.SHP

Introduction

The shapefile is the vector map format of Esri, the maker of ArcGIS, introduced with ArcView in the early 1990s. Esri published its specification in 1998, and it became the most common way to exchange points, lines and polygons between GIS programs. It is still everywhere, despite limits such as 2 GB per file and ten-character field names.

A shapefile is not one file but several with the same name:

• .shp: the shapes, their geometry
• .shx: an index giving the position of each shape in the .shp
• .dbf: the attributes, one row per shape, in dBASE format
• .prj (optional): the coordinate system, as well-known text

All of them are needed, so shapefiles are usually shared as a ZIP archive.

Structure

The .shp file starts with a 100-byte header. Some of its numbers are big-endian and some little-endian:

Records follow. Each has an 8-byte header, the record number and content length, big-endian, then the content, little-endian: the shape type, then the shape’s bounding box, parts and points. The .shx file has the same 100-byte header, then 8 bytes per record giving its offset and length.

Links