Now, let's define the lines/curves that give us the outfield fence.

Input := 

LeftFieldLine = ParametricPlot[{t,325},{t,0,250},
	DisplayFunction -> Identity];

Input := 

RightFieldLine = ParametricPlot[{425,t},{t,0,150},
	DisplayFunction -> Identity];

The quarter circle that defines the center field line has its center at (250,150) and has radius 175.

Input := 

CenterFieldLine = 
ImplicitPlot[(x-250)^2 + (y-150)^2 == 175^2,
	{x,250,425}, {y,150,325},
	DisplayFunction -> Identity];

Input := 

Show[LeftFieldLine, RightFieldLine, CenterFieldLine,
	AspectRatio -> Automatic,
	DisplayFunction -> $DisplayFunction]

Output =

-Graphics-