Strange behaviour in multipolygon

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Strange behaviour in multipolygon

Stefano Nannetti
Processing a small area in my region, I noticed a strange behaviour in the rendering of one multipolygon. In the original OSM map (see image A.jpg, bounds are 43.9978 to 44.0135, 10.2989 to 10.3228) the area along the street to "Cardoso" is a strip of empty land (untagged) sorrounded by a forest. This forest is a large and complex multipolygon tagged with natural=wood. Thus, the strip of empty land is defined by lines which are the outer members of the multipolygon (see file cardoso.osm). When processed with mkgmap, the result is somehow "inverted": the map shows the strip of land as wood and the area around as empty, both in Basecamp and in my eTrex 20x (see image B.jpg). To test the issue I created a simple style (see files polygons and points) containing only the file "version", the file "points" which defines only the rendering of places and the file "polygons" which defines only the rendering of the areas tagged with natural=wood. I cannot figure out if the problem lies in mkgmap or in the incorrect mapping of the area in OSM. By the way, the area is correctly shown in many online maps deriving from OSM data and also in the OpenMTBmap which I downloaded and installed in Basecamp. I'm definitely stuck. Any suggestion on how to solve the problem?

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A.jpg (239K) Download Attachment
B.jpg (74K) Download Attachment
polygons (114 bytes) Download Attachment
points (204 bytes) Download Attachment
cardoso.osm (1M) Download Attachment
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Re: Strange behaviour in multipolygon

Gerd Petermann
Hi Stefano,

the problem with your file cardoso.osm is that the multipolygon is incomplete. With this data mkgmap tries to guess the shape of the mp and that fails in this case.

I don't know how exactly you created the file, when I use JOSM to download an area from Overpass api the multipolygon is complete.

Gerd

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Von: mkgmap-dev <[hidden email]> im Auftrag von Stefano Nannetti <[hidden email]>
Gesendet: Mittwoch, 24. April 2019 14:49
An: [hidden email]
Betreff: [mkgmap-dev] Strange behaviour in multipolygon

Processing a small area in my region, I noticed a strange behaviour in the rendering of one multipolygon. In the original OSM map (see image A.jpg, bounds are 43.9978 to 44.0135, 10.2989 to 10.3228) the area along the street to "Cardoso" is a strip of empty land (untagged) sorrounded by a forest. This forest is a large and complex multipolygon tagged with natural=wood. Thus, the strip of empty land is defined by lines which are the outer members of the multipolygon (see file cardoso.osm). When processed with mkgmap, the result is somehow "inverted": the map shows the strip of land as wood and the area around as empty, both in Basecamp and in my eTrex 20x (see image B.jpg). To test the issue I created a simple style (see files polygons and points) containing only the file "version", the file "points" which defines only the rendering of places and the file "polygons" which defines only the rendering of the areas tagged with natural=wood. I cannot figure out if the problem lies in mkgma
 p or in the incorrect mapping of the area in OSM. By the way, the area is correctly shown in many online maps deriving from OSM data and also in the OpenMTBmap which I downloaded and installed in Basecamp. I'm definitely stuck. Any suggestion on how to solve the problem?
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Re: Strange behaviour in multipolygon

Stefano Nannetti
In reply to this post by Stefano Nannetti
Thank you Gerd, it works. I used to download the data from the Export section of openstreetmap.org, defining the bbox by hand and then clicking on "Overpass Api" (not just on the Export button). Probably the http call to Overpass misses an instruction to download the multipolygons completely.

Stefano

Il giorno sab 27 apr 2019 alle ore 13:00 <[hidden email]> ha scritto:
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---------- Forwarded message ----------
From: Gerd Petermann <[hidden email]>
To: Development list for mkgmap <[hidden email]>
Cc: 
Bcc: 
Date: Sat, 27 Apr 2019 05:42:17 +0000
Subject: Re: [mkgmap-dev] Strange behaviour in multipolygon
Hi Stefano,

the problem with your file cardoso.osm is that the multipolygon is incomplete. With this data mkgmap tries to guess the shape of the mp and that fails in this case.

I don't know how exactly you created the file, when I use JOSM to download an area from Overpass api the multipolygon is complete.

Gerd

________________________________________
Von: mkgmap-dev <[hidden email]> im Auftrag von Stefano Nannetti <[hidden email]>
Gesendet: Mittwoch, 24. April 2019 14:49
An: [hidden email]
Betreff: [mkgmap-dev] Strange behaviour in multipolygon

Processing a small area in my region, I noticed a strange behaviour in the rendering of one multipolygon. In the original OSM map (see image A.jpg, bounds are 43.9978 to 44.0135, 10.2989 to 10.3228) the area along the street to "Cardoso" is a strip of empty land (untagged) sorrounded by a forest. This forest is a large and complex multipolygon tagged with natural=wood. Thus, the strip of empty land is defined by lines which are the outer members of the multipolygon (see file cardoso.osm). When processed with mkgmap, the result is somehow "inverted": the map shows the strip of land as wood and the area around as empty, both in Basecamp and in my eTrex 20x (see image B.jpg). To test the issue I created a simple style (see files polygons and points) containing only the file "version", the file "points" which defines only the rendering of places and the file "polygons" which defines only the rendering of the areas tagged with natural=wood. I cannot figure out if the problem lies in mkgmap or in the incorrect mapping of the area in OSM. By the way, the area is correctly shown in many online maps deriving from OSM data and also in the OpenMTBmap which I downloaded and installed in Basecamp. I'm definitely stuck. Any suggestion on how to solve the problem?

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[hidden email]
http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev

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