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I need to use this raster data to extract elevation for points and polygons, but I am really confused about the unit of the pixel values

http://www.naturalearthdata.com/downloads/10m-raster-data/10m-shaded-relief/

The pixel values are between 49 and 252, and oceans have higher pixel values than some land areas. How should I interpret the pixel values and convert them to meters?

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    Shaded relief does not present heights/depths but shadows. Image is light where the sun hits the hill best. You need digital elevation model, not shaded relief. – user30184 Oct 23 '16 at 19:17
  • @user30184 If you can expand that with a bit more info you should add as an answer – Midavalo Oct 23 '16 at 19:48
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You can't get heights or depths from a shaded relief raster. The purpose of shaded relief is described in this tutorial:

Shaded relief models indicate terrain displacements using a shadow effect from evaluating the aspect and slope relative to the sun's azimuth angle and altitude achieved with varying grey scale tones resulting in the darkening of one side of terrain features, such as hills and ridges (the darker the shading, the steeper the slope).

The tutorial is worth reading. There is also a dedicated web site about shaded relief http://www.shadedrelief.com/.

For getting the real elevation values you need a digital elevation model (DEM).

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