Understanding scale with Pan and Zoom

Scale has been identified by the American Association of Science (1993) as a cross cutting concept uniting many science areas.  this appears to be borne out by an investigation of experts, the authors found that experts from a huge variety of subject areas all identified understanding scale as key to their topic (eg architects, biologists, cosmologists, physicists ...).
Meanwhile an investigation was done of the educational film 'Powers of Ten'.


 I posted a Google Earth version a while back.  The film has been used a great deal by science school teachers in the States to teach students about scale and the investigation tested its effectiveness in doing this.  Several important points came out of their work:

• Students 11-14 years old when asked to think of the largest thing they could imagine came up with ideas like football pitch and street.
• Students are good at comparing scales (which is bigger?) but poor at imaging absolute scale (wildly out when asked how many miles the United States is across).
• Their performance at understanding scale improved after watching the film.

My thoughts about this are that one of the unsung benefits of using Google Earth in education is its ability to convey scale easily via the power of zoom.  The Powers of Ten film effectively consists of a slow zoom out from human scale to the size of intergalactic space.  This communicates scale via the use of comparative scale - you can see the 10m square disappearing into the centre the centre of the screen as we zoom past the bigger 100m.  
However using Google Earth zoom to convey scale effectively assumes that we:

• Anchor Points: Provide 'anchor points' for users so they can compare sizes on screen e.g. A grid or set of squares (as in the Powers of 10 film) or showing the area of ice disappearing in the Antartic?  Then show an outline of the country of Wales (or similar) familiar to your users to give a sense of scale. [More on Wales as a unit area cliche]
• Zoom Speed: If you zoom out or in too fast user's will have trouble following the scales visible.  In the Power's of 10 film they fly at a gentle 1 change of magnitude per 10 seconds, you can go faster than this without confusing user's but it depends on the complexity of what is in view.