Cartography
Posted: 4 Jan 2022, 4:45pm
On terrain maps, why are the shadows on the south side of hills?
Please could you show an example.
Thanks
Jonathan
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Re: Cartography
Posted: 4 Jan 2022, 4:51pm
Re: Cartography
Posted: 4 Jan 2022, 5:09pm
Here is one.
https://commons.m.wikimedia.org/wiki/Fi ... d_1964.jpg
I have a copy of that very map, which I remembered.
The light appears to come from the NW.
The cover price is 50p.
Re: Cartography
Posted: 4 Jan 2022, 5:13pm
That's a good question. Here's one answer:
https://ramblemaps.com/why-does-sunligh ... from-north
https://ramblemaps.com/why-does-sunligh ... from-north
Re: Cartography
Posted: 4 Jan 2022, 5:20pm
Thanks for the example.
Shaded relief, or hill-shading, shows the shape of the terrain in a realistic fashion by showing how the three-dimensional surface would be illuminated from a point light source. The shadows normally follow the convention of top-left lighting in which the light source is placed near the upper-left corner of the map. If the map is oriented with north at the top, the result is that the light appears to come from the north-west. Although this is unrealistic lighting in the northern hemisphere, using a southern light source can cause multistable perception illusions, in which the topography appears inverted.
https://en.wikipedia.org/wiki/Terrain_c ... ded_relief
I think that's the same reason as the one that AndyK linked..
Jonathan
Shaded relief, or hill-shading, shows the shape of the terrain in a realistic fashion by showing how the three-dimensional surface would be illuminated from a point light source. The shadows normally follow the convention of top-left lighting in which the light source is placed near the upper-left corner of the map. If the map is oriented with north at the top, the result is that the light appears to come from the north-west. Although this is unrealistic lighting in the northern hemisphere, using a southern light source can cause multistable perception illusions, in which the topography appears inverted.
https://en.wikipedia.org/wiki/Terrain_c ... ded_relief
I think that's the same reason as the one that AndyK linked..
Jonathan
Re: Cartography
Posted: 4 Jan 2022, 5:25pm
another explanatory paragraph...
In addition to boundaries and contours, a map may also illustrate topography using techniques like hypsometric tinting and hill shading. Hypsometric tints use hue to encode the elevation at a point, while hill shading conveys slope by (approximately) shading the land surface as if lit from a canonical direction – generally north-west. Both techniques leverage the human visual system, taking advantage of our propensity for interpreting shape from shading, and our familiarity with colors found in nature as a diagramatic shorthand. Terrain features depicted in this manner can generally be perceived and interpreted with little conscious effort.
- "lit" from north west / top left
Re: Cartography
Posted: 4 Jan 2022, 5:36pm
Here is a French one.
Re: Cartography
Posted: 4 Jan 2022, 5:49pm
Now I see!
Re: Cartography
Posted: 4 Jan 2022, 5:50pm
Most people spent a childhood with stuff being lit from above and, to an extent, in front (since your own body is generally occluding light from behind a little) so when faking depth via shading that needs to be followed as that's what your brain has programmed itself to expect.
Note what's being faked here isn't the *real* lighting of the world's surface, it's the lack of relief of the map. Imagine the map were a 3d model on a wall, it would (odd interior design choices aside) be lit from above, not from the south.
Note what's being faked here isn't the *real* lighting of the world's surface, it's the lack of relief of the map. Imagine the map were a 3d model on a wall, it would (odd interior design choices aside) be lit from above, not from the south.
Re: Cartography
Posted: 4 Jan 2022, 6:15pm
Re: Cartography
Posted: 4 Jan 2022, 6:55pm
I used to make bespoke shaded relief maps for various professional reasons. I would routinely rotate the notional light source so that features I were interested in were preferentially picked out (eg N-S features are best shown lit from the E or W). However, often while the image looked fine to me (high points looked like highs), other people would swear the topography were inverted. Shading is surprisingly personal.Jdsk wrote: ↑4 Jan 2022, 5:20pm Thanks for the example.
Shaded relief, or hill-shading, shows the shape of the terrain in a realistic fashion by showing how the three-dimensional surface would be illuminated from a point light source. The shadows normally follow the convention of top-left lighting in which the light source is placed near the upper-left corner of the map. If the map is oriented with north at the top, the result is that the light appears to come from the north-west. Although this is unrealistic lighting in the northern hemisphere, using a southern light source can cause multistable perception illusions, in which the topography appears inverted.
https://en.wikipedia.org/wiki/Terrain_c ... ded_relief
I think that's the same reason as the one that AndyK linked..
Jonathan
Re: Cartography
Posted: 5 Jan 2022, 6:47am
As you observe, it is very wrong. But I was told that it was done that way because if it were done the correct way (shading to the north side of hills) the image isn't interpreted as well by the human eye. You could turn a map upside down to test this. But it is a curious thing.
Re: Cartography
Posted: 5 Jan 2022, 10:46am
I suppose now, when I look at terrain maps, I'll just have to imagine it's the summer solstice, half an hour before sunset.
Re: Cartography
Posted: 5 Jan 2022, 2:25pm
Wrong, but irrelevant.
The only people who are entitled to complain about it not being the way they are used to seeing mountains are pilots and astronauts.
Re: Cartography
Posted: 5 Jan 2022, 4:45pm
While i understand that shading is a lovely thing on maps used for decoration, i fail to see what it adds to a map being used for navigation. Those french topo maps are just cluttered up with shades and colours. I dont think think I've ever looked at an OS map and thought "if only it had lots of shading on"
This is very much imho of course, and perhaps influenced by my 1st year cartography and drawing practicals involving proper ink and drawing film...
This is very much imho of course, and perhaps influenced by my 1st year cartography and drawing practicals involving proper ink and drawing film...