GMT ASCII Vectors (.gmt)
OGR supports reading and writing GMT ASCII vector format. This is the
format used by the Generic Mapping Tools (GMT) package, and includes
recent additions to the format to handle more geometry types, and attributes.
Currently GMT files are only supported if they have the extension ".gmt".
Old (simple) GMT files are treated as either point, or linestring files
depending on whether a ">" line is encountered before the first vertex.
New style files have a variety of auxilary information including geometry
type, layer extents, coordinate system and attribute field declarations in
comments in the header, and for each feature can have attributes.
The driver supports creating new GMT files, and appending additional features
to existing files, but update of existing features is not supported. Each
layer is created as a seperate .gmt file.