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I'm looking at the enum defs in ogr_core.h. I see a bunch with comments that read:

2.5D extension as per 99-402.

What in the world does that mean? Presumably it's referencing a spec or RFC?

typedef enum 
{
  ...
  wkbPoint25D = 0x80000001, /**< 2.5D extension as per 99-402 */
  wkbLineString25D = 0x80000002, /**< 2.5D extension as per 99-402 */
  wkbPolygon25D = 0x80000003, /**< 2.5D extension as per 99-402 */
  ...
}
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3 Answers

up vote 2 down vote accepted

The reference is almost certainly to an OGC specification or change request for a specification, but I'm not able to find that specific one on the OGC web site. You can read more about it at GDAL web site (from a Frank W. email) or by looking in the OGC Simple Features spec.

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This is what I was looking for, thanks. This page (gdal.org/ogr) contains a link to Adam's 2.5 D Simple Features Proposal (OGC 99-402r2), which is at the link you mention, home.gdal.org/projects/opengis/twohalfdsf.html. –  jglouie Jan 16 '13 at 12:10
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Here is explanation from Wikipedia

A 2.5D image is a simplified three-dimensional (x, y, z) surface representation that contains at most one depth (z) value for every point in the (x, y) plane.

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It's generally a good idea to just take a snippet from the site and post it here as a quote so people don't have to follow the link. –  Nathan W Jan 15 '13 at 23:05
    
@NathanW Good point, I've taken the liberty to edit it –  mloskot Dec 2 '13 at 11:38
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This mean "Z values in Simple Features co-ordinates" which was originally proposed by Adam Gawne-Cain in 1999 as Simple Features Revision Proposal.

For sake of historical background, here is Martin Daly's post with reference to Adam Gawne-Cain's documents which use to be available from OGC site, thread [postgis-devel] OGC WKB/T - PGIS EWKB/T and another post in the same thread has the original 99-402r2.pdf attached.

That is the only existing online source of the original document I'm aware of. Reading it should answer where does the 2.5D geometries implemented in numerous software packages (GDAL/OGR, PostGIS, Safe FME, etc.) come from.

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