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

[a.furieri at lqt.it]

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