Under Construction Continued from page 53 sure the box next to Calculate Only Selected Fields is unchecked. These chord lengths will be joined to the road set, so make sure the numbers begin with 1 and end with 47. 6. Now, let's calculate each chord's length in U.S. Survey Feet. Right-click on the Length_ Ft header and choose Calculate Geometry. In the Calculate Geometry dialog box, set Property to Length, use the coordinate system of the data source, and choose Units to U.S. Feet. Notice that this operation yields length values rounded to two places. These values appear on the map just below each turn number. 7. Make sure all the numbers are in order and that lengths are reasonable and save the map. Don't end the edit session because the next step is building the Middle Ordinates. Task 3: Modeling the Middle Ordinate In a perfect curve, the Middle Ordinate is a two-node polyline extending perpendicular from the Turn Chord to the farthest point out on the curve. Use the CR MP 0.0 1:500 bookmark to return to the extent at the beginning of Chuckanut Ridge Road. 1. On the Editor toolbar, change the Target from Turn Chords to Middle Ordinates. Keep Create New Feature as the task. 2. Under Snapping in the Editor toolbar, check the boxes for Turn Chord edge and Chuckanut Ridge Road vertex. Since the Middle Ordinate is often very short, change the Snapping tolerance to five map units. 3. Starting with Turn 1, use the Sketch tool to draw a polyline connecting the farthest point on the curve to the Turn Chord. Slide the second point along the chord until it appears perpendicular. Double-click to save the feature. Continue up the road, building a Middle Ordinate for each curve. 4. Once all the Middle Ordinates have been sequentially built, save the project. 5. Number the Middle Ordinates to match each associated turn using the Field Calculator. Calculate the Middle Ordinate lengths using Calculate Geometry (the same procedure used for the Turn Chord table). Use CR MP 0.5 1:1,500 to zoom in, turn the image back on, and inspect the map. Be sure that all vectors are properly numbered. Save the edits and the map. This might be a good point to stop and take a break. Task 4: Splitting Lines and Separating Curves Now it's time to split the Chuckanut Ridge Road into curves and straight segments using the Turn Chord endpoints to locate each split point. When performed manually, this task is quite rigorous. If we could intersect all chords with the road, this would be an easy task. However, using the Chord Points and a special VBScript, the road can be split into curves and straight-line segments. See the accompanying article, "Road Repairs: Splitting Polylines with Visual Basic Scripting," for detailed information on how this procedure is accomplished. 1. Navigate back to Bookmark CR MP 0.0 1:500 once more. Make Chuckanut Ridge Road the Target layer and the only selectable layer and change the Task to Modify Feature. Change the snapping layer back to Chord Points vertex (or alternately, Turn Chords end). Start editing again and reset the Snap- Number curve segments to match the Turn numbers by selecting a turn segment and entering its number in the Chuckanut Ridge Road attribute table in the TURN_ NO field. 54 ArcUser Winter 2010 www.esri.com