# 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.

Note that when reading a Shapefile of type SHPT_ARC, the corresponding layer will be reported as of type wkbLineString, but depending on the number of parts of each geometry, the actual type of the geometry for each feature can be either OGRLineString or OGRMultiLineString. The same applies for SHPT_POLYGON shapefiles, reported as layers of type wkbPolygon, but depending on the number of parts of each geometry, the actual type can be either OGRPolygon or OGRMultiPolygon.

ESRI measure values (XYM) are read as XYZ geometries. MultiPatch files are read and each patch geometry is turned into a multi-polygon representation with one polygon per triangle in triangle fans and meshes.

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.

The read driver assumes that multipart polygons follow the specification, that is to say the vertices of outer rings should be oriented clockwise on the X/Y plane, and those of inner rings counterclockwise. If a Shapefile is broken w.r.t. that rule, it is possible to define the configuration option OGR_ORGANIZE_POLYGONS=DEFAULT to proceed to a full analysis based on topological relationships of the parts of the polygons so that the resulting polygons are correctly defined in the OGC Simple Feature convention.

An attempt is made to read the LDID/codepage setting from the .dbf file and use it to translate string fields to UTF-8 on read, and back when writing. LDID "87 / 0x57" is treated as ISO8859_1 which may not be appropriate. The SHAPE_ENCODING configuration option may be used to override the encoding interpretation of the shapefile with any encoding supported by CPLRecode or to "" to avoid any recoding. (Recoding support is new for GDAL/OGR 1.9.0)

## 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

Otherwise, the MapServer shptree utility can be used:

    shptree <shpfile> [<depth>] [<index_format>]

More information is available about this utility at the MapServer shptree page

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 10 characters long. Longer names will be silently truncated. This may result in non-unique column names, which will definitely cause problems later.

- Starting with version 1.7, the OGR Shapefile driver tries to generate unique field names. Successive duplicate field names, including those created by truncation to 10 characters, will be truncated to 8 characters and appended with a serial number from 1 to 99.

  For example:

  - a → a, a → a_1, A → A_2;
  - abcdefghijk → abcdefghij, abcdefghijkl → abcdefgh_1

- Only Integer, Real, String and Date (not DateTime, just year/month/day) 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 <tablename>' via the datasource ExecuteSQL() method.

## Spatial extent

Shapefiles store the layer spatial extent in the .SHP file. The layer spatial extent is automatically updated when inserting a new feature in a shapefile. However when updating an existing feature, if its previous shape was touching the bounding box of the layer extent but the updated shape does not touch the new extent, the computed extent will not be correct. It is then necessary to force a recomputation by invoking the SQL 'RECOMPUTE EXTENT ON <tablename>' via the datasource ExecuteSQL() method. The same applies for the deletion of a shape.

Note: RECOMPUTE EXTENT ON is available in OGR >= 1.9.0.

## Size Issues

Geometry: The Shapefile format explicitly uses 32bit offsets and so cannot go over 8GB (it actually uses 32bit offsets to 16bit words). Hence, it is is not recommended to use a file size over 4GB.

Attributes: The dbf format does not have any offsets in it, so it can be arbitrarily large.

### 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.
- ENCODING=value: set the encoding value in the DBF file. The default value is "LDID/87". It is not clear what other values may be appropriate.

### VSI Virtual File System API support

The driver supports reading from files managed by VSI Virtual File System API, which include "regular" files, as well as files in the /vsizip/, /vsigzip/ , /vsicurl/ domains.

### 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
- User Notes on OGR Shapefile Driver