From Steve Nix,
Your Guide to Forestry.
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American sycamore is a massive tree and can attain the largest trunk diameter of any of the Eastern U.S. hardwoods. The native sycamore has a grand branch display and its bark is unique among all trees - you can always identify a sycamore just by looking at the bark. The alternate maple-looking leaves are large and also unique to those familiar with sycamore.
Platanus occidentalis is readily identifiable with broad, maplelike leaves and a trunk and limb complexion of mixed green, tan and cream. Some suggest it looks like camouflage. It is a member of one of the planet's oldest clan of trees (Platanaceae) and paleobotanists have dated the family to be over 100 million years old. Living sycamore trees can reach ages of five hundred to six hundred years.
The American sycamore or western planetree is North America's largest native broadleaf tree and is often planted in yards and parks. It's hybridized cousin, the London planetree, adapts very well to urban living. The "improved" sycamore is New York City's tallest street tree and is the most common tree in Brooklyn, New York.

Start with the Tree Finder if you are not sure what kind of tree you have!
Consider Planting Sycamore
Sycamore - Not Just a Planetree