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ESRI Lunch Specials: Making city block polygons to create street void effects
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City blocks defined by wide street lines.
City blocks defined by wide street lines.
City Blocks using polygons that show the character of the curbs.
City Blocks using polygons that show the character of the curbs.

What: We defined the blocks or edges of pavement in this map with beige polygons (rightmost image above). Why: By contrast the blocks in image on the left appears unsophisticated especially when compared to the level of detail used to depict the buildings. In many areas of this map, a plain depiction of the blocks gives no clues to the reader as to which block is which. Since San Diego’s curb lines are distinctive and vary from block to block we felt this was a good way to help orient people using this map. Thus, one of our goals became ensuring that conference attendees, particularly those visiting San Diego for the first time, would be able to quickly determine which block or intersection they would be walking through.

How: To create your own detailed curb lines, follow this workflow:

  1. Curb line data:
    1. If you don’t have good curb line or edge of pavement data to begin with you will need to capture it. To capture this level of detail, we used high resolution imagery (three-inch pixel resolution color satellite imagery).
    2. If you do have curb line data, then you will need to create polygons. The obvious way to try is the Line to Polygon tool, however, any flaws in the curb line data’s geometry, like small breaks where the lines should be connected, will cause problems. Because this map was small we deemed it would be faster to create the block polygons with the Editor’s trace tool.
  2. For this map, there was no edge of pavement data available, but since high resolution imagery was available, and since it would only take a day of effort, we decided to capture the block polygons. This was the basic procedure we used:
    1. In ArcCatalog, we created a new polygon feature class. We added an integer field to denote the type of block. At the time we were capturing the blocks, we did not know whether the blocks that were under construction would be drawn with a different symbol on the map; it turned out that they did not.
    2. Because the map scale was going to be between 1:5,000 and 1:7,500, the scale that the blocks needed to be captured at in order to have the expected look of geographic features versus coarse geometric figures, was 1:500. A good rule of thumb when capturing features from imagery is to zoom in by a factor of 10; the result will look finished and any geometric oddities will be too small to notice.
    3. The block geometry was captured by using two of the sketch tools in the Editor. To learn the basics about creating features in the Editor click here. First the straight edges of the blocks were captured using the Sketch tool, which is the default editing tool for creating new polygons. While sketching the block edges, some curves were encountered. Typically these were unbalanced S-shapes where one half of the S was considerably smaller than the other half. This meant two curves were needed; to create these, the Arc tool was used. While sketching a new polygon the sketch tool can be changed at any time. The first arc was created to be tangent to the last straight segment that was drawn. The second curve was created tangent to the endpoint of the first Arc (see below). There were also curves that effected round-overs at corners and just one curve was used for these.
    4. Example of sketching curves with the sketch construction tools.
      Starting with the sketch tool the rightmost vertex was first created, then the Arc tool (or Endpoint Arc tool) was used for the next two vertexes, then the Sketch tool for the next, and then the Arc tool for the last vertex at the left.
    TIPS
    1. To make it easier to switch between the Sketch tool and the Arc tool, a custom toolbar was created. The tools that were commonly used while editing the blocks were added to this toolbar, which while editing, was kept floating close at hand, the blocks, making it easy to switch tools (no extra clicks, drags, or long journeys for the mouse).
      Custom Editing Toolbar image
    2. When creating polygons that contain curves and straight segments, start sketching the polygon in the middle of a straight segment, because finishing a curve at the starting point of the polygon is very difficult to accomplish correctly. If the side of the polygon that was used to start and end the sketch is not straight, use the Reshape Edit Task to connect the vertexes on either side of the endpoints.
    3. When making compound curves it sometimes helps to zoom in a bit more and add a very short straight segment. This is shown in the picture above; the segment to the right of the red vertex is straight.
    4. When creating a curve that is unbalanced, such as the one shown above, create multiple shorter curves along the path of the unbalanced curve. Try to see the curve as a series of smaller curves that link up at tangent points—this will guide where curves should start and end.
    5. Because sketching curves with the Arc tool takes some practice, even with the above tips, it may be easier for beginners to use the Endpoint Arc tool, which works by first defining the start and end points of an arc, then defining the amount of deflection as the last operation.
    6. Sometimes the shape of one block was identical or very close to being so with another block. In those cases it was much easier to copy, paste, and move the new block rather than repetitively digitize the same shape.

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