Country Centroids - GBIF Data Blog

#1

Country Centroids are a known data quality issue within the GBIF network.


This is a companion discussion topic for the original entry at https://data-blog.gbif.org/post/country-centroids/
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#2

Interesting post. Determining the centre of a country, however, is not always easy. For example, for Australia there are five different (accepted) methods for determining the centre of the country:

Centre of Gravity Method 23° 07’ South, 132° 08’ (-23.1166667, 132.33333) (0 in the ALA records within 1 km)

Lambert Gravitational Centre 25° 36’ 36.4"S, 134° 21’ 17.3"E (-25.610111, 134.3548056) (In the ALA, Lots of records within 1km - also it comes up on the ALA map about 300 m from where Geosciences Australia places it! which sounds like it may be a Datum shift?)

Furthest Point from Coastline 23° 02’S, 132° 10’E (-23.033333, 132.1666667) (0 records in ALA Database within 1 km)

Geodetic Median Point 23° 33’ 09.89"S, 133° 23’ 46.00"E (-23.5527472, 133.396111) (8 records in ALA within 1 km)

Johnston Geodetic Station 25° 56’ 49.3"S, 133° 12’ 34.7"E (-25.9470278, 133.2096389) (lots of records in ALA within 1 km)

Some of those records maybe good records with a low uncertainty. The place where ALA places records that just say “Australia” or “Nova Hollandia” doesn’t coincide with any of the above
ALA -25.2744, 133.7751333 (24 records within 1 km in the ALA Database)

Incidentally - the Getty Thesaurus of Geographic Names (TGN) rounds the location (for Australia) down to -25.0000, 135.0000

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#3

Thanks for pointing out these other centroids, @ArthurChapman .

I was using the centroids published with the R package CoordinateCleaner. I am not sure what method they used to calculate their centroids, but obviously more centroids need to be added.

CoordinateCleaner uses these two centroid locations for Austrailia:

"CoordinateCleaner" (-27.000, 133.000)
"CoordinateCleaner" (-24.973, 136.189) 

As you point out, these two points are not in your list:

“Centre of Gravity Method”, -23.1166667, 132.33333
“Lambert Gravitational Centre”, -25.9470278, 133.2096389
“Furthest Point from Coastline”, -23.033333, 132.1666667
“Geodetic Median Point”, -23.5527472, 133.396111
"Johnston Geodetic Station",-25.9470278, 133.2096389 (lots of records in ALA within 1 km)
“Getty Thesaurus of Geographic Names”, -25.0000, 135.0000

This is probably why I do not detect a significant country centroid in Australia even though there is one at “Johnston Geodetic Station”,-25.9470278, 133.2096389

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#4

No idea where Coordinate Cleaner got those centroids. Nowhere near where any of the other methods detect it - ?maybe they used the external territories (Norfolk Is, Christmas Island etc.) as extremes and then got the centre. That wouldn’t explain being 2 degrees south of the others. Also, in the first case, rounding to a degree of latitude and longitude doesn’t make sense. I will chase up when I get home in a few weeks time. I am working on another project in Argentina at the moment.

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#5

From the R docs: "Coordinates are based on the Central Intelligence Agency World Factbook https://www.cia.gov/library/publications/ the-world-factbook/ and http://thematicmapping.org/downloads/world_borders.php."

So I’d say @ArthurChapman is right on the money, external territories are skewing the centroid. If I remember correctly, the world_borders shapefile includes Macquarie, Heard & McDonald Islands as part of Australia, which could explain the 2° southward skew.

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