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When you add content to ArcGIS Explorer, the way it’s displayed on the map is often defined by the source of the content. For example the control that creates a note will assign a symbol to that note, KML layers define how their features are drawn, and ArcGIS layers' symbology have been defined by their authors.

In some cases, you’ll decide how the GIS information that you add to the map should be displayed, whether it’s to assign a symbol to a place that you find, or to specify how the features in a shapefile should be drawn.

When you add content to the map (i.e. create a note, add a shapefile, etc), the application assigns a default symbol for you to display the data. You can change the symbol used by clicking the item’s Appearance tab.

The Appearance tab includes a Symbol group that contains various controls for selecting symbols to draw the GIS data with and controls for modifying properties of the symbol. Notice the different galleries for points, line, and areas (polygons):

The Symbol Gallery
The Symbol Gallery displays the list of symbols available for the type of GIS feature that you are symbolizing (i.e. point, line or polygon). You can scroll through the symbols or drop the gallery down to see more symbols. When you click a symbol, that symbol is assigned and the map updates to display it.

When displaying point symbols, you can click the Browse… or Add from a Url... button to specify a local file name or Url to use as the source of the point symbol. Symbols specified in this way will be added to a group named My Symbols and remain a part of your Symbol Gallery. You can also add or remove custom symbols using the Manage My Symbols dialog, which can be accessed by clicking on Manage My Symbols....

Changing the symbol properties
Once you choose a symbol, you use the other controls in the symbol to change some characteristics of how it’s displayed. Note that the controls are specific to the geometry of the symbol; in other words, some controls only display when you've selected points or lines or polygons.

Transparency: clicking the transparency control displays a slider control that allows you to change how transparent the symbol displaying the data is. The more transparent, the more you can see the map content that is drawn under the symbolized data. The transparency control is available for all geometry types in feature layers: point, lines, and polygons:

Point Style: the point style menu gives you the option of displaying the point symbol as a billboarded 3D symbol when the map is in 3D displaymode, or as a rasterized symbol draped on the surface of the map:

Size: when you add points to a map, a symbol size is assigned for you based on the scale ranges the data is displayed in, what the data represents (that is, a map note versus a GIS feature), etc. In ArcGIS Explorer this point size is represented in terms of pixels or a point to map units conversion. In most cases, you just want to adjust the symbol to be a little larger or smaller. Two controls, one to increase, the other to decrease the point size, are available in the Appearance tab's Symbol group:

A similar pair of controls is available for changing the width of lines or the outlines of fill symbols:

Line Color: you can change the color of a line symbol or the outline of a polygon symbol by clicking the Line Color control to display a selection of standard palette of colors and variations on those colors that you can pick from.

You can choose to have no color for the line or outline by clicking No Color.

Fill Color: you can change the color of a fill symbol by clicking the Fill Color control. This will display the same color selection control as described above for Line Color.