Page 2 of 2 Previous Image standards should address the data resolution needed at different scales of mapping, referencing to the selected datum and map projection, planimetric accuracy and in case of multispectral data the tolerable band to band registration error. It should be noted that the image standards should not be tied to any one remote sensing system. The specifications should be generic and keep the final objective in view as remotely sensed images are an important input data source these specifications will impact the further products and the final database parameters. Images will be converted to thematic maps through a process of interpretation. The maps will need to be referred and tied to the national framework. Other than the standards which are inherited from the source image it is also necessary to specify the minimum mappable unit and the planimetric and thematic classification and mapping accuracies. Where height data is involved it is necessary to specify the height accuracy and the resolution. Finally the digital format in which the maps are to be generated should be specified. The next important step is to specify the geo-database standards. Since the national framework is defined this becomes the geo-database framework. Tie points must be identified on which the input images and maps can be referenced. The tie points are the corner references for the map tiles in the national spatial framework. A typical tie point scheme for a typical database is shown in figure 2  Fig. 2 A tie point scematic for different scales of mapping Other reference specifications are the coordinate units to be used, the coordinate precision and the accuracy with which the input map registers to the framework. Specifications on the registration accuracy, planimetric accuracy and the limit of coordinate movement tolerance resulting from GIS operations are important to ensure that the input maps are consistent and compatible. Other specifications are weed and sliver tolerances. Weed tolerances indicate the minimum distance between two vertices in a feature. Sliver polygon are generated during GIS operations and are removed if they are smaller than the minimum mapping unit. In the case of raster data like images and digital elevation models the pixel size has to be defined for each scale and is usually several orders better then the minimum mapping unit. Output standards define output formats such as soft copy, hardcopy and output media types. The framework (datum and projection) is usually specified by the user. The data format can also be specified in the case of digital data. An important element is the symbology to be used, the colour schemes and patterns for different features. This is usually standardised nationally but in rare cases the user may desire a unique symbol set. The quality of data in a data base is of prime importance. Quality parameters must be defined for each level beginning with the national framework, planimetry of the input data, classification and mapping accuracies of the input maps, the minimum mapping unit and the scale distortion for analogue outputs. It is necessary that quality checks are carried out at each level to ensure the integrity of the database. Each input map has to have its contents defined. Predefined terminologies for features is essential to maintain semantic integrity. Classification should be hierarchical to enable map generalisation. Classification should also be flexible to enable addition, deletion and modification of classes and sub-classes. Content classification can be contentious and requires the participation of data generators and users to arrive at an acceptable solution. An important feature of the database is the catalogue information which provides data about the database contents. This is metadata and it also must be standardised to enable easy access to the catalogue information. Metadata must be sufficient to unambiguously specify the data without being too detailed. Typically metadata includes data source, date of creation, accuracies, extent and access information. In closing it is important to note that the development of standards is a gradual and inclusive process. Standardisation may appear to slow down the work on hand but it pays dividends in the long run with ease of access, consistency and reliability. All stakeholders must be a party to the standards. Further, no standard can be static. It has to evolve as new requirements and new technologies emerge. Thus a standing mechanism to periodically review the standards and bring in changes when needed is an important part of the standardisation process.
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