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[EDIT: In my original post, I had received an error that was based on a typo, so I scrapped that part of the question, but left the questions that still apply.]

In the ogr2ogr docs it says:

-nlt type:
Define the geometry type for the created layer. One of NONE, GEOMETRY, POINT, LINESTRING, POLYGON, GEOMETRYCOLLECTION, MULTIPOINT, MULTIPOLYGON or MULTILINESTRING. Add "25D" to the name to get 2.5D versions.

Does this part: Add "25D" to the name to get 2.5D versions apply to both loading things into postgres as well as exporting data from postgres to .shp files?

Additionally, I am assuming 25D to meant that one can have a z value that corresponds to each pair of xy coordinates (as is the case with PolygonZ shapefile types), but that these coordinates may not overlap. Is this correct? What is the intended distinction in this case between 2.5D and 3D?

Thanks

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To answer the first question: it is not necessary to use the -nlt with 25D for output to shapefiles. It appears that if I load shapefiles into Postgres using the -nlt MULTIPOLYGON25D option, and later export the resulting features as shapefiles without the -nlt option, the coordinates of each shape retains its individual z values. –  BenjaminGolder Mar 28 '11 at 23:22
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1 Answer

up vote 6 down vote accepted

The term 2.5D is used instead of 3D because, although you have Z values, they are not taken into account when doing any of the spatial operations. Intersections, buffers, any of the spatial predicates (within, overlaps,etc) operate with by ignoring the Z value.

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