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Digital Image Interface with Spatial Databases - Requirements and Procedure
Automatic Geo-referencing
After image extraction, the next step is to
perform automatic geo-referencing of the extracted image using satellite
ephemeris information available in form of corner points latitude-longitude
information and line number-pixel number information. During automatic
geo-referencing the following steps should be performed:
- Conversion of the corner points latitude-longitude information to the node
database map co-ordinate system.
- Generation of image corner points data file containing pixel number, line
number and their co-ordinates in the map projection system for each corner
point.
- Generation of coefficients file for mapping polynomial by making use of the
image corner points.
- Automatic geo-referencing of the extracted image using mapping polynomial.
The process of automatic geo-referencing changes the co-ordinate
system of the image to that of spatial database and removes the geometric
distortions up to the accuracy of the ephemeris information, which could be of
the order of several meters or a few kilometers. Since the automatic
geo-referencing process uses only first degree mapping polynomial, higher order
distortions other than translation and scaling are still not removed. In order
to remove the higher order distortions left in the image and register the image
as accurately as possible with the spatial database layers, the automatically
geo-referenced image is still required to be further geo-referenced using
interactive geo-referencing process.
Interactive Geo-referencing
The objective of the interactive geo-referencing is to remove geometric
distortions present in the auto geo-referenced image due to inaccuracy of the
ephemeris information. The major steps for interactive geo-referencing are as
follows.
- Identification of links between image and spatial database features.
- Evaluation of links identified in terms of Root Mean Squared (RMS) error
between positions of links in the spatial database and transformed positions of
links in the image.
- Geo-referencing the image using the evaluated links and a higher order
mapping polynomial to register the image more accurately with the database
layers and storing the geo-referenced image into an image catalogue.
Image Catalogue Creation
An image catalogue is a logical
collection of images normally belonging to an area of interest. The image
catalogue contains information about the images stored in the catalogue as well
as other information about the image such as date of image data acquisition,
sensor name, geometric resolution of the image, geographical extents of the
image etc. After image integration is done, the images could be stored in the
image catalogue for future use.
Spatial Databases Layer
Updation
Satellite images provide latest information about various
land-cover types like roads, urban areas, canals, railway lines, forests, barren
land etc. and hence they are very useful in updating the spatial data elements
containing above information. This facility is especially of very much use when
for a particular area; the information is obtained from the maps, which are more
than 25 years old. The spatial database layer updation procedure could be used
for updating a spatial data layer using the satellite data of the recent dates.
The updation should not only include spatial part but its corresponding
descriptive part also.
The layer updation process could consist of
database layer selection for updation and image using which it is to be updated,
performing screen management utilities required and carrying out feature editing
actions and session management actions. For this purpose, an easy-to-use and
menu-driven procedure should be developed and it should have the following
facilities.
Layer Selection and Background Image Setup
The
spatial database layer updation menu should offer choice of layer to be updated
from the available spatial data layers and background image to be selected from
the image catalogue.
Screen Management Utilities
The utilities
for screen management should include displaying of the features having image in
background, zooming the area of interest, displaying the full view and inquiring
about the attribute of the selected feature.
Feature Editing
Actions The feature editing facilities should include adding a new point,
line or polygon feature and its attribute, selecting a feature for editing
operation, splitting a feature into two parts, codifying a feature, removing a
feature from the database and undoing the last delete operation.
Session Management Actions
Session management actions should
provide facility for saving the updated layer in to the target database after
performing necessary actions such as line or polygon topology building.
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