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48 lines
2.0 KiB
Markdown
48 lines
2.0 KiB
Markdown
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# README World Map GeoPackage
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The `world_map.gpkg` contains the following layers:
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- countries
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- states_provinces
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- disputed_borders
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These layers were processed from source data. The purpose of this readme is to explain briefly the data sources and the processing steps.
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### Data source
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Natural Earth 1:10m detail level Cultural vectors: https://www.naturalearthdata.com/downloads/10m-cultural-vectors/
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The specific layers used for input are:
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[0]: Admin 1 – States, Provinces v4.1.0
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[1]: Admin 0 – Countries v4.1.0
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[2]: Admin 0 – Breakaway, Disputed Areas v4.1.0 (boundary lines)
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### Processing
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The layer-specific processing steps are described below.
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##### _countries_ and _states_provinces_
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These two layers are derived from data source [0].
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In order to keep the size small, the geometry was simplified with GRASS `v.generalize`, then dissolved
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on state/province level and on country level respectively. These above steps were done with a QGIS model `world_map_generalize.model3` that is included in this folder.
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For the **countries** layer, the attribute table of data source [1] was joined in manually and the country attribute fields were copied.
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*Side notes to the QGIS model:*
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- *Input data are in WGS84 (EPSG:4326); re-projection to a meter-based CRS was necessary
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in order to set meaningful thresholds in `v.generalize`.
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I chose Robinson (EPSG:54030) for that purpose. I think it is an OK
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compromise for a global projected CRS in this case.*
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- *The model also creates a safe extent polygon for WGS84 coordinate space
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and clips the simplified layers to that extent before re-projecting them back to WGS84. This was necessary in order to avoid polygons
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crossing the 180˚ meridian. Those would be rendered as large invalid polygons crossing the entire world.*
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##### _disputed_borders_
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This layer was created by importing data source [2] as-is.
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##### Manual clean-up of attribute tables (all layers)
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Finally, in the GeoPackage the less meaningful fields in the attribute tables were dropped.
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