Topographic Maps With SVG

Abstract

Peter Pavlicko
Department of Cartography,GIS and Remote Sensing,
Comenius University, Bratislava, Slovakia
pavlicko@fns.uniba.sk

Michael P. Peterson
Department of Geography/Geology
University of Nebraska at Omaha,
mpeterson@unomaha.edu

keywords: USGS Large Scale Topographic Maps, SVG, interactive topographic web maps

Large scale topographic maps portray detailed information about the landscape. They are used for a wide variety of purposes. United States Geological Survey (USGS) large scale topographic maps at a scale of 1:24,000 have been traditionally distributed in paper form. Scanned versions of these maps are now available through the Internet as raster representations. Instead of the common raster format presentation, the solution presented here is based on a vector approach using SVG. The vector format provides many advantages compared to the use of a raster-based presentation, such as the quality of the graphical representation, maintenance, actualization, interactivity, and extensibility through other Web programming languages. This study shows that SVG is a promising technology for delivering high-quality, fully-vector topographic maps via the Internet, both in terms of graphic quality and interactivity.

The purpose of this research is to propose an optimal structure of a SVG document that would be universally applicable for all USGS large scale topographic maps. The emphasis was placed on creating a logical structure of the final SVG document with a minimal file size. The separation of the content of the document from its format represents the initial step in this procedure. By content, we strictly understand data that is defined by its geometric structure. It refers to objects, coordinates (geometry) as well as their geometric properties like area, line, or point representation. A format of a document simply refers to the symbology of large scale topographic maps as defined by USGS standards. This format or style can be applied to individual objects in many ways. Common solutions are based on using internal or external CSS styles. By applying such an approach, we should be able to easily create topographic web maps in SVG that would be graphically identical to its paper version.

However, there are still several shortcomings in publishing topographic maps over the Internet. Foremost among these is that the structure of the source DLG data, in which topographic maps are distributed, is not very appropriate for direct conversion to the SVG format. Source data contains many unneeded properties that interfere with the graphic quality of the resultant map. Therefore, data has to be first edited to the required structure through the use of a GIS program.

The file size of a SVG document is another important limitation in the distribution of large scale topographic maps over the Internet. Ideally, the distribution of whole topographic maps (topo-sheets) would logically represent the best solution. However, the file sizes of such individual topo-sheets are too big and therefore not appropriate for distribution through the Web at this time. According to the analyses of file sizes for topographic maps from such varied topographies in the states of Nebraska, Montana, and Wyoming, we can conclude that file sizes are too big not only for their distribution through the Internet but they are also too large for the manipulation of such files. We propose that, at a maximum, approximately only one quarter of a topo-sheet is appropriate for presentation through the Web. To distribute topographic maps for a large area encompassing many map sheets using SVG, it would be necessary to create a database that could distribute just that part of an individual topographic map on demand. Methods for delivering larger geographic datasets for large scale topographic maps are proposed by other studies.

Interactivity represents an important method that allows users to better extract topographic information from a map. The number of such interactive features that can be implemented is basically unlimited and can be developed to serve specific applications. Turning layers on and off, providing coordinate readouts, and displaying attributes through a mouseover function represent basic functions that would allow interactive manipulation with a topographic map.

The approach described here can be applied not only to USGS topographic maps but essentially to any similar type of map. It also can serve as a basis for the design of a database that would facilitate the distribution of large scale topographic maps in SVG through the Internet.

Link to the project site: http://ptolemy.unomaha.edu/~pavlicko/