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[closed] Distorted map view

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uuderzo

[closed] Distorted map view

Post by uuderzo »

Hello,

I used Alpine quest for several days without a glitch, then it subtly started to show a distorted view of the map. Everything is stretched horizontally (tracks, points, map, radial distance indicator).
It sure started due of a map configuration modification but I cannot recall what I did because I been aware of that distortion after a while (I deactivated the radial tool so it was subtle and simply strange).
I recall it happened also several years ago and after some stumbling on the options I gave up and uninstalled the application. Now that I reinstalled it and want to give another go I wish I'd love to understand what's the cause so I can fix when it happens.

P.S. it's not perspective view, if I use it the map get more distorted because of the perspective. It simply looks like the app starts to apply a scaling that is not proportional.

Thanks!
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Re: Distorted map view

Post by Psyberia-Support »

Hi,
A screenshot would help a lot.

However, if you have stacked multiple maps (check the Maps menu -> Displayed maps), the application will use the map projection of the bottom most map (excluding the default world map), and re-project over maps accordingly. If over maps have a different map projection, it will make a kind of "stretching" so that all images matches together.
But in this case tracks and points should not be affected, only the over maps.
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uuderzo

Re: Distorted map view

Post by uuderzo »

Sorry. I attach a screenshot.

Yes there is map stacking. I discovered that if I place the DEM hillshading map just over the base map I get the distortion. If I shift the map up one or more levels everything is ok.

So can I assume that the DEM hillshading map has wrong projection data in it?

Thanks!
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Re: Distorted map view

Post by Psyberia-Support »

The DEM hillshading does not have a wrong projection. It's just that your 3 maps uses different projections, so the application must stretch the ones displayed over in order that the map features correctly match on all maps.

For information hillshading uses the "plate carrée" projection, Cycle/Hike map the "pseudo-mercator" one, and the italia topo a "transverse mercator".

Anyway displaying hillshading at the bottom does not make real sense because it's not visible at all.
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uuderzo

Re: Distorted map view

Post by uuderzo »

I used darken blending modes on the overlays. Anyway it makes more sense to put the DEM map over all the others.
I'm not expert on maps projections, just wish to understand if all those maps are ok to be displayed in stack? At first glance, excluding possible proportion issues that now I don't perceive, they look to be correctly aligned to each other.
Is there some article I can read to help me avoiding those kind of mistakes is future?
Thank you again for the support!
Umberto
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Re: Distorted map view

Post by Psyberia-Support »

There is no problem to stack all those maps, it's just that it is not visually nice because this "stretch" effect appears, which is normal.

To make things easier to understand, imagine you have 2 paper maps of the same area, but they don't have the exact same scale.
If you want to display one over the other, and have all the locations on both maps to match, you'll have no other choice than squeeze the large one or stretch the small one.
The application does exactly this. Exact that based on how your particular maps are made, it only need to stretch the horizontal axis.

Now what is a map projection? It a "technique" (transformations, distortions, stretches, ...) that allows you to put on a paper (which is flat), a part on the Earth (which is round). More technical details here.

Hope it helps
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uuderzo

Re: Distorted map view

Post by uuderzo »

Thank you, very helpful, I'll read asap!

Please allow me a last question... Why the DEM map if used alone leads to a distortion anyway? I mean... The radial tool appears distorted also if DEM map only is used. Shouldn't it look normal if used alone?

Is just that the projection used simply appears stretched in my location, and it's not possible to adjust that until there is a more local map with proper projection that drives the DEM map stretching?

Thanks!
Umberto
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Re: Distorted map view

Post by Psyberia-Support »

This is how this map is made. Distances on the X axis (longitude) are not equal to the distances on the Y axis (latitude). At least at your particular location. If you go to the equator, it will be real circles.
Look at the Tissot indicators here (2nd image), it shows circles all around the Earth, and how they appear on a flat map.

Imagine you have a scale model of the Earth in your hands, the size of a ball. Now I ask you to make me a (flat) paper map from it. How would you do?
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uuderzo

Re: Distorted map view

Post by uuderzo »

Yep!

Now it's perfectly clear! Now that I understand how Alpine quest handles projections, I appreciate more and more how it's designed.

Thanks again and keep up this great app!
Umberto
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