Just to let you know, we have added many new symbols—the tally is now at 229—and have regrouped them in somewhat more logical categories. The free symbols are available at the link below.
We create new symbols, some which are pretty esoteric, based on requests from park staff. We also occasionally tweak the design of existing symbols. For example, the updated amphitheater symbol now looks less like the ubiquitous Wi-Fi symbol.