USGS KMZ warped

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abwx
Posts: 5
Joined: Fri Sep 25, 2015 2:57 am

USGS KMZ warped

Post by abwx » Thu Apr 22, 2021 5:50 pm

Importing a USGS 1:62500 topo from https://ngmdb.usgs.gov/topoview/viewer/ ... /-122.3268 ("63K" button, 1943 map) displays the rectangular map as a square. Adding a layer might warp the added layer to correspond to the warped square, but maybe almost always corrects the KMZ to the new layer?

Experimenting with a layer set of OSM "Standard Carto," plus the KMZ, plus Open Railway "Infrastructure," when I move various layers up or down, the map sometimes goes back and forth between warped and okay - even if the layer I am moving is hidden. I'm not convinced if the behaviors are entirely consistent, but it seems likely that they are.

In the course of all this, I've figured out how to get what I want, so I wouldn't call it a priority for fixing.

It's actually been fun.
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Psyberia-Support
Site Admin
Posts: 4957
Joined: Wed Apr 14, 2010 9:41 pm

Re: USGS KMZ warped

Post by Psyberia-Support » Fri Apr 23, 2021 9:12 am

Hi,

Yes what you describe is the intended behavior.
When you stack multiple maps with different map projection, the application uses the most bottom (excluding the default world map) one as the reference.
All layers above this maps are re-projected ("warped") so that all locations matches correctly.

Because the KMZ map is a square, if it's used as the reference map, it is displayed as a square and all maps above are re-projected accordingly.
When it's not the reference map, it's stretched so that it matches the OSM map projection (vertical deformation is higher latitudes).
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abwx
Posts: 5
Joined: Fri Sep 25, 2015 2:57 am

Re: USGS KMZ warped

Post by abwx » Sat Apr 24, 2021 8:42 pm

Thank you for your quick response.

AlpineQuest details on the imported KMZ map says "LATLONBOX" which is a bounds, not a projection, and "EPSG:NONE (AFFINE)" which is a clue, not actual information.

Being a little slow, I wasted quite bit of time trying to look inside the .KMZ file before it occurred to me that I just needed to rename it to a .ZIP. Then I had more problems getting into the KML, Where there is no clue to projection anyway.

Well later, I found that the KML standard is EPSG:4326, which goes with your "affine" and probably should replace the "EPSG:NONE."

Are any of AlpineQuest's internal or community maps using a projection other than EPSG:3857?

Is there a list somewhere of the EPSG codes that AlpineQuest supports?

Psyberia-Support
Site Admin
Posts: 4957
Joined: Wed Apr 14, 2010 9:41 pm

Re: USGS KMZ warped

Post by Psyberia-Support » Mon Apr 26, 2021 2:54 pm

Here are some more details on this subject.
EPSG:4326 is not a projection, just a geodetic coordinate system. The EPSG gives IDs to various things, not only map projections.
The Lat/Long information in the KML files only tells you the locations on the Earth of the four corners of the attached image, given in the EPSG:4326 format (1 axis is longitude, 1 axis is latitude).
But there is no explanations on how to display the image, how it needs to be stretched (and if it needs). This is usually the job of the map projection. How to turn something spherical (the Earth) into something flat (A map sheet, a phone screen, etc). This is done with various parameters, for example see the "Definition: Well Known Text (WKT)" section here for EPSG:3857 : https://epsg.io/3857
So when using KML maps ("kml overlay images"), the application will simply display the image in its original format. Your image is a square (5300px X 5300px), so it's displayed as a square.
Concerning the name displayed by the application, it's "none" simply because there is no epsg code provided in the kml file. But this is only for user information.
Are any of AlpineQuest's internal or community maps using a projection other than EPSG:3857?
A lot of them. For example you can check the "Netherlands topo map" (which uses the stereographic map projection).
Is there a list somewhere of the EPSG codes that AlpineQuest supports?
You can check this website that list a lot of epsg codes: https://epsg.io/
The app supports most of them.
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