[gradsusr] Questions with plotting cross section plots

[Cypresslin]

Hello!
When I plot a N-S cross sections with:
set lev 1000 97.5
set lon lon1
set lat  lat1 lat2
I got this plot:
You can see the terrain comes with undefined value.
http://img844.imageshack.us/img844/1382/nocollect.png

But with the following method:
http://www.iges.org/grads/gadoc/usingstationdata.html
(see: Drawing Arbitrary Cross Sections)
The undefined area was gone
http://img713.imageshack.us/img713/6015/42627027.png

Why there is such a difference?
Thanks