SDTS Topological Vector Profile and Raster Profile (TVP, DDF) File Description
Surfer can import USGS SDTS Topological Vector Profile .TVP or .DDF data sets.
SDTS File Information
The Spatial Data Transfer Standard, or SDTS, is a robust way of transferring earth-referenced spatial data between dissimilar computer systems with the potential for no information loss. It is a transfer standard that embraces the philosophy of self-contained transfers, i.e. spatial data, attribute, georeferencing, data quality report, data dictionary, and other supporting metadata all included in the transfer.
The purpose of the SDTS is to promote and facilitate the transfer of digital spatial data between dissimilar computer systems, while preserving information meaning and minimizing the need for information external to the transfer. Implementation of SDTS is of significant interest to users and producers of digital spatial data because of the potential for increased access to and sharing of spatial data, the reduction of information loss in data exchange, the elimination of the duplication of data acquisition, and the increase in the quality and integrity of spatial data. SDTS is neutral, modular, growth-oriented, extensible, and flexible--all characteristics of an "open systems" standard.
The SDTS provides a solution to the problem of spatial data transfer from the conceptual level to the details of physical file encoding. Transfer of spatial data involves modeling spatial data concepts, data structures, and logical and physical file structures. To be useful, the data to be transferred must also be meaningful in terms of data content and data quality. SDTS addresses all of these aspects for both vector and raster data structures.
There are two separate types of SDTS DEM files: topological vector profile SDTS and raster profile SDTS files.
TVP
The topological vector profile SDTS files contain boundary line information and can be used as a base map (use with Home | New Map | Base).
DDF
The raster profile SDTS files contain gridded elevation information and can be used to directly create a grid-based map (i.e. Home | New Map | Contour) or to perform grid based functions (i.e. Grids | Edit | Assign NoData). An uncompressed SDTS contains several files with the .DDF extension. All of the .DDF files are necessary to produce a map (i.e. you cannot copy just one .DDF file and create a map from it).
Remarks
It is not necessary to unzip the .TAR.GZ, .TAR, .ZIP, or .TGZ file. Surfer can read the information in the various files directly from the compressed file.
If you do unzip the .TAR.GZ, .TAR, .ZIP, or .TGZ file containing the DDF files, there is an option in WinZip (or other unzipping software) that needs to be disabled. Use the 
WinZip settings to disable this option. All .DDF files must be extracted into the same directory.
See Also
SDTS TVP Import Options Dialog