I found the answer here Updating OpenGL ES Touch Detection (Ray Tracing) for iPad Retina?
- (void)handleTap: (UITapGestureRecognizer *)recognizer
{
CGPoint tapLoc = [recognizer locationInView:self.view];
tapLoc.x *= [UIScreen mainScreen].scale;
tapLoc.y *= [UIScreen mainScreen].scale;
bool testResult;
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);
float uiKitOffset = 113; //Need to factor in the height of the nav bar + the height of the tab bar at the bottom in the storyboard.
GLKVector3 nearPt = GLKMathUnproject(GLKVector3Make(tapLoc.x, (tapLoc.y-viewport[3]+uiKitOffset)*-1, 0.0), modelViewMatrix, projectionMatrix, &viewport[0] , &testResult);
GLKVector3 farPt = GLKMathUnproject(GLKVector3Make(tapLoc.x, (tapLoc.y-viewport[3]+uiKitOffset)*-1, 1.0), modelViewMatrix, projectionMatrix, &viewport[0] , &testResult);
farPt = GLKVector3Subtract(farPt, nearPt);
....
}