Wednesday, February 08, 2006

The Dynamic Display in GTViewer

Since the GTViewer Data format was designed to support data from different GIS sources (ESRI, Smallworld, Intergraph, etc.), it has always provided a variety of tools to manage the view in a dynamic fashion. For some GIS data, like Smallworld, it is common place for feature symbology to be dynamic and change as the view scale changes; however, for CAD based systems like FRAMME, the symbolgy is instance based and does not readily support dynamic styles. From GTViewer’s perspective, the source of the GIS data is irrelevant, so the dynamic style concept it uses with Smallworld can also be used with FRAMME data or any other GIS data source giving the user a much richer viewing experience. GTViewer provides two mechanisms to support Dynamic Display capabilities: Dynamic Style Rules and Display Threshold. These two approaches will be discussed in detail below.

In GTViewer, Dynamic Style Rules provide for different symbology (color, linestyles, weight, symbol characters, scale, etc.) as the view scale (zoom level) changes. Features viewed from far out can use one symbology while using a different symbology when viewed close up. The Dynamic Style Rules associate a named Style Definition with a particular feature for a particular zoom level range; there can be any number of Style Rules applied to feature as long as the zoom level ranges do not overlap (so only one style rule is in effect for a feature at any given zoom level). For example, a simple Dynamic Style Rule may define something like the following:

For Zoom Levels 0 to 2500, use Style Definition: Switch – Large
  • ColorValue=2550255
  • Scale=5.0

For Zoom Levels 2500 to Max, use Style Definition: Switch – Small

  • ColorValue=127640
  • Scale=1.0

The screenshot below shows the “Switch – Large” style rule in action. It causes the switch features (the Circled “S”) to be displayed much larger and in a brighter color (magenta) so that the features can be easily identified from far out. This sample data is from FRAMME which could not support this ease-of-viewing functionality. The Style Definition can be used to change the feature in a more dramatic way, such as changing the symbol character, rotating, offsetting, or any other number of characteristics (see the Style Manager for more info).

When zoomed in closer to the features, a different style rule is used with the switches:

Filter and Category Display Thresholds are also nice features for creating Dynamic View characteristics. Generally, Display Thresholds are used to boost performance by reducing the number elements drawn for each view update; however, Display Thresholds have a very nice side-effect of decluttering a view automatically as the user zooms in or out. Category Display Thresholds define a maximum and minimum zoom level for each Category of data in GTViewer. The Category will only be rendered when the view’s zoom level is within the Category’s Display Threshold range. So, if a category contains features that are only useful (or clearly visible) when the view is zoomed into, then it is always a good idea to set a maximum Category Display Threshold that will turn the Category off when zoomed out past a certain level. This idea is carried one step farther with Filter Display Thresholds. Each Feature Component in a Category has its own independent minimum and maximum Display Filter Threshold. A Filter Display Threshold is always superseded by its parent Category Display Threshold; however, if your Category contains a large display range, then Filter Display Thresholds can make a substantial difference in the usability of the data.

The following screenshot shows data with all features turned on as it would be in the original GIS:

The next screenshot uses Filter Display Thresholds to turn off all features that clutter the view or are not big enough to be of use. This example goes to the extreme by turning off most features to make a point, but as the user moves in closer, more features can be turned on automatically:

Both Dynamic Style Rules and Display Thresholds can be used in GTViewer to ensure that the users have the most usable view of the GIS data without any effort on their part. The users do not have to concern themselves with turning on and off features to get an unclutter view of the data and the symbology can automatically adjust to be provide the most useful display. The Dynamic Display characteristics are common to some GIS systems and GTViewer will carry these characteristics forward; however, the viewing requirement for mobile users or for specific applications may have different needs than the original GIS system, so alternate Dynamic Display characteristics can be provided on a user-class basis. Also, for GIS systems that never supported dynamic display characteristics in the first place, these features can provide an even richer viewing experience making the data more useful and the users more productive.

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