How to Interpret Satellite Imagery for Backcountry Navigation using Hillmap.com

Satellite imagery is a powerful source of information for planning backcountry routes but it takes practice to interpret what you’re looking at, and whether it depicts features you can use to make your route easier or whether you should avoid them.

The best way to view satellite imagery is side-by-side with a topographic map of the area you’re interested in. I use a free online navigation planning tool called Hillmap.com is ideal for this purpose because it keeps both views of the same area in synch with one another. For example, if I’m viewing a topographic map that has a river and a highway drawn on it (above left), I can easily correlate that with the imagery associated with them (above right.) I can also draw a route on a topographic map and have it mirrored on top of the satellite image.

If I drill down further into the satellite view (left above), I can make out three landscape features not shown on the topographic map of this area.

1. The river does not fill the entire channel drawn on the topographic map and it looks like there are large areas of dry rock or sand along its banks.
2. There is a lighter green line of vegetation that runs parallel to the highway (red dashed line) depicted on the topographic maps. This is probably a power line or a railway line that’s been cut open through the forest. Chances are good that the vegetation is thinner along it or that there’s some sort trail that runs over it.
3. Below it is a set of polka-dot like shadows that look like depressions scooped out of the forest cover. These are likely logging cuts that have small trees growing in them and would be difficult to walk through. They can be easily skirted though, but it’s good to know that they’re there.

With practice, you can teach yourself how to recognize a wide range of landscape features by matching what’s shown on topographic maps and what’s not. Features like cliffs, ravines, landslides, trails, paved roads, dirt roads, rivers, streams, open fields, potential property boundaries, buildings, bridges, dams, bogs, and so on. Some of this information can be quite useful, especially if you don’t have up to date topographic maps or road maps for backcountry areas, something that’s quite common in the more rural areas of the backcountry.

Like all maps and mapping data sources, you need to be a little cautious about the completeness and age of the satellite imagery data you use to plan routes in navigation planning tools. For example, the satellite imagery shown in these examples is labelled in Hillmap as being sourced in 2017, which hardly seems likely, given that it’s still winter in this area and much of it’s still covered in snow. When in doubt, use Google to find trip reports or other data sources, including paper maps, if you require more information.

Written 2017.

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