Contents: Hillshading

What is hillshading?
Optional hillshading
Why is hillshading good for vector maps?

What is hillshading?

‘Hillshading’ describes the shadows that would be cast over the ground if the sun’s rays were shining over it. Although not essential, it does help quickly judge the lie of the land. Some phone apps offer hillshading that may be toggled on or off to taste and it can be an option on GPS handhelds and watches too.

Here in the UK we’re used to the sun shining from the southern half of the compass but maps work much better placing an imaginary light source to the north west. The topography can appear inverted with the light source placed at the bottom of the map. We like our shadows under things. More on Wikipedia.

Online OSM maps usually have hillshading. For example OpenTopoMap, Mapy.cz Outdoor and Thunderforest Landscape.

Phone apps that provide those web-based maps offline download those website image tiles. The map will appear in the app exactly as it does online. You cannot turn the hillshading off.

Optional hillshading

Greyscale shadows are hard to represent in a typical OSM vector map. Vectors are good for points, lines and shapes but not so good for the gradual changes in hillshading. To solve this, a separate layer of semi-transparent shadows must be placed above a vector map. The mostly transparent overlay only bumps up storage requirements a little. A vector map plus hillshading overlay still takes up much less space than converted paper mapping.

Some apps offer a separate hillshading layer which can be applied to any ‘flat’ map. If you don’t want hillshading then switch it off.

Hillshaded OS maps are optional with Locus Map 4.

Although a hillshading overlay is easily provided by mostly-transparent fixed images, more sophisticated apps like Locus Map and Oruxmaps generate a hillshading layer on-the-fly from stored NASA ‘height’ files. These are the same .hgt files that are used to calculate contours for OSM maps. Those same ‘Digital Elevation Model’ (DEM) files make it possible for an app to provide altitude related information and warp maps in 3D etc.

Good hillshading for the UK needs a modest 250MB (roughly) worth of .hgt files in an app like Locus Map 4.

Why is hillshading good for vector maps?

Zooming with a converted paper map just causes the map to change size. That’s it. The number of contours for any given slope doesn’t change.

However, the zoom range is large on a vector map and some, or all, contours must disappear when zooming out, to avoid cluttering the map and slowing the app down. Hillshading helps keep the lie of the land obvious as the contour density varies. Take a look…

Being able to switch a hillshading layer on or off is handy (eg Locus Map 4 or OsmAnd). I much prefer having hillshading ‘on’ if I’m using vector mapping but I might turn it off for OS mapping.