int _high = map(xval, xaxis[xpos-1],xaxis[xpos], zvalues[xpos-1,ypos], zvalues[xpos,ypos]);
This isn't the right way to index into a 2-dimensional array:
zvalues[xpos,ypos]
int _high = map(xval, xaxis[xpos-1],xaxis[xpos], zvalues[xpos-1,ypos], zvalues[xpos,ypos]);
This isn't the right way to index into a 2-dimensional array:
zvalues[xpos,ypos]