How to use aerial imagery on a bushwalk
Aerial imagery is remarkably easy to come by these days and can be extremely useful in identifying interesting bushwalking features, potential routes and vegetation thickness. It’s easy to spot urban areas, constructed buildings and tracks and possible to determine elevation via shadows.
Aerial imagery means any images taken from an elevated position, including photography from helicopters, fixed wing aircraft or drones, right through to air balloons, parachutes and satellites.
A few free sources include:
Google Maps also gets its high-resolution images of cities from aircraft flying at 240 to 460 m, and the rest from satellites. LANDSAT satellites have been pivotal in providing accurate environmental data about the earth’s surface and in 2016 google earth has released worldwide images sourced from Landsat 8. Most of the satellite images are no more than three years old, meaning that the user can be confident that the information is reasonably accurate and current.
Reading and interpreting How to read and interpret aerial imagery
Just like topographic maps, manmade and natural features can be identified on aerial images. Aerial images show texture, shadow and patterns only. They do not usually include names of landmarks, roads or settlements, or information about elevation.
Shadows help to identify elevation and relief on aerial photos. Shadows that fall outwards indicate higher elevations such as hills or mountains, whereas shadows that fall inwards indicated riverbeds, creeks and gullies. The rate of shadow appearance and disappearance along a ridgeline can determine the slope and steepness of features.
Rivers are wide dark grey lines with many curves and bends in them. Side creeks flow into them. The v-shape of the connection between a side creek and the main creek always points downstream. Hence the reader can determine flow direction.
The colour of clear and muddy water bodies is different due to different amounts of reflected sunlight. Since clear water reflects less sunlight than muddy water, muddy water appears lighter in colour. Sandy soil also looks lighter than humid soil surfaces.
Aerial images can also give an indication of whether a water source is perennial or non-perennial, helpful when planning water sources on a walk.
Natural vegetation and grasslands usually have dark tints and a diverse pattern. Crops or plantations are identified by their unnaturally straight pattern of growth and patchwork appearance of cultivated lands. Tall crops appear darker in colour than small crops. Cultivated vegetation usually has a smooth, fine appearance, whereas natural bushland is dotted, and mountainous areas rough.
Manmade objects tend to be evenly distributed with straight edges. Broad grey lines with few bends in them are major bitumen roads. Secondary roads are generally narrower with more curves. Fire trails may have a brown/orange/red tinge to them depending on the soil and road surface. Railway lines are small grey lines with long, smooth curves. Roads connect up perpendicular to each other while railway lines gradually merge.
Planning Using aerial imagery to plan a trip
Aerial imagery can be helpful in conjunction with topographic maps. They give an overview of the area, and able the user to see different patterns than on a topographic map.
From a bushwalking perspective, aerial images can be helpful to identify:
- Clear ridges for easy walking
- Dense scrub vegetation to avoid
- Potential canyons, caves and other rock formations of interest
- Perennial and non-perennial water sources for managing water needs
- Cultivated (i.e. private) properties where bushwalkers cannot go without permission
- Clear areas as potential campgrounds, or equally cliff-lines with potential camp-caves
- New or altered fire trail or roads not shown on topographic maps
In NSW, bushwalkers use aerial imagery to detect canyoning areas. Often, the 10-20m topographic contour maps provided by the lands department are not accurate enough to determine if a creek line has or doesn’t have canyon features. Aerial imagery gives a reasonable indication of the depth of the cliff and can be helpful to figure out if the area is worth exploring.