LayerDescription

Base Type: MapTableDescription

A class that describes the contents of a layer in a map service.

Remarks

This object can be used to modify the presentation and use of a layer in a map.  For example, you can use it to turn layers on/off when generating a map image or limit a query by applying a layer definition.  These settings include a unique ID for the layer (LayerID), an ID the corresponds with a Geoprocessing job (SourceID), a boolean indicating if the layer is visible as defined in the table of contents, i.e. the layer is marked on (Visible), a boolean to indicate whether the layer should display its dynamic labels (ShowLabels) if labels for this layer are available to the map service, a boolean to indicate whether the layer should scale it symbols according to a reference scale set in the map (ScaleSymbols).
 

SourceID is used for map service interaction with the Geoprocessing Server. It is a unique job id assigned to a geoprocessing service each time it runs on the server. This property can be used to find out which geoprocessing job has created this layer.  It is important to note that LayerID, SourceID and the index of layers in map document are not the same.
 

In order for symbols to scale, the map must have a reference scale set and the layer must support symbol scaling. Use ScaleSymbols to activate or deactivate a layer's symbol scaling. If a map does not have a set reference scale setting, ScaleSymbols will have no effect. Use CanScaleSymbols on MapLayerInfo to verify if a layer supports symbol scaling. A reference scale is set within the original map document.
 

Not all map service layers have available labels. To see if the layer has labels use HasLabels on MapLayerInfo.

The LayerDescription includes a DefinitionExpression.  A DefinitionExpression can be set on a layer in order to limit layer features available for display or query. A default definition expression can be set for a layer in the source map.  An expression set in the LayerDescription will override one set for the layer in the source map. This only applies for the request and does not replace the definition query set in the source map.
 

Let's look at some examples. You have a layer in your map that represents sales regions. The layer includes fields: REGIONS, SALES and MANAGER.
 

Example #1: In the source map the layer has a definition query, "REGION = 'North'". No DefintionExpression is specified in LayerDescription. Only those features where REGION = 'North' will be displayed in your map.

Example #2: In the source map the layer has a definition query, "REGION = 'North'". You apply a DefinitionExpression in LayerDescription as "SALES > 1000". The LayerDescription DefinitionExpression overrides the definition query set in the source map. Only those features where SALES > 1000 will be displayed in your map.
 

If you wish to use the DefinitionExpression as a way of getting a subset of the features already filtered by the definition query in the map you will need to save the original expression and then add it to the new expression to form one compound expression, REGION = 'North' AND SALES > 1000. This will display map features that are belong to the North region and have sales over 1000.

Example #3: In the source map the layer has no definition query. You apply a DefinitionExpression in LayerDescription as "SALES > 1000". Only those features where SALES > 1000 will be displayed in your map.

 

The following MapServer methods will honor DefinitionExpression: Find, Identify, QueryFeatureCount2, QueryFeatureIds2, QueryFeatureData2.

The following map service methods will only use the definition expression set in the source map since a MapDescription or LayerDescription is not provided as a parameter: QueryFeatureCount, QueryFeatureIds, and QueryFeatureData.

Note that DefinitionExpression does not affect spatial extents.

Please note that the following properties only honored in ExportMapImage and Identify operations:

• UseTime

• TimeDataCumulative

• TimeOffset
  TimeOffsetUnits