Introduction
Realization of National Spatial Data Infrastructure (NSDI) is the current national focus. NSDI is proposed as a single window mechanism for providing access to the spatial data being generated and managed by various agencies in the country. It is visualized as a network of databases consisting of the domain specific databases created and managed by various agencies like Survey of India, Geological Survey of India etc. and a central database having metadata to be managed by NSDI secretariat. Towards this, a national task force has been set up by Department of Science and technology which is looking at all the related issues including evolution of spatial database standards at various domain servers as well as the metadata standards for providing single window access to the domain databases.

Evolution of Mechanisms for exchange of spatial data sets is an integral part of such a system. Towards this, efforts have made to define the National Spatial Data Exchange (NSDE) format. In this connection a format has been evolved and it has been presented to a sub-group on NSDI standards working under the guidance of NSDI task force. This in under discussion and evaluation amongst the various participating agencies like Survey of India, Geological Survey of India, Department of Science and Technology and Department of Space.

This paper describes the proposed NSDE format as it stands today and is offered for feedback from larger section of seminar participants. The proposed NSDE format is a set of ASCII files which is independent of platform or GIS software and is structured to incorporate various types of data elements like vector data on topography as well as various types of thematic elements along with associated attributes data, Image data, DEM data, Coded raster GIS data etc.

Proposed NSDE Format
The format is an 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. In addition to the topographic data sets, 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 various types of thematic data sets along with the associated attribute data in tabular form. The proposed format along with the data organization and contents of each type of record is described.

The various agencies concerned with the data generation and supply could use the proposed National Spatial Data Exchange Format for supply of data as a set of files as shown below. The data could be supplied on-line (through a transfer media like internet, FTP e.t.c.) or on various kinds of physical media. When supplying the data on Cartridge, all these files would be copied unlabeled in the same sequence as shown below with the End-Of-File (EOF) mark after each file and one extra EOF mark at the end of all files to indicate End-Of-Volume. In case of data on Cartridge/CD-ROM the entire volume will be contained in one single Cartridge/CD. For data on floppies, the volume will comprise the entire set of floppies containing all files.

As shown above in the NSDE format organization, there are totally 6+2*p files in general namely VOLDIR, GENINFO, COMPIND, ADMNIND, SPLFTNT, CROSMRK, DATACAT and DATAFIL where p is number of base categories for SOI-DVD vector data or number of themes for NRIS vector data or number of spectral bands for Image/DEM/Coded raster data. However, the files COMPIND, ADMIND and SPLFTNT are applicable only for SOI-DVD data and not for other data types. VOLDTRY file indicates weather these files exist or not. The DATACAT and DATAFIL repeat p times as shown in the above table. All the files have fixed-length records of 72 Bytes.

The first two records of all the files are identification and control records and are exactly identical in structure. The first record contains associated SOI topographic sheet number along with edition number and edition year. The second record contains file name as mentioned above, Volume code which is unique ID for the dataset product, serial number of floppy if applicable, total number of records in the file and serial number of the base category/theme serial number in case of multiple files, band number of raster images.

The description and format of these two records identical to all files is given in Table 1.

Table 1. Description and format of first two Records Common to all

Rec No	Field	Contents	Type (Fortran)	Format	Starting Byte	Ending Byte	Remarks
1	1	SOI Toposheet Number	CHARACTER	A60	1	60	The Map Sheet Number As per Scale, e.g.1:25,000 :- 56G/12/NE1:50,000 :- 56G/121:250,000 :- 56G
	2	Edition Number	INTEGER*2	I4	61	64	Edition of the source material viz., 2 for second Edition
	3	Edition Year	INTEGER*2	I8	67	72	Edition Year, e.g., 1964
2	1	File Name	CHARACTER	A7	1	7	7- Digit file name as per figure 1.
	2	Filler	-	-	8	9	2 Spaces
	3	Volume Code	INTEGER*4	I6	10	15	6- Digit Code, Unique for Data Set. Provided for future use.
	4	Data Type	INTEGER*2	I2	16	17	Indicates type of Spatial Data1 if data is base category wise (Vector Format).2 if data is Theme Wise ( Vector Format)3 if raster multiband image ( band wise)4 if Single band raster image values5 if Single band coded raster image
		Filler	-	-	18	18	1 Space
	5	Serial Number of Floppy	INTEGER*2	I6	19	24	For data on Floppies only. Otherwise this value is zero
	6	Total Number of Records in the file	INTEGER*4	I12	25	36
	7	Base Category / Theme serial No.	INTEGER*2	I6	37	42	Corresponds to field 1 of Record number 4 to 3+p of VOLDTRY. Applicable only for files DATACAT and DATAFIL. FOR Other files this value is Zero.
		Filler	-	-	43	72	30 Spaces

The description and format of all the eight files of NSDE from third record onwards are described below:

File Voldir
This file contains volume directory information. It indicates weather data are organized base category wise, theme wise or band wise. The description and serial number of all the base/theme category/band exists,, whether attribute data exists or not and those included in the volume along with the serial number of floppy (applicable for data on floppies) containing it are also contained in this file.

The detailed description and format of record number 3 onwards of this file is given in Table 2.

Table 2. Volume Directory (VOLDTRY) Records

Rec No	Field	Contents	Type (Fortran)	Format	Starting Byte	End Byte	Remarks
3	1	Availability of Data as One Single file or Multiple Files Or No. of ThemesOr No. of Bands (p)	INTEGER*2	I6	1	6	This Value is zero for single file and non-zero (p) for multiple files, where p is the total number of base categories/ Themes/Bands mentioned in the volumes. Record no. 4 onwards will appear only if this value is non-zero. Also the next field of this record is meaningful only if this value is non-zero. In case of Raster data, each file will contain data for a particular band. In case of Theme data, the file will contain data pertaining to a theme.
	2	Whether data is base category wise / theme wise/ Image band wise	INTEGER*2	I6	7	12	This value is 1 if data is base category wise (Vector Format).2 if data is Theme Wise ( Vector Format)3 if raster multiband image ( band wise)4 if Single band raster image values5 if Single band coded raster image
	3	Whether File COMPIND Exists	INTEGER*2	I6	13	18	1 for yes, 0 for NONot relevant for raster data & User Domain Theme data
	4	Whether File ADMIND Exists	INTEGER*2	I6	19	24	1 for yes, 0 for NONot relevant for raster data & User Domain Theme data
	5	Whether File SPLFTNT Exists	INTEGER*2	I6	25	30	1 for yes, 0 for NONot relevant for raster data & User Domain Theme data
		Filler	-	-	31	72	42 Spaces
4 to 3+p	1	Base Category/ Theme/ Band Serial Number	INTEGER*2	I6	1	6	Order of listing of Base Categories, Themes, Bands
	-	Filler	-	-	7	7	1 Space
	2	Whether Attribute table Exist	INTEGER*2	I2	8	9	0 indicates no attribute data attached in DATAFIL, ELSE number of attribute tables associated
	-	Filler	-	-	10	10	1 Space