The process of preparing base map by overlaying satellite imagery over cadastral maps has been referred from chapter-6 of ‘Space based information support for decentralized planning (SIS-DP) Manual: Preparation of Geo-spatial layers using (Cartosat – 1 Pan + LISS-IV Mx) Orthorectified Satellite Imagery’ of ISRO. The manual (part -2) is available for public and can be downloaded from the link: http://www.bhuvan-panchayat.nrsc.gov.in/assets/Manual-Part2.pdf.
H.2 Process of base map formulation by overlaying Satellite images over cadastral maps
For overlaying cadastral map with satellite image it is required that cadastral map to be generated in vector mode. In this process the main tasks are acquisition of cadastral maps, scanning and digitization of cadastral maps and generation of vector data. Once the cadastral maps in vector mode are available, georeferencing of these maps can be done.The geo-referencing of digital cadastral maps and overlaying with satellite image consists of the following steps:
Comment [T1]: Section 6.3.2.4.2
Acquisition of GCP’s
Transformation model development and assessment
Geo-referencing of cadastral maps
Validation of Geo-referenced map, in Isolation
Validation of Geo-referenced map, with neighbourhood
Mosaic generation at revenue inspector (RI), taluka and district level
Comment [T2]: Section 6.3.2.4.3
H.2.1 Acquisition of ground control points:
Sufficient numbers of GCP’s shall be identified on the vector cadastral map and on satellite image. The characteristics of the GCP’s are intersections of parcel boundaries, river/stream with parcel boundary, of roads with parcel boundaries, roads with rivers and corners of water tanks. The GCPs should spread uniformly in the entire map.
H.2.2 Transformation model assessment:
Transformation model is applied for geo-referencing the cadastral map. The area of the village, number of sheets covered in a single village, condition of the cadastral map sheets, number and characteristics of GCPs collected and terrain conditions the affect polynomial model. The transformation model is accepted when the actual root mean square and residual errors arrived are less than the threshold values i.e. 6 m in either direction.
H.2.3 Geo-referencing of cadastral maps:
Once the transformation model is accepted, the vector cadastral map is geo-referenced through affine transformation4 in GIS environment. The outputs are further validated both visually and mathematically.
H.2.4 Validation of Geo-referenced map, in isolation:
Output product validation is an essential element in development of land information system. The georeferenced vector file of each village is validated with reference to the merged product. The geo-referenced vector file is overlaid on the reference image and initial validation is carried out through visual checking. If the parcel boundaries coincide with the image features and deviations/displacements are not observed, the geo-referencing is successful. Feature matching is checked in the following order of priority; tanks, water bodies, tank bunds, ponds, road, rail, canal, stream, nalla, river, field bunds, forest boundary / vegetation boundary, abadi (village settlement).
H.2.5 Validation of Geo-referenced map, with neighbourhood reference:
This validation is carried out to ensure that the village boundary is matching with all adjoining village boundaries. The boundary should match perfectly. However, in accordance with the terrain conditions, the boundary (overlap/under lap) error tolerance in undulating terrains is kept around 15 m and in hilly areas, the tolerance is around 25 m (Srinivasa Rao et al., 2003b).
H.2.6 Mosaic generation at Revenue, taluka and district level:
After geo-referencing the cadastral maps to the required accuracy standards, a number of maps are mosaiced at the next higher administrative level. While mosaicing, feature continuity as well as attribute accuracy is to be maintained. These are the final products of geo-referencing of cadastral maps and can be used as base map. These products are directly linked to LIS and GIS databases.
Comment [T3]: Section 6.3.2.4.3
4Affine Transformation: A geometric transformation that scales, rotates, skews, and/or translates images or coordinates between any two Euclidean spaces. It is commonly used in GIS to transform maps between coordinate systems. In an affine transformation, parallel lines remain parallel, the midpoint of a line segment remains a midpoint, and all points on a straight line remain on a straight line.