Most modern maps are generated from multiple map layers stored in Geographic Information Systems (GIS). Each map layer represents a different group of geo-referenced data points that are superimposed one on top of the another to create a composite map image. For example, Google maps using this technology to display businesses and public building on top of street maps. It’s also used widely in business applications to visualize relationships between different data layers and geographic areas.
Many modern phone-based backcountry navigation apps including Gaia GPS, Guthook’s Guides, and ViewRanger use map layers to display different views of the same geographic areas and hiking routes. Map layers are also used by Caltopo.com, a free and very powerful web-based mapping application that lets you use incorporate you own data layers.
One of the most powerful features in Caltopo and other navigation apps is the ability to switch between multiple base maps of the same areas, including satellite imagery, so you can view and compare the same route using different types of backgrounds maps. For example, I can draw a hiking route to climb a mountain and then superimpose it on top of different backgrounds that emphasize different aspects of an area to help visualize what I’m likely to encounter on foot.
These base layers starting from top left, clockwise are: USFS 2016, Satellite Imagery, USGS 1885-1912, and Google Terrain:
- the USFS (United States Forest Service) 2016 map is very up-to-date and includes nearby trailheads and roads
- the Satellite imagery is from 2013-2015 and probably based on NASA imagery which is published into the public domain
- the USGS (United State Geological Survey) 1885-1915 is a historic map that shows me what the area looked like 100 years ago
- the Google Terrain map is probably fairly recent and derived from the current USGS Topographic National Map. It’s hard to know for certain, but topography changes fairly slowly compared to human artifacts like buildings, trails, or roads, so it’s probably good enough for navigating.
One of the thorniest issues with backcountry navigation today is knowing which digital base maps and data layers you can trust to be up to date and what you can’t. Digital map publishers and the apps that use digital maps don’t go out of their way to provide this information for you and the copyright notices published on digital maps or map layers rarely correspond to the dates in which the data was gathered. I’ll cover this topic more in another article.
You best bet is to carry a topographic map created by a local map publisher or outdoor organization that has a vested interest in publishing reliable maps. Maps published by national publishers, like National Geographic, are often based on outdated data from last generation USGS maps. It can also pay to carry or plan your trips with multiple paper maps as a way to check or augment the accuracy of digital sources.