Came across a very useful app called CopyIt. ($9)
On the iPad CopyIt uses the camera to take photos or uses existing stored or found web images and superimposes a useful grid from which it is easy to scale up when drawing on canvas or paper. CopyIt also allows griding of 'live' views through the camera.
Handy to have some assistance with proportion and perspective or to check your freehand work.
Lots of options including your choice of background music.https://itunes.apple.com/au/app/copyit-grid-drawing-method/id377185812?mt=8