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:
- 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.
- 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.
- 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.
- 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.
- 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).
- 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).
- 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.
- 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.
- 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.
- 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
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.