[Proj] Slightly off-topic: The phantom island that is real

strebe at aol.com strebe at aol.com
Sat Nov 24 18:00:13 EST 2012

Joaquim: Thank you for the explanation. But I think all you have done is confirm that the region is probably a large sand bar, sometimes submerged or partially submerged. There seems to be two reasons data is so sparse there: frequent cloud cover hindering remote sensing, and very shallow water that makes soundings by ship impossible.

Here is a representative sample of MODIS data that I culled from USGS’s Global Visualization Viewer:

The particular MODIS data set is MCD43A4 (MODIS/Terra+Aqua Nadir BRDF-Adjusted Reflectance 16-Day L3 Global 500m SIN Grid). If you cycle through the months and years, you can clearly see that MODIS usually sees something there but sometimes does not. That means (to me) that sometimes the sand bar is submerged too far for the satellite to detect it but usually is shallow enough to yield a signal. None of the data are consistent with deep water.

— daan Strebe


-----Original Message-----
From: J. Luis <jmfluis at gmail.com>
To: strebe <strebe at aol.com>
Sent: Sat, Nov 24, 2012 6:36 am
Subject: Re: [Proj] Slightly off-topic: The phantom island that is real

    Although we never phrased it out in those mails, it's quite obvious    that what appears to an island in the SRTM+ is nothing but an    interpolation artifact introduced by a constrain imposed the    coastline itself. Notice how close the coastline and the 1 m meter    contour lines are. Furthermore the maximum height in the SRTM+ data    is 1m meter, which is obviously very suspicious. I'll add it also    that the 'Walter' that appears in one of those mails is Walter Smith    from the Smith & Sandwell bathymetry.
    HOWEVER, that does not rule out the possibility that the island    exists. Only says that there is no reliable data to constrain the    grid construction at that location. The 1400 m difference also    puzzled me a bit and moved by your mail I searched also the MODIS    data for Sea Surface Temperature ... and the mystery continues MODIS    does not seam to find water over there. See the image that I posted    in GMT mailing list.
I don’t            follow your assertion. The e-mail you cite states the island DOES                  appear in GTOPO30 and SRM30+. I see the same thing. When I look at the raw values in                  SRTM30 Plus, the entire area of the                    island consists of sea-level DEM values. Plus, NASA Blue Marble satellite                        imagery clearly shows shallow water over the                        entire 30_km x 6_km area that is                                called “Sandy Island” on                                  innumerable maps.
      Whether or not there is an “island”          there (as opposed to a barely submerged            sandbar) is perhaps open to                debate or depends on the tides.                    But the crew of the                        research vessel is claiming 1,400_m soundings,                                    which just                                        does not coincide with that area                                        at all.
          — daan Strebe
-----Original          Message-----
          From: J. Luis <jmfluis at gmail.com>
          To: PROJ.4 and general Projections Discussions          <proj at lists.maptools.org>
          Cc: strebe <strebe at aol.com>
          Sent: Fri, Nov 23, 2012 6:40 pm
          Subject: Re: [Proj] Slightly off-topic: The phantom island          that is real
            We discussed that on the GMT list too and no, the island            does NOT exist in the SRTM+ or GTOPO30 series
            See for example
The                  crew of a research ship claims to have undiscovered a                  large island in the South Pacific’s Coral Sea near New                  Caledonia, called Sandy Island. The news feeds have                  picked up on this and it’s making its way around the                  geeksphere with great rapidity. Here is an example                  article:
                  However, it is quite clear from satellite imagery and                  multiple data sources that the “island” in fact                  exists, though possibly it is a barely submerged                  sandbar. Yet the crew claims depths there are around                  1,400 meters.
                  Obviously the ship was not where they say it was. The                  question is, how did the crew of this research vessel                  convince themselves they were sailing in the same                  area? I don’t think a datum mismatch can account for                  this, given the size of the sandbar. 
                  I have started a thread here:
                  — daan Strebe

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