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Re: [TowerTalk] Terrain data for HFTA

Subject: Re: [TowerTalk] Terrain data for HFTA
From: Jim Lux <>
Date: Mon, 22 Aug 2011 15:57:09 -0700
List-post: <">>
> I  would not trust Google-Earth elevation data.To add an example like
> Bimini,  Bahamas: Check Malpelo Island, HK0TU, 400 km away from the colombian
> Pacific  coast (4.0 N, 81.6 W) According to Google earth the island is
> submerged under  the water surface a least 340 ft. In reality, this island has
> elevations over  1000 ft ASL.

I think that's a good example of which ellipsoid is being used.  if you 
move your cursor across some of the islands (little more than rocks 
sticking up, it looks like) at 3-49-57.46 N 81-36-15.28W it seems to be 
pretty consistently around -80m

If you traverse across the island (center roughly at 4-0-12N 81-36-27W)

you see the altitude smoothly vary from -80 to the SE of the island, 
getting gradually lower to -229m (at least) to the NW of the island

In fact, if you look at all the elevations in the area, it runs around 
-240m everywhere, and that -60 to -80 seems to form a hump to the SW of 
the island.  It looks more like it's misregistered between the elevation 
grid and the island.  Again, that could be how the original photo image 
was georeferenced vs what the elevation data was referenced against in 
terms of datum

Given the extreme depths (<-1000m) not too far away, I'm going to guess 
that Google Earth is returning bathymetry data.

So, I'd say a combination of spheroid and data set mismatches.

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