Production of 1:50,000 scale image maps using satellite imagery
Merging SPOT Panchromatic and Landsat TM data
The Landsat TM image is now a layered image over the SPOT panchromatic image. Digital numbers (DN) associated with the Landsat TM image may be combined or merged with those for the panchromatic reference image using techniques as previously referred by R Welch and Manfred Ehlers[1] (Photogrammetric Engineer and Remote Sensing, vol53, No3 March 1987, pp 301-303) over the work of saint and well Cliche et al., and Chave (1984-86). These methods may be summarized in the following equations:
Mi = Ai * SQRT ( M'i *P ) + Bi .........................(i)
Mi = Ai * ( W1 * M'i + W2 * P ) + Bi ........................(ii)
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Where Mi is the DN for a pixel in the i-th band of the merged image; M'1 is the DN for the corresponding pixel of the i-th multispectral image; P is the DN for the corresponding panchromatic reference image pixel; W1 &W2 are weighting factors and Ai and Bi are scaling factors to place the resulting DN within the dynamic range (0,255) . The author used the second algorithm with responsable success. Unit weighing factors and default scaling factors as in the software for pieces of 512x512 size, were used. Performing this job for each 512x512 frame in piece meal for the full image, is best done by designing a system-600 command procedure.
Registering the merged image with map
Polyconic projection over the modified Everest spheroid is used for the image map. The central latitude and longitude coordinate pair (in degrees), for the region of mapping is assigned the equivalent Lambert Grid Coordinate pair. The software can now output and internal grid coordinate pair for each input coordinate pair in degrees of latitudes and longitudes for the sheet. The internal grid coordinate for the limiting graticule corners are obtained in this way for subsequent identification during georeferencing.
The existing 1:50,000 scale topographical maps were used for selection of Ground Control points (GCP). The map is mounted in the Intergraph environment for digitisation of GCPs. The corner coordinates in degreed for the latitude and longitude of the map are entered to obtain the coorinates of the GCPs in same terms as needed in the IIS environment. The GCPs are selected from the image and digitized over the map to obtain their degree coordinates, since our IIS has no digitizer in the configuration. These coordinate are fed into the IIS by keying in over the terminal.
Same technique of least squares regression as described before is used to attain optimum selection of GCPs. We used 21 GCPs with RMS error as 0.853 SPOT pixels . The warping is done using full bivariate fourth order Legendre Polynomial . The image is then warped to register with map coordinate system. For both registrations the error status as found is appended in the enclosure-2.
Geocoding the warped image
Internal grid coordinate values for the corners of the sheet of mapping is obtained from the machine after we input the Lambert Grid values for the central latitude and longitude of the sheet as described in the previous para. These values are easily located in the warped image. Now the monitor will show the column line coordinates of the corners of the sheet for mapping The georeferencing graticule tick marks are placed in these locations.
Superimposition of map data over the warped image
The following topographical map layers processed in the Intergraph environment are used for superimposing over the image map.
- Contours at 200m vertical interval
- Settlements and lines of communication
- Glacial features.
- Annotations.
The layers are required to the rotated to make the parallels of latitude parallel to the X-axis of the machine. The IIS software on the other hand rotates the image at the warping stage achieving the same. Therefore the internal corner grid values for the sheet of mapping as given by the machine and as may be obtained from the Lambert Grid Tables, will differ and it will have no effect in the placement of latitude and longitude ticks.
