[gdal-dev] Missing features after copying layers from Shapefiles to SQLite

[Nik Sands]

Thanks for this information.

I'd like to replicate that ogr2ogr PROMOTE_TO_MULTI functionality in my code, but I'm having trouble figuring out how to do it.  I guess I can take a look at the ogr2ogr code.

(I could also just include the entire ogr2ogr tool bundled into my app and call it from the app, but that seems a bit of overkill for an iOS app, which is best as lean as possible.)


On 4 Mar 2014, at 9:50 pm, a.furieri at lqt.it wrote:

> Hi all,
> 
> just yo better clarify some relevant details:
> 
> a Shapefile can legitimately contain one the following shapes:
> Point, PolyLine, Polygon and MultiPoint; and there are further
> shape-codes intended to support extended XYZ or XYM dimensions,
> such as PointZ and PolyLineM
> 
> please note: accordingly to the SHP model there is a very strong
> distinction between Point and MultiPoint; but a PolyLine can
> indifferently contain just a single line or a collection of
> many individual lines, and the same is for Polygons.
> 
> on the opposite side, the standard OGC-SFS data model supports
> the following geometry classes:
> Point, Linestring, Polygon, MultiPoint, MultiLinestring,
> MultiPolygon and GeometryCollection; in this case too we have
> further classes supporting extending XYZ, XYM and XYZM
> dimensions, such as PointZ, LinestringM or PolygonZM.
> 
> please note: the OGC-SFS model introduces a very sharp distinction
> between e.g. Linestring and MultiLinestring; a Linestring can
> contain just a single line, a MultiLinestring can contain any
> arbitrary number of individual lines ... THIS INCLUDING JUST ONE !!!
> and the same is for Point/MultiPoint and Polygon/MultiPolygon.
> 
> quick conclusion: SHP geometry types doesn't exactly match
> OFC-SFS types, and there is enough room to introduce some
> dangerous inconsistency, e.g.:
> 
> - the SHP PolyLine type could possibly match the Linestring type
>  if (and only if) all PolyLines into the SHP will contain just
>  a single line.
> - when some SHP PolyLine does actually contain more individual
>  lines a MultiLinestring will always be required on the OGC-SFS
>  side
> - anyway OGC-SFS MultiLinestring will always exactly match the
>  SHP PolyLine type, even in the case of "mixed" Polylines
>  sometimes being elementary and sometimes being complex.
> - the same considerations obviously apply to Polygon/MultiPolygon
> - conclusion: always assuming a Multi?? type is the safest approach,
>  if you aren't absolutely sure about the actual SHP payload.
> 
> ogr2ogr does actually has an argument nicely supporting
> all this (just pasted from the ogr2ogr man page):
> 
> -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. Starting with GDAL 1.10, PROMOTE_TO_MULTI
>    can be used to automatically promote layers that mix
>    polygon or multipolygons to multipolygons, and layers
>    that mix linestrings or multilinestrings to multilinestrings.
>    Can be usefull when converting shapefiles to PostGIS
>    (and other target drivers) that implements strict checks
>    for geometry type.
> 
> it works for PostGIS, and it works for SpatiaLite as well.
> 
> bye Sandro
> 
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