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Top 10 Benefits of Using GML

Supplied by Galdos Systems Inc., August 06, 2001

The GML approach is a great improvement over the historical reliance on simple GIF/JPG image maps for the following reasons:
  1. Better quality maps. GML encodes information about geographic features or objects, and these can be displayed to as fine a resolution as required. Thus, screen- based maps generated from GML appear crisp and easy-to-read. Such maps can also be saved as local files, emailed, or printed.

  2. Works on a browser, without the need to purchase client-side software. When a GML file is received at the client, it is converted to a set of drawing objects and rendered as a map on the browser. Typically, Scalable Vector Graphics (SVG) is used as the drawing object language. As long as the browser supports vector graphics, then the map can be displayed without any additional software. Currently, SVG is supported by means of a free downloadable plug-in from Adobe Corporation (just like their PDF reader). No other special software is needed to view GML files.

  3. Custom map styling. GML contains map "content" only (e.g., where features are, their geometry, type and attributes), but it does not provide any information about how that map data should be displayed. This is actually a benefit because different "stylesheets" can be applied to the geographic data to make it appear however the user wishes. One user may choose thin black dashed lines for roads and a cross symbol to depict churches, whereas another might choose thick solid red lines for roads and a CH sign for churches. The important point is that the GML data is the same in both cases, each group just invokes a different stylesheet to render the map. Choosing the stylesheet may be done automatically, or users may be given a choice of stylesheets to use.

  4. Editable maps. It is quite straightforward to annotate GML-based maps that have been downloaded and rendered on a browser. Once GML has been converted to SVG, the user can apply graphic editing tools on the client to add text (of any font, size and color), highlight features, and draw virtually any kind of shape on the map. The annotated map graphic can then be saved as a file, emailed, or printed.

  5. More sophisticated linking capabilities. One of the benefits of GML is that you can embed links associated with features. These links can be simple URL addresses, or they can be more sophisticated. But, at its simplest, this means that you can associate any Web address with a feature. When a user clicks on a feature, the user is transferred to that address. This capability can be used to initiate a feature-based query or to take a user to a new Web page (e.g., click on the opera house and be taken to its home page).

  6. Better query capability. Often users want to be able to click on a feature on a map and find out more about it (e.g., what is the name of this river?). For GIF/JPG-style maps either this cannot be done at all, or if it can be done, the query mechanism is quite rudimentary and involves measuring the pixel position of the cursor, translating this to geographic coordinates on the server, and then looking for the required feature in the server-side GIS. Limited pixel resolution and the vagaries of different browsers often limit the accuracy of this method, meaning that frequently the user does not get back information about the feature they clicked on. This problem does not arise in the world of feature-based GML. When you click on a feature, you will always unambiguously identify that feature. And, by means of turning on and off different feature themes, it's easy to identify features within features (e.g., a house within a lot).

  7. Control over content. Because GML is feature-based, it is quite easy to provide a filtering function that allows users to download only the feature-types that they want to appear on their maps. For example, if you don't care about railway lines, then you don't have to download them from the server. This filtering can reduce data transfer time. Map content can also be controlled after the geographic information has been delivered to the client's Web browser. Using a clickable legend, a user can display/hide information themes instantly, and without the need to call the server to generate and deliver a new map. This cannot be done with GIF/JPG maps.

  8. Animated features. Objects and features that change over time can be accommodated in GML, and can be rendered as animated graphics using SVG. For instance, if you want to show the path of an aircraft, its position at different times can be recorded in GML as separate features. Simple code on the client can be used to create an icon of a plane and display it as a moving object against a background map. The user might use a time slide bar to move the plane. GML also lets you define an object's position algorithmically. In your GML file, you could have a feature representing an oil spill, and include a dispersion algorithm as an attribute of the feature. Then you would just need a simple client-side utility to render the oil spill and show how it spreads over time.

  9. You don't have to target just a Web browser. GML is a non-proprietary geographic file format that can encode most types of geographic information. As such, you can use it as a general geo-spatial data interchange format. In fact, geographic data in GML can be sent to any device with an XML interface. So, for instance, you could use GML to send geographic data from one GIS to another. GML can also be displayed on XML-enabled devices like the new-generation PDAs and cell phones. The benefit for the provider is that one format suits all uses.

  10. Service chaining. An example of service chaining is where you take some geo- spatial data, send it to a site to convert from the NAD27 to the NAD83 reference system, send it to another site to convert from geographic coordinates to UTM, send it to another site to add administration boundaries and demographic data, and finally pass it along to another site for display or storage. Each site is providing a discrete service. This concept is supported well by GML because (a) GML is a general format, so sites don't need to support lots of proprietary data formats, and (b) GML is extensible and XML-based, which makes it easy to manipulate, change, and add to its contents.

Galdos Systems, Inc. is a privately-held Canadian company based in Vancouver, B.C. We are a leader in the development of non-proprietary, standards-based tools for managing geo-spatial data over the Internet. These software tools are based on the Geography Markup Language (GML).

Customers include geo-spatial data holders and private and public sector organizations who are seeking to provide enhanced services, whether over the Internet, intranet or extranet, based on leading-edge geo-spatial technologies.

Phone: (604) 484-2750

Entire article (c) 2001, Galdos Systems, Inc. Reproduction or redistribution in whole or in part without contacting the author and The Geocommunity is strictly prohbited.

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