Do It Yourself! Continued from page 51 will support geocoding and a time/distance network. This dataset contains speed limit [SPEED_MPH], street class [CLASS], and school zone [SCHOOLZONE] fields that can participate in the network. However, it does not contain any numeric values for distance. The TYPE field provides clues that it contains some directional streets. Select fields where [TYPE] = ‘FRWY' or [TYPE] = ‘RAMP' and notice that these records do not contain geocoding attributes. They can be edited without upsetting the geocoding fields. Impedance—Distance Notice that there is no distance field in the source dataset. The Length_Mi field added earlier needs to be populated, but before calculating street segment lengths, many of the freeway vectors need to be edited. Impedance—Speed Sort the [SPEED_MPH] field in ascending order and carefully review the values. Notice that nine records have 0 speed limit. Before building the network, these records must be updated or deleted. Just remember that they are now in the street dataset. The [SPEED_MPH] field will provide the primary impedance and a zero speed will not contribute to a meaningful time. Once directionality and connectivity issues are resolved, the segment length in miles and travel time in minutes can be calculated. Crossing Relationships Because this is a geocoding dataset, it does not contain fields for nonintersecting crossings (which are also called z-elevations). Although two z-elevation fields could be added to this table, it would require careful editing of all the values for each crossing and intersecting street segment—a complex, time-intensive process. Instead, geometry—not attribution— will be used to define and manage crossing relationships. Connectivity To properly create a network, all street segments must share common endpoints. Make the Streets with Speed layer selectable and zoom in or use the Magnifier Window to inspect intersections. Select street segments to verify connectivity. Notice that even freeway interchanges display connectivity where they cross city streets. Later in the exercise, freeway segments will be spanned across city streets to limit connectivity. Directionality Open the Layer Properties dialog box for the Streets with Speed layer, click on the Display ArcUser Summer 2009 Study the endpoint arrows and notice that some segments are properly oriented for right-hand travel, while some are not. Delete Redlands Mall and the Santa Ana Wash; set all other speeds to 10 mph. tab, and set the transparency for the layer to 70 percent. Turn on the Streets with Speed and Direction layer. Zoom in and inspect streets near the I-10 and I-210 freeway interchange. Study the endpoint arrows and notice that not all segments are properly oriented for right-hand travel. This must be fixed. The OneWay field will be used to code all one-way segments. Turns and Turn Relationships Turns and intersection slowdowns are important in a time-based network. After updating streets in the next section, the next step will be building a network using global turns. Save the Redlands01.mxd before going on to the next section. www.esri.com