I started to do some tests with Back Country Navigator to see the differences between the types of maps against the same area. The test area was about 1/5 the area of Oregon. Here's what I've found so far:
map data-
USGSTopo: 4.9GB
ArcGIS: 2.49GB
CalTopo: 15.78GB (extrapolated)
To put into perspective, here are rough estimates to cover the entire state-
USGSTopo: 24.5GB
ArcGIS: 12.45GB
CalTopo: 78.9GB
Natl Geo TOPO!: 3.14GB
BCN might render tiles for each zoom level (though it looks digitized after level 15). Topo! has 5 zoom levels using distinct map data, with USGS 500k, 100k and 24k map data for the 3 closest zooms. That approach appears to be much more efficient for storage. I don't think BCNs many levels of zoom gets you much on a small screen as you begin to zoom out. Topo type maps get very busy when lines begin to squish together and make maps unusable. If BCN used a digitized zoom to fill the gaps between actual map data it would go a long ways toward making large areas portable. Likewise if BCN allowed you to download *only* a certain zoom level (vs downloading "down to" a level), it might go a long ways to make the data stretch and give BCN's servers a breather. At least they haven't disabled area downloads (yet).
Even Nat Geo has abandoned stand-alone TOPO! State Series for download subscriptions, and the alternatives to actual USGS type maps are (like Terrain Navigator) are expensive per state. Overland Navigator has done a good job at providing very fairly priced 24k state data using less space (the cost being the data looks to use more compression), but it isn't out for iPad yet and no plans for Android.
Still waiting for an offline wide area solution as well.
