# ESRI Shapefile

All varieties of ESRI Shapefiles should be available for reading, and simple 3D files can be created.

Normally the OGR Shapefile driver treats a whole directory of shapefiles as a dataset, and a single shapefile within that directory as a layer. In this case the directory name should be used as the dataset name. However, it is also possible to use one of the files (.shp, .shx or .dbf) in a shapefile set as the dataset name, and then it will be treated as a dataset with one layer.

Currently the shapefile driver always returns polygons, rather than multipolygons, even for polygons with multiple outer rings. This is a violation of the OGR (and Simple Features) data model, and will be corrected at some point in the future. The new ESRI measure values are discarded if encountered.

If a .prj files in old Arc/Info style or new ESRI OGC WKT style is present, it will be read and used to associate a projection with features.

## Spatial and Attribute Indexing

The OGR Shapefile driver supports spatial indexing and a limited form of attribute indexing.

The spatial indexing uses the same .qix quadtree spatial index files that are used by UMN MapServer. It does not support the ESRI spatial index files (.sbn / .sbx). Spatial indexing can accelerate spatially filtered passes through large datasets to pick out a small area quite dramatically.

To create a spatial index, issue a SQL command of the form

    CREATE SPATIAL INDEX ON tablename [DEPTH N]

where optional DEPTH specifier can be used to control number of index tree levels generated. If DEPTH is omitted, tree depth is estimated on basis of number of features in a shapefile and its value ranges from 1 to 12.

To delete a spatial index issue a command of the form

    DROP SPATIAL INDEX ON tablename

Currently the OGR Shapefile driver only supports attribute indexes for looking up specific values in a unique key column. To create an attribute index for a column issue an SQL command of the form "CREATE INDEX ON tablename USING fieldname". To drop the attribute indexes issue a command of the form "DROP INDEX ON tablename". The attribute index will accelerate WHERE clause searches of the form "fieldname = value". The attribute index is actually stored as a mapinfo format index and is not compatible with any other shapefile applications.

## Creation Issues

The Shapefile driver treats a directory as a dataset, and each Shapefile set (.shp, .shx, and .dbf) as a layer. The dataset name will be treated as a directory name. If the directory already exists it is used and existing files in the directory are ignored. If the directory does not exist it will be created.

As a special case attempts to create a new dataset with the extension .shp will result in a single file set being created instead of a directory.

ESRI shapefiles can only store one kind of geometry per layer (shapefile). On creation this is may be set based on the source file (if a uniform geometry type is known from the source driver), or it may be set directly by the user with the layer creation option SHPT (shown below). If not set the layer creation will fail. If geometries of incompatible types are written to the layer, the output will be terminated with an error.

Note that this can make it very difficult to translate a mixed geometry layer from another format into Shapefile format using ogr2ogr, since ogr2ogr has no support for separating out geometries from a source layer. See the FAQ for a solution.

Shapefile feature attributes are stored in an associated .dbf file, and so attributes suffer a number of limitations:

- Attribute names can only be up to 12 characters long. Longer names will be silently truncated. This may result in non-unique column names, which will definately cause problems later.

- Only Integer, Real and String field types are supported. The various list, and binary field types cannot be created.

- The field width and precision are directly used to establish storage size in the .dbf file. This means that strings longer than the field width, or numbers that don't fit into the indicated field format will suffer truncation.

- Integer fields without an explicit width are treated as width 11.

- Real (floating point) fields without an explicit width are treated as width 24 with 15 decimal places of precision.

- String fields without an assigned width are treated as 80 characters.

Also, .dbf files are required to have at least one field. If none are created by the application an "FID" field will be automatically created and populated with the record number.

The OGR shapefile driver supports rewriting existing shapes in a shapefile as well as deleting shapes. Deleted shapes are marked for deletion in the .dbf file, and then ignored by OGR. To actually remove them permanently (resulting in renumbering of FIDs) invoke the SQL 'REPACK ' via the datasource ExecuteSQL() method.

### Dataset Creation Options

None

### Layer Creation Options

SHPT=type: Override the type of shapefile created. Can be one of NULL for a simple .dbf file with no .shp file, POINT, ARC, POLYGON or MULTIPOINT for 2D, or POINTZ, ARCZ, POLYGONZ or MULTIPOINTZ for 3D. Shapefiles with measure values are not supported, nor are MULTIPATCH files.

### Examples

A merge of two shapefiles 'file1.shp' and 'file2.shp' into a new file 'file_merged.shp' is performed like this:

    % ogr2ogr file_merged.shp file1.shp
    % ogr2ogr -update -append file_merged.shp file2.shp -nln file_merged

The second command is opening file_merged.shp in update mode, and trying to find existing layers and append the features being copied.
The -nln option sets the name of the layer to be copied to.

### See Also

- Shapelib Page