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Digital Image Interface with Spatial Databases - Requirements and Procedure
P. D. Yadav, Ketki Kharod, Bharat Vaishnav, R. K. Goel
Geomatics Technology Division Space Applications Centre Ahmedabad 380
053
Remote Sensing (RS) data
obtained from satellites and aerial flights are valuable source of information
providing up-to-date information about natural resources like land, water,
forests, urban areas and infrastructure facilities like transportation network,
canal network etc. Geographical Information Systems (GIS) are widely being used
for management, monitoring and planning of natural resources. In order to make
this information available to the GIS database, the image containing RS data has
to be integrated with the GIS, it is possible to make use of these data by a GIS
in terms of spatial data layers updation, analysis of spatial data with
attributes in conjunction with RS data and generation of maps containing
combination of RS image and other types of cartographic and thematic
information.
This paper discusses and describes requirements and
procedure for digital image integration into GIS databases. The issues related
to image integration are discussed and a procedure of geo-referencing image
within the GIS database framework, GIS database layer updation and composition
and plotting of map containing image data as well as selected database layers
has been described in detail with special reference to Natural Resources
Information System (NRIS). Ultimately, this approach will lead to development of
a user friendly software Shell for carrying out the tasks of image registration
with spatial databases (e.g. NRIS), generation of image-maps by overlaying
selected database elements at desired scales and paper sizes and updation of
existing spatial database layers like landuse, transportation network, drainage,
canal network etc. Towards this, a prototype is under development, which
conforms to NRIS database standards.
Image Integration With Spatial
Databases
Issues Related To Image Integration A satellite image or
aerial image can not be integrated as it is with a spatial database because of
two reasons. The first reason is that a satellite image suffers from geometric
distortions because of platform instabilities and attitude effects and second
reason is that the coordinate system of the image and the node database are
different. Because of the different co-ordinate systems, an image can not be
displayed along with a spatial database layer because their extents do not
match. Due to geometrical distortions, features of the image do not match with
the spatial database features because of differences in scale, translation and
rotation effects. Unless these issues are resolved, image integration can not be
performed.
Geo-referencing An image can be integrated with the
spatial databases using geo-referencing technique. Using this technique
geometrical distortions are removed from the image and the co-ordinate system of
the image is changed to the co-ordinate system of the database. Geo-referencing
is done by establishing links between image features and corresponding features
in the spatial database. Features used for establishing links are called Ground
Control Points (GCPs) which should be clearly identifiable on the image as well
as on the spatial database. From the GCP positions on the image and spatial
database, the image is geo-referenced by making use of a mapping polynomial.
This mapping procedure removes geometric distortions of the image and changes
the co-ordinate system of the image to spatial database co-ordinate system.
Normally, the geo-referencing is done in interactive mode. The user identifies
GCPs interactively on the image and database and establishes links. Based on
these links geo-referencing process calculates mapping polynomial coefficients
and image is mapped with the spatial database. However, this process could be
made automatic if image corner points are known in terms of latitude and
longitude values. Since no user interaction is required, this geo-referencing
technique could be called automatic geo-referencing.
The process of
image integration consists of three steps. These steps are image extraction,
automatic geo-referencing and interactive geo-referencing. The steps required
for image integration are described below.
Image Extraction The process
of image extraction involves extraction of the image data from the DAT/CD media
and storing in the disk file, preparation of the image header file containing
information about image size, number of bands etc., preparation of image
statistics file giving information about minimum and maximum gray levels, mean
and standard deviation for each band and extraction of latitude-longitude
information for all four image corners into a file from the ancillary
information provided on the media. The image extraction program can generate
these four files which are used in subsequent steps.
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