Proposed NSDE Format
The proposed National Spatial Data Exchange (NSDE) format is a set of ASCII files, independent of platform or GIS software, which is structured to incorporate various types of data elements like vector data on topography as well as various types of thematic data along with associated attribute data, remotely sensed data, DEM data, coded raster GIS data etc.

The proposed NSDE format is extension of Digital Vector Data (DVD-3) format devised by the Survey of India to represent SOI toposheet data in digital form along with the point, line and polygon topology describing relationships among the spatial features point, line and polygon. The proposed format also has provision to include digital images acquired by satellites and Digital Elevation Model (DEM) and coded raster data. This format is further modified for accommodating attribute data associated with spatial features and other information.

Study Area Identification
In order to carry out NRIS to NSDE conversion and vice-versa, the study area belonging to toposheet number 46F/11 covering a part of Panchmahals district of Gujarat state is selected. Its boundaries are as follows:
Latitude (dd-mm-ss ) : 22-25-00 to 22-30-00
Longitude (dd-mm-ss) :73-30-00 to 73-37-30

The NRIS spatial data are of three categories : Point, Line and Polygon. The conversion exercise is carried out for each of the data category. Settlements representing point data, Roads representing line data and Landuse data representing polygon type are taken from NRIS database, created using ARC/INFO GIS software, for the study area belonging to Panchmahals District of Gujarat.

NRIS to NSDE Conversion
The NRIS to NSDE conversion process has two major steps:

• Generation of intermediate files giving information about constant parameters of the district or study area (distconst.dat), variable parameters of the district or study area (dist_vari_param.dat), files containing information about theme n (DATACAT_n.dat) and files containing actual spatial and attribute data about theme n (DATAFILE_n.dat) where n is the serial number of the theme.
• Generation of VOLDIR, GENINFO, DATACAT and DATAFIL files as per proposed NSDE format by making use of the files generated in step (1).

The two steps for converting NRIS database contents to NSDE are described below:

Generation of intermediate files
The process of generating intermediate files required for NRIS to NSDE conversion is accomplished by a set of Arc Macro Language (AML) based procedures. The coverages selected for conversion are provided as input and intermediate files are generated in the following manner:

• Extraction of spatial data belonging to AOI: If the area of Interest (AOI) does not belong to entire district, it is required to extract spatial data belonging to AOI from the district database. This is done by clipping the coverages against the AOI coverage and using the clipped coverages for further processing.
• Generation of AOI Tics: This process is required to be carried out only when tic file of the NRIS coverage is for full district and the area of interest for conversion to NSDE is part of that district. This process selects only those Tic points which belong to area of interest.
• Generation of AOI Attribute Tables: This process is also required to be carried out when the area of interest for conversion to NSDE is subset of the district. The attribute information belonging to area of interest only is retained. For example, if the full district have 1000 villages, attribute files will contain information about all 1000 villages. If the area of interest have only 50 villages, this village will remove attribute information about 950 villages lying outside the area of interest and retains information relevant to 50 villages.
• Preparation of Inputs: This process prepares inputs required for generating file DATAFILE_n.dat. It generates NODE information, adds labels to Polygon Attribute Table (PAT) if the coverage type is polygon, ungenerates coverage and stores ungenerated co-ordinates to intermediate file for further use.
• Generation of Constant Prmameters: This process generates constant parameters for NRIS entire district node or for area of interest like name of the state, name of the district, projection related information, data source, tic points information and writes all these parameters in distconst.dat file.
• Generation of DATACAT and DATAFILE: This process generates dist_vari_param.dat file containing information about number of coverages (p) selected for conversion, their serial number n, name of the coverage as per NRIS standards and number of attribute tables in the coverage. For each coverage, separate data category file (DATACAT_n.dat) file is generated giving information about coverage name, number of major categories as per NRIS standards, their code and description, number of NODE, LINE and AREA records, structure of each attribute table in terms of field description, number of data records in each table etc. Similarly for each coverage, separate data file (DATAFILE_n.dat) is generated by this process which contains data about NODE, LINE AREA and ATTRIBUTE records.

Generation of VOLDIR, GENINFO, DATACAT and DATAFIL files
The intermediate files generated in step (1) contain all the information required for NSDE conversion. Using the distconst.dat, distvari_param.dat and DATACAT_n.dat and DATAFILE_n.dat files of respective coverages, this process generates VOLDIR, GENINFO, DATACAT and DATAFILE files as per the structure and contents.

The flowchart for NRIS to the NSDE conversion is given in Figure 1.


Fig 1: Flowchart for NRIS to NSDE conversion