problem with opentopo source NASADEM

[mdsumner]

the issue is the NODATA value is-32768, but then the zeros that fill each tile override the GEBCO source

so, we might set NODATA to 0 ... it'll ruin some of the data but can always separate the sources in two calls

txt <- readr::read_file("https://opentopography.s3.sdsc.edu/raster/NASADEM/NASADEM_be.vrt")
txt <- gsub("<NODATA>-32768</NODATA>", "<NODATA>0</NODATA>", txt)
## sub out the relative paths while we're at it
txt <- gsub("relativeToVRT=\"1\">NASADEM_be", "relativeToVRT=\"0\">/vsicurl/https://opentopography.s3.sdsc.edu/raster/NASADEM/NASADEM_be", 
            txt)
  
file <- tempfile(fileext = ".vrt")
writeLines(txt, file)

library(whatarelief)
gebco <- "/vsicurl/https://public.services.aad.gov.au/datasets/science/GEBCO_2019_GEOTIFF/GEBCO_2019.tif"
nd <- file

ex <- c(147, 148, -45, -42)
e <- elevation(extent = ex, dimension = c(512, 512), threshold = 14, projection  = "OGC:CRS84")
e1 <- elevation(extent = ex, dimension = c(512, 512), threshold = 14, projection  = "OGC:CRS84", source = c(gebco, nd))

par(mfcol = c(2, 1))
ximage(e, extent = ex)
ximage(e1, extent = ex)

figure shows the tile masking in the upper figure, "fixed" in the lower figure

[mdsumner]

• replace NASADEM_be VRT url with the installed file above
• provide more opt-out of this by having file getters 'gebco19_source()', 'NASADEM_source(opentopo = FALSE)' that elevation() uses by default
• do that thing with the VRT where you get tile extents and build an index

[mdsumner]

these figures show the data that missed a bit better