In the very beginning, I started what would ultimately become CalTopo as a hobby project focused on offline environments - a command post or tailgate somewhere off in the woods far away from the nearest internet connection. As CalTopo grew, that ability for entirely self-contained operation has remained as a little known but fundamental building block of the codebase. Flip out a few of the underlying bits - Google Maps with OpenLayers, a standalone database with HSQLDB, Tomcat with Winstone - and you'd get a free-standing Java web app with no external dependencies.
For search and rescue, those who knew the secret handshake could get a similarly configured copy of SARTopo, delivered via physical thumb drives along with statewide map data. While this system worked for a while, it's been chugging along on a donut spare and one headlight for some time. Although releasing copies of a subscription-driven website into the wild feels a bit risky, it's past time to make the offline functionality a supported feature that doesn't require my involvement with every install.
Welcome to CalTopo (and SARTopo) Offline.
Due to the costs of delivering map data and the extra support footprint it will present, there was no way I could roll the offline functionality into the existing subscription levels. Instead there is now a $100/yr offline subscription level that also includes all pro-level features. First responders who have upgraded to a SAR account will see a $50/yr offline upgrade option.
After upgrading, your account dialog will have an offline access tab with links to downloading both map data and a copy of the program:
Map data is provided in 1 degree by 1 degree blocks, in MBTiles format. Available layers include USGS 7.5' topos, forest service maps, slope angle shading, elevation data for profiles and viewsheds, and NAIP aerial imagery down to 1 meter resolution; most blocks include 2 years' worth of imagery.
Each account is allowed 300GB worth of map downloads per year, with no geographic restrictions other than being limited to the lower 48 states. It's up to you whether you want to skip the imagery and cover a larger footprint, or get 2 years worth of 1 meter imagery for a smaller coverage area. At the moment data is only available for the red-shaded areas below, but that should grow to full lower-48 coverage within another week or two.
I'd post some screenshots, but there's not much to show - the offline version looks very much like caltopo.com. For more information on how this all works, check out some of the very thorough documentation written Patty Lindsay (there is hopefully more documentation of this caliber coming to help.caltopo.com).