Create the GPS data set. The X and Y variables are converted from decimal degrees to radians. The X variable is also multiplied by -1 to match the values in the MAPS.COUNTIES data set.

data gps;
  input x y site $;
  x=x*arcos(-1)/180;
  x=x*(-1);
  y=y*arcos(-1)/180;
datalines;
-77.0348 40.0454 a
-78.4437 39.1623 b
-78.4115 39.3751 c
-78.7646 40.6354 d
;
run;