Visual Interfaces for Geographic Data

Transcription

1 Visual Interfaces for Geographic Data Robert Laurini, INSA-Lyon, SYNONYMS Cartography, Visualizing spatial data, interactive capture, interactive layout DEFINITION Geographic data are in essence visual multimedia data. To entry those data with keyboards and to print them as tables of data are not very practical actions. For entry, special devices exist (theodolites, lasers, aerial photos, satellite images, etc.) whereas visual layout is currently named cartography or mapping. Visual interfaces have two facets, allowing the user to present their output as maps, possibly with very large printers, and to present spatial queries visually. HISTORICAL BACKGROUND In the 70ies, the used expression was computer-aided mapping emphasizing the idea that printing maps was the sole scope of using computers. Then, in the 80ies, the more important was considered as structuring geographic database, and then the expression Geographic Information Systems was coined. From this period, research has been done on presenting maps as well as presenting queries. For centuries cartographers have created a large corpus of rules for manual mapping. Those rules were progressively integrated and enlarge to compose maps. For instance, zone hatching was considered as a boring task when manually done, whereas with computer, this is straightforward. In contrast, name placement was considered as a relatively easy task although by using computing, this is still a challenge. SCIENTIFIC FUNDAMENTALS It is common to say that approximately 80% of all government data has some geographic component 1. This affirmation states the importance of geographic information, not only for administration but also for companies, not only for environmental and urban planning, but also for geo-marketing, as all those data are stored in databases, or more exactly geographic or spatial databases. In addition to non-spatial attributes, geographic objects are characterized by geometric shapes and coordinates, usually in two dimensions, but more and more with three dimensions and time. Those characteristics imply three consequences: the necessity of special data models for storing which are presented in another sections of this encyclopedia, the necessity of distinguishing storage format and layout format, 1 Langlois G., Federal Computer Week, Jan. 08, 2001.

2 2 and the necessity of specialized interfaces for both querying against databases and presenting the results, essentially by means of cartography. The scope of this presentation is overall to study those interfaces. But before presenting those interfaces, it is necessary to remind that geographic data are not entered via key-board but are acquired by different devices such as theodolites, aerial photos, satellite images and more recently GPS (Global Positioning System). Those devices are essentially characterized by different resolutions and error levels. As theodolites can measure objects within accuracy less than 0.1 mm, some satellite systems or GPS systems have accuracies of 100 meters or less. In the other hand, according to the size of the used screens and the size of the territory to map, different scales must be used. In other words, the resulting map must be simplified or more exactly generalized in order to reach readability and completeness, so passing from storage format to some other layout format. This paper will be organized as follows: after a short introduction, visual interfaces first for cartographic output will be examined, and then for querying. After, a small section will conclude on new barriers for mobile handheld devices. Visual Interfaces for Cartographic Output The classical way to respond a spatial query or to layout the results of some spatial analysis is a map. However, during centuries cartographers and geographers have elaborated sets of knowledge and know-how to make maps. One of the main problems is known as generalization, i.e. the way to simplify a map; for instance, in our geographic database let us have a country described with 1 million points/segments, but must be presented in a thumbnail with only 30 points/segments. Another aspect is called graphic semiology, i.e. the way to select the symbology for mapping. Generalities One fundamental rule in conventional cartography states that any object, once reduced after scaling to less than 0.1 mm cannot be mapped. For instance an house or a road having 10 m wide, at 1:1000 scale will be represented by 1cm, whereas at 1:100,000 scale they will not be represented al all. Another rule concerns details to be aggregated. For instance two houses separated by a road, at some scale they will be represented separately, whereas at other scales they will be aggregated. As a consequence, at some scales a city is seen as a collection of separated houses, then as a set of city-blocks, then as a compact area, and finally as a point.

3 3 Fig. 1. Example of generalization (1:25000, 1:35000, 1:50000). Source: [4]. Generalization By generalization, a piece-wise line can be simplified into a single piece, especially by using Douglas-Peucker algorithm [3] or variants based on multi-agents systems [4]. Figure 1 gives some examples. Graphic Semiology Graphic semiology, invented by J. Bertin [1] is the study of the meaning of graphics. In other words, it deals with the signification of drawings, the choice of captions, symbols and icons, and together with a methodology to transmit visual messages. For that six visual variables have been proposed for representing spatial objects (See Figure 2); shape, size, orientation, pattern, hue and value; for instance to represent a church, a cross symbol (shape) can be used; the symbol s size can be selected according to the initial size of the church, etc. Fig 2. Bertin s Visual variables. Source: view Animation For some applications, animated cartography can be used characterized by movement of objects, flickering, mutation, modification of shape or of color, velocity, etc. An excellent example is given by animated maps for weather forecast where some iconized clouds are slowly moving.

4 4 Chorems Chorems are a new way of representing schematized territories. Indeed, for several applications, it is not necessary to restitute the complete database contents, but rather map the more important aspects. Usually those chorems were designed manually, but by means of spatial and spatio-temporal data mining, geographic patterns can be discovered and mapped. In other words, chorems are a new visual representation of geographic database summaries [10] and a way to represent geographic knowledge. The following example (Figure 3) emphasizes the water problem in Brazil: it is easy to understand that a conventional river map (Fig 3a) does not show the more crucial aspects as given in Fig 3b with caption in Fig 3c. Fig. 3. The water problem in Brazil using a conventional river map (a) and a chorem map (b) issued from [7]. Query input Visual interfaces for GIS are not only for cartography, but also for query input, i.e. by means of interaction; present systems can be classified into three categories: - textual queries, such as by using spatial extensions of SQL/ORACLE - tabular queries, by means of forms in which some queries are preprogrammed - and graphic queries based on widgets such as icons, mouse and clicks. Some basic queries such as point-in-polygon, region or buffer queries are usually given visually and interactively. However, more complex queries are also usually made such as queries regarding intersection or adjacency by means of Egenhofer s spatial relation. For instance the LVIS system [2] is a visual system based on those relations (See Figure 4).

5 5 Fig. 4. Example of a visual query asking for all cities crossed by a river [2]. But a fourth method seems more interesting based on a tangible table in which several persons can collaborate. Figure 5 shows such a table (from the Geodan Company); see [11] for details. Fig. 5. Example of a tangible table from the Geodan Company ( Final remarks: challenges for small mobile devices. In early 70ies the main problem was producing maps, and them more and more maps were seen as results of spatial queries or as results of spatial analysis techniques. However, as it is simple to layout map in conventional screens or in very big screens, the size of the screen of the new handheld mobile devices implies the discovering of

6 6 new modes of representing maps and of acting interactively with them, essentially for Location-Based Services. A very common example is representing the way to go from one location to another location, as given Figure 6. Our opinion is that those new mobile handheld devices will imply new techniques for visualizing geographic data, essentially due to screen size Fig. 6. Example of a map presented on a mobile device. CROSS REFERENCES Geographic Information Systems, Geographic Databases, Spatial Information Systems, Spatial Databases, Cartography, Spatial indexing, KEY APPLICATIONS Any domains in cartography, from urban to environmental planning, geology, archaeology, real estate mapping, location-based services, etc. RECOMMENDED READING 1. Bertin J. (1970) Sémiologie graphique, La Haye, Mouton, Bonhomme C., Trepied C., Aufaure MA, Laurini R. (1999) A Visual Language for Querying Spatio-Temporal Databases. Proc. of the 7th Intern l Symposium on GIS, ACMGIS'99, Kansas City, November 5-6, 1999, edited by C. Bauzer-Medeiros, ACM-Press, pp Duchêne Cécile (2004) Généralisation Cartographique par Agents Communicants : le modèle CartACom, PhD dissertation, University Paris VI, 11/06/ Douglas D., Peucker T., (1973) Algorithms for the reduction of the number of points required to represent a digitized line or its caricature, The Canadian Cartographer 10(2),

7 , 5. Kraak M.-J., Brown A. (2000) Web Cartography Published by CRC, 208 p. 6. Kraak M.-J., Omerling F. (2003) Cartography: Visualization of Geospatial Data, Pearson Education; 2 nd edition, 205 p. 7. Lafon B., Codemard C., Lafon F. (2005) "Essai de chorème sur la thématique de l eau au Brésil" Laurini R., Thompson D. (1992) Fundamentals of Spatial Information Systems. Academic Press, Février p. 9. Laurini R. (2001) Information Systems for Urban Planning: A Hypermedia Co-operative Approach. 308 p. February Del Fatto V., Laurini R., Lopez K., Loreto R., Milleret-Raffort F., Sebillo M., Sol-Martinez D., Vitiello G. (2007) "Potentialities of Chorems as Visual Summaries of Spatial Databases Contents", VISUAL 2007, 9th International Conference on Visual Information Systems, Shanghai, China, June Edited by Qiu, G., Leung, C., Xue, X.-Y., Laurini, R., Springer Verlag LNCS, Volume 4781 "Advances in Visual Information Systems", pp van Borkulo E., Barbosa V., Dilo A., Zlatanova S., Scholten H.(2006) Services for an Emergency Response System in The Netherlands, Second Symposium on Gi4DM, Goa, September 2006, 6 p. 7