Question: why the number is so big?

The concrete procedure is as follows:

PDE := diff(u(x,t),t)+diff(u(x,t),x)=0;  IBC := {u(x,0)=x,u(0,t)=0};

pds := pdsolve(PDE,IBC,numeric,time=t,range=0..0.1);

pds:-plot3d(t = 0 .. .1, x = 0 .. 1, axes = boxed, orientation = [-110, 60], color = [0, 0, u]);

and the greatest number in the graph is over 4,500. I do not understand why the number is so high?  The former PDE and the initial value are very small.   many thanks!

 

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