c# - Create circle around point in dynamic display data map by radius and angle -

i using microsoft visual 2010 dynamic data display dll need circle around point angle , radius. have been successful it's wrong, think so. first of all, source code:

i got preps mouseclick (it's not problem point perfect working can see next in picture)

 // x position of pointclicked                     cx = (double)preps.x;                     // y position of pointclicked                     cy = double.parse(this.plotter.viewport.transform.datatransform.viewporttodata(preps).y.tostring());                      // new x position of pointclicked angel math calculation                     xendp = (float)(double.parse(txt_enterradius.text.tostring()) * math.cos(a * math.pi / 180f)) + cx;                     // new y position of pointclicked angel math calculation                     yendp = (float)(double.parse(txt_enterradius.text.tostring()) * math.sin(a * math.pi / 180f)) + cy; 

secondly actualy got : @ middle got perfect circle, in north , south circle type of ellipse. picture:

http://sizmedia.com/my.php?i=hm2zuv5yyenj.png 

i happy understand : why? good? or need change something? becuase thought , earth circle , type of reasonable.