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Map Projections

Does the map projection matter?

No, not when you are zoomed in on a country or further in, then most decent projections will work.

swerefvsmercator

A local projection (Sweref99) left and a global projection (Mercator) right.

Can you see the difference? To the left, a projection that is specifically tailored for the area and to the right Mercator. If you look carefully you can se a slight rotation between the two but nothing that disturbs the presentation.

I often hear that “We must present the data in our local projection so that it is displayed correctly”. This usually is a misunderstanding. It is difficult to see differences between decent projections at a local scale. What matters is that the data presented on the map ends up at the correct location, which it does if the data is reprojected properly.

The problem comes when you zoom out. Since the Earth is round it can not generally be viewed in a nice way on a flat screen. What makes a good projection? Traditionally cartographers talk about the parameters shape, distance, direction, scale and area. All can not be correct for a projection but different projection often focus on getting one or two correct at the expense of the others. When zoomed in, most parameters are OK for most projections. However, a good projection can also be classified as giving the viewer the right impression of the area.

Lets look at some common projections.

mercatorworld

The World in Mercator projection.

Mercator is the completely dominant projection for web maps. Advantages are that it shows the entire earth and angles are correct in each point (shape). The big disadvantage is that areas near the poles are extremely exaggerated. Greenland looks like it is the same size as Africa when it actually is just one fourteenth of the area!

globe

Globe projection.

I think the globe projection, where you can dynamically rotate the earth, is one of the nicest. It gives a correct impression since the eye interprets it as a globe. However, the big disadvantage is that only half the world can be shown at a time and most of the visible half is shown at a “bad angle”.

equirectoutin

Equirectangular projection. Notice how the road network looks stretched. The roundabout is an oval.

The Equirectangular projection has same scale in all directions only at one latitude, in this case +-40 degrees. It somewhat compensates for the exaggerated size of Greenland. Probably you can live with the distortions when you are zoomed out (as discussed above, you have no choice, you just can chose which distortions) but when you zoom in on areas not close to latitude +-40 degrees you would expect the same scale in all directions. In this projection the map will look stretched.

iaoutin

Idevio Adaptive projection. Aspect ratio is correct when zoomed in.

Idevio have introduced an adaptive projection that have the properties of the Equirectangular projection when zoomed out but still looks correct when zoomed in. Since we are not dealing with static paper maps and that Idevio WebMap (IdevioMaps) uses vector data, it is possible to adapt the projection to the viewed area. It is a great improvement over the Mercator projection which gives a totally wrong impression of sizes. Also, at least as important for practical use, Greenland and north Canada does not take up half the screen when you display results on the map. It also keeps a constant scale when panning so that at a particular level the scale is not changed. It looks familiar with north always up (in contrast to for instance the globe projection). Except for the poles, Idevio Adaptive projection works in all scales and for all places on earth.

mercatorVsIdevioAdaptive

Mercator vs Idevio Adaptive. The two maps have the same upper left corner and the same scale.

The Idevio Adaptive projection combines the good properties of the Equirectangular projection when looking at a global scale with minimal distortions when zoomed in.

 

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