Mapping Center

For the past few years, when time allowed, I've been reconstructing a map of colonial Boston in ArcGIS, using source maps from 1775 to 1784.  The image on the left is an excerpt of one of these maps.  I like a number of the effects that 18th century cartographers had drawn on their maps, including little pictographs for trees (today cartographers call these mimetic symbols).  Anyway, I wanted to add some of these effects to my map.  This posting explains how I showed trees on my map.

The picture marker symbols for the trees on the left (below) appear to be arranged on the map by a more skilled and patient hand than those in the example to the right.  The trick wasn't to practice long hours at adding point features hoping to hone my digitizing skills, or to use some clever system of determining the exact coordinates of the tree point features as they were created. (Click on the images to see more details.)

The trick was not to use point features at all.  Instead line features were used, and they were symbolized using cartographic representations to give the idealized appearance of what walking through the Mall along the Boston Commons must have seemed like--I think that was the intention of the original author as well.  Another upside to this technique is that it is much faster to create and merge a few line features than to painstakingly place several dozen tree points.

To create this effect, I used a representation marker line and applied the "Along line" geometric effect (this is the default setting).  However, the default parameters for this effect were changed:

• In order to have the trees oriented with the map, the "Angle to line" option was unchecked.
• Control points were added to the effect.
• The Endings setting was changed to "With marker". Since each line feature starts at the same distance from an adjacent feature, a uniform start to the side-by-side line patterns was achieved.
• The setting for control points was changed to "With marker".  This forced a marker to appear on vertices at the corners defined by control points, allowing this effect to echo the street corners.

Here is what my settings looked like:

It took me almost twice as long to create the point features in the lower quality example shown above, and with many fewer points, as it did to create the lines and representations in the more polished example.  It took roughly the same amount of time to figure out the correct symbol size and spacing for the point features as it did to determine the same (size and step) for the representations.  Once I knew that it was possible to do this and how to do it, I've saved ten times the amount of time and effort as it took to place markers one at a time in my work since learning this technique.

Incidentally, the British burned many of those trees for firewood during the winter of 1775/76 while they occupied Boston.  Another map, completed in 1777 shows that a new mall had already been replanted.