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

2.5 Grid correction of scanned cadastral map

2.5.1 Even after appropriate quality checks during the scanning process, there can be few errors due to machine specification and scanning techniques. There can be also some distortion in the input manuscript (analog cadastral map). The scanned map may carry forward the errors due to differential scanning, wear and tear or differential shrinkage/expansion. The net result may be non-uniform scale at parts of the map, deflection in north orientation, etc. To make the map plani-metrically accurate, rectifying the map with the vector grid is suggested.

2.5.2 Prior to the correction, selection of the appropriate grid has to be made. The grid is scale-dependent. Generally, the grid found on 1:3960 (16”=1mile) will be placed at a distance of 10 zaribs (i.e., 50.8 cm or 2.00000008 inch); the maps with 1:4000 scale will have a metric system grid and will be placed at a distance of 25 cm. Each sheet will have 10 X 14 grid blocks in 1:3690 scale, or 20 X 28 grid blocks in 1:4000 scale sheet. After ascertaining the scale of the maps under consideration, appropriate mathematical grid has to be generated.

2.5.3 After selecting the appropriate grid, the cadastral scanned maps have to be registered with the grid.

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  Gridlines and tics are available - The highlighted tics in the scanned map have to be stitched/tagged with its appropriate intersections of the gridlines of the mathematical grid and then transformed.
  
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  Four corner tics available – The highlighted four corner tics in the scanned map have to be stitched/tagged with its appropriate intersections of the gridlines of the mathematical grid and then transformed.
  
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  No gridlines and tics available – This needs to be executed in two steps:
  
  1. 
     The transferred tics before scanning are to be stitched/tagged with its appropriate intersections of the gridlines of the mathematical grid which was used to transfer the tics. Then first transformation is made. This will remove the scanning distortion, if any.
     
  2. 
     Comparing the available graphical scale of the map with the mathematically generated graphic scale of the same type and then registering the scale and transform. This may correct the distortion of the map scale, if any.
     

2.5.4 The process should eliminate the possible warping effect of cadastral maps. High accuracy and low residual error are to be achieved in grid correction of cadastral images. The transformed rectified cadastral scanned image is termed as ‘registered scanned cadastral map’ and can take the name such as R120501007035101.tif. The quality check DQC-3 is performed on the image to see whether the gridlines overlay with the tics of the map. If the errors are not within the acceptable limits, re-registration has to be performed.