These symbols are used by the U.S. National Park Service:
Airport
Boat launch
Campground
Canoe access
Drinking water
First aid
Fishing
Food service
Four-wheel drive access
Gas station
Hospital
Information
Litter receptacle
Lodging
Parking area
Picnic area
Ranger station
Recycling
Restrooms
RV campground
Self-guiding trail
Shelter
Store
Telephone
Trailhead
Bridge
Hiking trail
Marsh
Rapids
Spring
Unpaved road