Texture transform problem

I am trying to simulate the effect of the camera moving over an infinite textured plane. I am doing this by placing a stationary surface(centred at 0,0) and applying texture transforms to give the appearance of movement. I have set the original texture coordinates such that the texture origin lies at the centre of the surface. I can now happily rotate the texture to look in different directions. The problem is when I move forward. I am calculating what I think ought to be the correct translation in x and y to produce the desired effect but whilst the texture moves it doesnt move in the right direction so as to simulate a camera moving forward. Getting desperate. Here is the relevent code. Any suggestions greatly appreciated.

if (direction == ‘l’) // left
theta = theta + 3 ;

if (direction ==‘r’) // right
theta = theta - 3 ;
if (direction == ‘f’) // forward
txinc = sin(0.0175theta) ;
tyinc = cos(0.0175
theta) ;
txpos = txpos + txinc ;
typos = typos + tyinc ;

glMatrixMode(GL_TEXTURE) ;
glLoadIdentity() ;

glTranslatef(txpos, typos, 0) ;
glRotatef(theta, 0.0,0.0, 1.0) ;
glMatrixMode(GL_MODELVIEW) ;