网络评估类型-选自desktop help

网络评估是对网络属性的定义，使用的是各种数据源，如边，点等，meters属性直接取自field ，可以是 shape—length，也可以是其他的。网络评估类型包括以下：

Evaluator type	Description
Field evaluator	The most common way to assign values for a network attribute is by identifying a single field that will be used to evaluate the network attribute when the network dataset is built. For example, a field evaluator may be used in cases where the value describes a measurement, such as meters.
Field expression evaluator	The field evaluator can be modified to take a field expression as the value instead of a single field. In this case, you'll build an expression on the Field Evaluator dialog box. For example, if the units of the network attribute are in meters but the units of source data are in feet, you can create an expression to translate the feet into meters when the network dataset is built.
Constant evaluator	Attributes can be assigned a constant value. The value can be numeric (0, 1, 2) for cost, descriptor, and hierarchy attributes, or a Boolean expression (Traversable or Restricted) for a restriction attribute.
Function evaluator	The function evaluator calculates attribute values by performing a multiplicative or logical function on another attribute value or parameter value. For numeric attribute types, the values are derived from an expression that multiplies the value of another attribute by some value. For example: DriveTime * 1.25. For Boolean attribute types, the values are derived from an expression that compares another attribute value to a parameter value. For example: MaxHeight < VehicleHeight.   NOTE: Since the function evaluator is new in ArcGIS 9.3, using it in your network dataset will make your network dataset unusable in previous ArcGIS releases.
Global turn delay evaluator	The global turn delay evaluator assigns a cost value for transitioning between two edge elements based on the deflection angle between the two edges and the hierarchy attribute value of each edge. For example, it may take longer to make a left turn from a local road on to a secondary road than it does to go straight through an intersection on a secondary road. Unlike other evaluators, the global turn delay evaluator can only be assigned as the default evaluator for turn elements.     Learn more about establishing global turns     NOTE: Since the global turn delay evaluator is new in ArcGIS 9.3, using it in your network dataset will make your network dataset unusable in previous ArcGIS releases.
VBScript evaluator	Attributes can be assigned from the result of the execution of VBScript. This provides a way to model complex attributes. Unlike other evaluators, the VBScript evaluator does not assign values when a network is built. Instead, it assigns values when a particular network analysis requires the use of that attribute. If the values of an attribute change constantly, the use of a VBScript evaluator can ensure the use of updated attributes for network analysis.   NOTE: Since the VBScript evaluator calculates its values at the time of analysis, this evaluator can affect the analysis performance.