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tilezen / joerd

Joerd can be used to download, merge and generate tiles from digital elevation data
MIT License
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Missing ice sheets? #103

Open meetar opened 7 years ago

meetar commented 7 years ago

Compare the SRTM views of Greenland and Antarctica (equirectangular):

greenland-srtm antartica-srtm

With our own data, converted to grayscale with black = sea level and white = 1500m: greenland-mapzen antartica-mapzen

It appears we're seeing these landforms as they'd be minus the ice:

https://en.wikipedia.org/wiki/Greenland_ice_sheet#/media/File:Topographic_map_of_Greenland_bedrock.jpg http://www.geo.utexas.edu/courses/371c/Labs/Lab10/bed_elev_water_zero_contour.jpg

What's the source of this data? I'm quite surprised to see it, and could imagine situations where it would be useful, but I assume most people would expect the ice to be there (for the next few years, anyway... 😎)

zerebubuth commented 7 years ago

The data source for low zooms is likely to be ETOPO1 which does come in "ice sheet" or "bedrock" versions, and it looks like we used the "bedrock" version.

This probably changes as one zooms in, as it will likely switch to GMTED. I'm not sure, but from reading the docs (PDF) it seems like it includes the ice sheet.

meetar commented 7 years ago

Antarctica does appear to switch over at z7, however Greenland never does – it may be that GMTED doesn't have this data, I see in the tech sheet that "Some areas, namely Greenland and Antarctica, do not have data available at the 15- and 7.5-arc-second resolutions because the input source data do not support that level of detail" – and it also shows that Antarctica has a separate data source which contributed to the GMTED source. Not sure how all of that translates to zoom levels but that may explain the discrepancy.