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GenericMappingTools / gshhg-gmt

Scripts and raw data that combine to produce the GSHHG data sets for GMT
GNU Lesser General Public License v3.0
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Northern Greenland coastlines are inaccurate #12

Open evangowan opened 1 year ago

evangowan commented 1 year ago

Hello,

I am compiling data to document deglacial sea level changes in Greenland, and I noticed there is an error in the coastlines in GSHHG. I have illustrated this error by comparing the shorelines to the GEBCO 2021 topography dataset (the black lines are the GSHHG shorelines):

north_greenland

The red dots are locations where radiocarbon dated fossil shells and driftwood have been found in glacial isostatically uplifted marine and shoreline deposits. Many of these data were reported in a paper by Funder et al., 2011 (https://doi.org/10.1126/science.1202760). I asked Dr. Funder about this, and he said that the locations of data they collected were measured using GPS, so they are undoubtedly accurate. The map in the paper shows the correct shoreline positions.

At its worse, the coastline in GSHHG dataset is about 35 km from the (apparent) true location. This error also affects Google Earth. The georeferencing of the satellite imagery in Google Earth appears to have used GSHHG as a base. It appears that the coastline west of about 53W and south of about 78N is accurate

I have attached the script I used to create the above image (though you will have to download GEBCO separately).

test_greenland.zip

PaulWessel commented 1 year ago

We have been aware of this for a decade or more but nobody has the time or energy to fix such bad data. If I had personally needed that coastline fixed then probably I would have but as it is, lots of work. Not just the coastline of main Greenland, but changes to islands, and from other data, lakes.

evangowan commented 1 year ago

Hi Paul,

Thank you for letting me know. I understand what an undertaking this would be.

I will contact the Geological Survey of Denmark and Greenland and see if anyone there might be able to fix this.

evangowan commented 1 year ago

I guess if anyone runs into this, as a workaround, I extracted the coastlines from the BedMachine Greenland topography dataset using the attached script. It outputs GMT formatted files converted to geographical coordinates using ogr2ogr. It outputs separate files for the coastline using both the ice shelf edge and the grounding line where the ice enters the ocean.

greenland_shore.zip