Draft operational guidelines of digital india land records modernization programme (dilrmp) introduction

7.2 Scanning and dimensional accuracy
7.2.1 The original map is scanned to produce the raster form. For cadastral map, scanning may be done in 400 dpi (dots per inch). Special attention should be given to see that the map is not deformed dimensionally. For this purpose, “X” (cross) marks are placed at corners of the original map before scanning. In the next step, the lengths between the “X” marks of the original map are compared with those of the scanned map to check whether any differences exist. Finally, raster editing is done for the elimination of unwanted patches in order to enhance vectorisation of the raster data.
7.2.2 Dimensional accuracy of the raster data implies total correspondence between the raster data and the original map. The following procedures are used to achieve dimensional accuracy:
1. Checking for expansion.
2. Checking for contraction.
3. Checking for translation.
4. Checking for rotation.
Raster form of the map may appear to be expanded or contracted as compared to the original map, which in turn affects the vector data. The checking for expansion and contraction is based on the principle that ‘area is invariant’. The Land Records Department has Jurisdiction List (JL) in which the total mouza area is defined and the land records information contains each plot area of an owner. These two are compared with the vector data of the map to check for the expansion or contraction of the map with respect to the original map. Using a GIS tool, the NIC West Bengal State Unit has developed software which can easily integrate the land records data with the digitized data based on the plot numbers. Using the software, one can easily compare the total mouza area (as defined in JL) with digitized mouza data as well as digitized plot area with the individual plot area defined in the land records database.
Translation and rotational error may occur during the scanning process. Checking for this purpose is done using the check point. A calibrated plotter HP 1050C or higher is used to plot a map from the vector data. This map is then compared with the original map to find complete correspondence between the lines of the two maps. Any mismatch between the two reveals the existence of the abovementioned errors. These errors may be removed by accurate scanning of the original map and confirming its correctness with the original.
7.2.3 While digitizing, the scale should be maintained accurately, so that the output corresponds 1:1 with the original. Either flat-bed scanner or roller-type scanner can be used for scanning. For maps that are brittle, flat-bed scanners would be more suitable.