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Graphics are points, lines, or polygons that are drawn on top of your map in a layer that is independent of any other data layer associated with a map service. Most people associate a graphic object with the symbol that is displayed on a map to represent the graphic. However, each graphic in ArcGIS Server can be composed of up to four objects, including the geometry of the graphic, the symbology associated with the graphic, attributes that describe the graphic, and an info template that defines the format of the info window that appears when a graphic is clicked on. Although a graphic can be composed of up to four objects, it is not always necessary for this to happen. The objects you choose to associate with your graphic will be dependent on the needs of the application that you are building. For example, in an application that displays GPS coordinates on a map, you may not need to associate attributes or display info window for the graphic. However, in most cases, you will be defining the geometry and symbology for a graphic.
Graphics are temporary objects stored in a separate layer on the map. They are displayed while an application is in use and are removed when the session is complete. The separate layer, called the graphics layer, stores all the graphics associated with your map. Just as with the other types of layers, GraphicsLayer also inherits from the Layer class. Therefore, all the properties, methods, and events found in the Layer class will also be present in GraphicsLayer.
Graphics are displayed on top of any other layers that are present in your application. An example of point and polygon graphics is provided in the following screenshot. These graphics can be created by users or drawn by the application in response to the tasks that have been submitted. For example, a business analysis application might provide a tool that allows the user to draw a freehand polygon to represent a potential trade area.
The polygon graphic would be displayed on top of the map, and could then be used as an input to a geoprocessing task that pulls demographic information pertaining to the potential trade area.
Many ArcGIS Server tasks return their results as graphics. The QueryTask object can perform both attribute and spatial queries. The results of a query are then returned to the application in the form of a FeatureSet object, which is simply an array of features. You can then access each of these features as graphics and plot them on the map using a looping structure. Perhaps you’d like to find and display all land parcels that intersect the 100 year flood plain. A QueryTask object could perform the spatial query and then return the results to your application, where they would then be displayed as polygon graphics on the map.
In this article, we will cover the following topics:
- The four parts of a graphic
- Creating geometry for graphics
- Symbolizing graphics
- Assigning attributes to graphics
- Displaying graphic attributes in an info window
- Creating graphics
- Adding graphics to the graphics layer
The four parts of a graphic
A graphic is composed of four items: Geometry, Symbol, Attributes, and InfoTemplate, as shown in the following diagram:
A graphic has a geometric representation that describes where it is located. The geometry, along with a symbol, defines how the graphic is displayed. A graphic can also have attributes that provide descriptive information about the graphic. Attributes are defined as a set of name-value pairs. For example, a graphic depicting a wildfire location could have attributes that describe the name of the fire along with the number of acres burned. The info template defines what attributes should be displayed in the info window that appears when the graphic appears, along with how they should be displayed. After their creation, the graphic objects must be stored inside a GraphicsLayer object, before they can be displayed on the map. This GraphicsLayer object functions as a container for all the graphics that will be displayed.
All the elements of a graphic are optional. However, the geometry and symbology of a graphic are almost always assigned. Without these two items, there would be nothing to display on the map, and there isn’t much point in having a graphic unless you’re going to display it.
The following figure shows the typical process of creating a graphic and adding it to the graphics layer. In this case, we are applying the geometry of the graphic as well as a symbol to depict the graphic. However, we haven’t specifically assigned attributes or an info template to this graphic.