Web map services, such as Google Maps and MapQuest, are among the most popular sites on the Internet. One can easily access these services through a Web browser on a personal computer or mobile device. The high accessibility and efficiency offered by these sites is possible, in part, by the use of standard image formats. The present review is a description of the most common image formats available from web map servers nowadays, as well as other formats with great possibilities for the future. We describe raster and vector formats and highlight advantages and disadvantages in each case. We also refer to protocols and image formats supported by the Open Geospatial Consortium (OGC) standards.
A raster or pictorial image consists of a 2-dimensional array of pixels. An uncompressed raster image is typically referred to as a bitmap. The most commonly used raster formats on the Web are GIF (W3C, 1990), JPEG (W3C, 2003a), and PNG (W3C, 2006). These formats can be visualized in every Web browser without the need of any additional plug-in or component. Another raster format that has been largely used in GIS desktop applications is TIFF. A TIFF image can contain tags with additional information such as the georeferencing information of GeoTIFF. It can also contain multiple layers in a single image or different types of compression. These, among other features, make TIFF a very flexible but complicated format. Therefore, common Web browsers do not support TIFF and its use on the Internet is very restricted.
JPEG shows its advantages over the rest of the Web suitable raster formats by the compression ratio that can be achieved. It is the best choice for aerial photographs and satellite images. However, GIF and PNG are better for compression of images with evenly colored areas; their compression is loss-less as opposed to JPEG. Some examples of images that can be compressed as GIF or PNG are those generated out of geometric elements such as topography or road maps, also those with homogeneous areas such as bathymetry images. Furthermore, GIF and PNG support transparency which is very useful when the elements in the image do not cover the whole space so several images can be superimposed on top of each other. This is often the case when the elements included in the maps are points and polylines. Even though it is possible to superimpose images, it causes problems of its own. The overlapping needs to be implemented on the client side application, which usually runs on a Web browser. Because of differences in the way Web browsers layout the page contents, and other incompatibilities, the solution may be somewhat tricky.
Although GIF and PNG are very similar, some differences can be pointed out. PNG is more recent than GIF; it was created to fulfill the patent related problems with the algorithm used in the compression of a GIF image. In addition, GIF is limited to a maximum of 256 colors. Although the patent for the compression of the GIF format has already expired, PNG still seems to be the preferred format on Web map servers. See Table 1 for an outlined comparison among the raster formats.Table 1.
Comparison of raster formats supported by WMS
|Feature \ Format||JPEG||GIF||PNG||TIFF|
|Supported by Web browsers||Yes||Yes||Yes||With the use of plug-ins|
|Number of colors||Over 16 million||256||Over 16 million||Over 16 million|
|Compression||Lossy, better for photographs||Loss-less, better for images with uniformly colored areas. Formerly patented||Loss-less, better for images with uniformly colored areas||Different methods|
|Georeferenced||No||No||No||Yes, with special tags for it|
Key Terms in this Chapter
SVG: Scalable Vector Graphics. Vector-based open standard format developed by the W3C Consortium.
XML: Extensible Markup Language. Defines an extensible way to structure files containing textual information.
JPEG: Joint Photograph Group. Standard image format created to store photograph as a compressed rectangular array of pixels. The compressed images usually have lower quality compared to the originals but, for some compression ratios this is not noticeable by a human eye.
OGC: Open Geospatial Consortium Inc., founded in 1994, after the OpenGIS Project, is an organization that involves entities from the government, academics and private sector whose mission is to develop standards for geospatial products.
W3C: World Wide Web Consortium. Organization that defines the standards for the Internet.
TIFF: Tagged Image File Format. Image format designed to store high resolution images. Its contents may or may not be compressed. It can contain additional information in the form of tags.
PNG: Portable Network Graphics. Image format similar to GIF except that it has been developed more recently and is able to store one of over 16 million of colors per pixel.
WebCGM: Web Computer Graphics Metafile. Internet version of the Computer Graphics Metafile created in 1987 as a standard format for the interchange of graphic information.
SWF: Macromedia Flash format. Vector-based format developed by Macromedia. A SWF file can contain videos and animations including audio.
WMS: Web Map Service. The latest available version of the WMS specification is 1.3.0 and it is also available as ISO 19128 since 2005.
GIF: Graphics Interchange Format. Image format stored as a compressed rectangular array of pixels. The pixel colors are limited to a maximum of 256. This format can contain transparent areas.