or value not only do not serve to arrange the data in some kind of logic known from maps but on the contrary they act in the opposite way. Oddly represented on photographs is, for example, the relationship between land and water. The essential information about the terrain for which on topographic maps the most appealing graphic contrast is preserved here on the photograph is barely visible (Fig.1A). In return, we get useless in this case but quite substantial information about borders of parcels and the diversity of the land use (Fig.1B). There is more such cartographic oddities on aerial photographs. Lakes are represented in various shades of blue and green because they really differ in, e.g. the depth, water clarity or the angle of observation. Buildings on photographs are also represented in various colors, which is natural as their roofs have not only different colors but also are made of different materials and and have different slopes.

Figure 1. Cartographic oddities on aerial photographs: A. barely visible coastline of the lake B. �excessive� accentuation of the land use diversity

If we analyse the photographic image from the side of its spatial frequencies, we notice that at least the highest frequency range can be more emphasised at the cost of other ranges and this action results in better sharpness of the image (Fig.2D). Image sharpening is commonly used in many graphic softwares. However, we do not know clearly how essential role play the other ranges, of medium and low spatial frequencies. At the moment this can be searched experimentally. From comparison of histograms of the topographic map and of the aerial photograph we notice something that we expected but now our visual impression gains mathematical basis. On the aerial photograph the range of medium shades prevails forming "a mountain" in the gaussian curve shape of the histogram (we are not mistaking medium shades with medium frequencies). On the map histogram the white together with light colors are most numerous samples. The white of the paper and light colors make the map favourable in comparison to photographic image. They ensure clarity of the drawing and facilitate the placement of names and signs. The photograph, on the contrary, make use of all shades including medium and dark in continuous ranges because its fragile light and shadow drawing is based on shade differences.

Figure 2. Aerial photograph (A) and three out of five of its spatial frequency components: lowest (B), medium (C) and highest (D). SEARCHING FOR A BETTER COMPOSED IMAGE Only now with the use of a computer system we can attempt to work out few ideas concerning the improvement of the Earth's surface images, where the improvement means here desired visualization characteristics. The conception presented here is a search for an intermediate form of the photographic representation being half way between the raw image and the cartographic image of the terrain. The recognized directions of this search include the elimination of this part of information which could be regarded as excessive or useless and accentuation of the other parts of information estimated as valuable. To preserve valuable part one has to get aware of its presence. Because of some natural properties of photograph type images they can not be usually regarded ready to use in their raw form. The acquisition technique prevent us to see from the beginning what is in the image either because of the air was not clear or the situation on the ground was not favourable to some parts of the information. So the raw image can be regarded as only a file of information about the real image of the terrain. To get the real image several processings may be required. This concerns both satellite and aerial images which usually need at least the adjustment of contrast and color saturation and the accentuation of the high spatial frequency range to make the image slightly sharper. Only after these basic processigs we get the probable form of the "true" image (Fig.3).

Figure 3. Aerial photograph before and after accentuation of the high spatial frequency range. Only after correction the

subtle relief of dune area becomes apparent. Being aware of the true image values we may continue our search by analysing what could be omitted from the image information content without reducing its integrity. The true image may be now regarded as a file of essential information about the ideal image on which, for example, the forests would not be bluish. Such an image or a view

could be observed and registered in conditions of perfect transparency of the air which really never happens, but our experience tells us that the phenomenon has its own dynamics and so the point of perfect transparency of the air is only the limit of the range. The conception of the ideal image being different from the real one is quite familiar to cartographers. It is associated with the very idea of the map being essentially a model of the geographic reality displayed in a form of an image. A model which is far from being a mirror reflection of this reality. A model which include elements of idealization. The map as a model with its basic attribute of miniaturization, is being created in a process of continuous selections and simplifications both in content and form and the prime reason to do so is the lack of space to contain everything. This direction of thinking when applied to aerial photographs can also give desirable results. In selecting and classifying the image information into important and not so important it seams natural to accept traditional classification of the terrain objects practiced in topographic maps. For equally important one should additionally regard these information which underline the unique characteristics of the area and which are typically photographic, like in Fig.3. To carry out the plan of classification of the terrain objects in the photographic image is a tedious task but worth doing it as the results are remarkable. Unfortunately it requires accurate interpretation of the image which can not be performed by automatic classification solely. The result is a decomposition of the image area into segments associated with different classes of topographic objects. Marking the classification result in the image is a delicate task. Each class of objects should be in a new version of the image easily noticeable. It is now the time for simplification of graphic forms of particular objects belonging to one class. This can be done by eliminating their less important characteristics together with emphasizing one characteristic common for all, like e.g. blue color for waters. One should estimate which visual variable would serve as the best common characteristic for each class of objects to make this class suitably prominent (Fig.4).

Figure 4. After classification of the image objects the original colors of lakes were replaced by the common blue color.

The second area of possible selections and simplifications is the spatial frequency domain which enables us to decompose the image "vertically". If we succeed in eliminating or suppressing some components of the image after its decomposition shown in Fig.2 we can obtain much lighter image because dark shades of lower frequency components occupy much of the image space. In such case making the image more bright does not cost us loosing the delicate drawing of the situation on the ground and the value of the image is somehow preserved. It is the time now to find out the final shape of the image with the use of cartographic design rules (Fig.5). CONCLUSIONS Remote sensing data present interesting potential for making maps of a new kind. These photograph type images are still not fully examined and exploited especially by map makers. They show too many important things to be treated as

only a tool for measurements. New photographic maps can be an interesting supplement to traditional tourist maps. With new designs they can offer mach more than a regular map and this is something like a taste of the represented landscape.

Fig. 5. Aerial photograph before (A) and after (B) suppressing the low spatial frequency component. ACKNOWLEDGMENTS This paper is based on the research conducted in the project No 9 T12E 042 26 founded by the Polish Committee for Scientific Research in 2004. REFERENCES 1. Bertin, J., 1981. Graphics and Graphic Information-Processing. Walter de Gruyter, Berlin New York, pp. 176-232. 2. Drachal, J., "A message from the Tatra. Geographical Information Systems and Remote Sensing in Mountain

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Jacek Drachal was born in 1954, graduated in 1978 from the Warsaw University of Technology with a Master of Science in Photogrammetry and Cartography. Since 1978 employed at the Institute of Geodesy and Cartography, Warsaw, Poland, first at the department of Remote Sensing then at the department of Cartography. In 1991 PhD degree for digital image processing applied to cartography. Experience in Cartography, Remote Sensing, Photogrammetry and Digital Image Processing. Software Expertise in Intergraph Microstation and image processing systems of OVAAC8, Intergraph, Adobe, PCI and Erdas. Publications, mainly in Polish scientific journals, concern scanning and processing of satellite images, the resolution and legibility of remote sensing digital images and the application of cartographic design to photomaps. Author and co-author of several satellite image maps of Warsaw at scales 1:25000, 1:50000 (two awarded for innovative technology and design), road atlas of Poland at scale 1:250000 (5 editions), tourist photomaps of Slowinski National Park and Polish Tatra at scale 1:20000. Member of ICA commissions on Mountain Cartography and Mapping from Satellite Imagery. Consultant in several international projects in Algeria (1985), Iraq (1985), Belgium (1992, 1993, 1994) and China (1997). His research interests include digital cartography, application of remote sensing to natural resource mapping, photographic maps.