By Charlie Frye, Esri Chief Cartographer
Using cartographic representations to make line symbols like those in the image to the right, where markers are displayed in conjunction with a dash pattern, requires more than just casual knowledge of the geometric effects. In fact, to make a line symbol like the one shown to the right, the default settings won’t work.
Making this sort of representation symbol can be tricky because settings that work for a straight line will not work for a line that curves or changes direction. The lower example on the right shows what can happen as lines become progressively more complex.
Here is how we made this symbol:
First, the dashed line: this is pretty simple, but the key is to set the Endings to “No constraint”. This is necessary because any other constraint will cause the length of the dashes and gaps to be adjusted to fit between the end points of the line. Changing the length of the dashes independently of the position of the marker symbols will cause the markers to not align with the dashes.
Next, the marker line symbol, which requires two additional effects. First is the Cut curve, which in this example is set to draw the first marker 16.5 points along the line instead of right at the beginning of the line. An end cut value of 4 is also specified to prevent the final marker on any line symbol from overhanging the end of a line.
The second required effect is Dashes, which is not an obvious effect to use. A dash pattern of “1 38″ is used, which means one marker and then a gap of 38 units (starting from the center of the marker). Here the Endings are also set to “No Constraint” to match the dashed line. Notice that a marker appears on every third dash. The reason that works is because the sum of the numbers in the pattern is 39, which is exactly three times the sum of the numbers in the dashed line pattern, which is 13. In order for markers to line up, the sum of their pattern must be evenly divisible by the sum of the dashed line’s pattern
This is a good place to note that using the control points effect will not work given how this line symbol is set up. The control points effect will add points to the constraints, so instead of just the end points of a line, additional points based on the angle threshold will also constrain the dash or marker pattern, but they will do so independently and cause misalignments that are not acceptable for this symbol.
Notice the “Step” value in the Marker line is set to 1; which is also 1/39th of the pattern. The segment of the feature’s geometry this 1-length mark occupies determines the angle of the marker and the location of the marker. Because the step exactly matches the mark length in the dash pattern, the marker is placed in the center of the mark segment. This is also why the cut curve setting had to be 16.5 instead of 17. Also note, because the dashes effect was used, the Endings setting for the marker is ignored.
Last, instead of adding an offset curve effect to the marker line, we edited the marker and moved it relative to its insertion point. In the representation marker editor, the X represents the insertion point and the center of rotation, and is circled in red. This allowed the based of the triangle to sit on the line.
Given this method of setting up line symbols it is possible to create many different combinations of dashes and markers, including creating patterns where dashes are skipped or where markers appear on or in between dashes.