Digital image interface with spatial databases- requirements and procedure
P. D. Yadav, Ketki Kharod, Bharat Vaishnav, R. K. Goel
Geomatics Technologies Division, Space Applications Centre
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.
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.
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.
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.
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.
Digital image interface with spatial databases- requirements and procedure
Spatial Database 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 vailable 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 in to 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.
A spatial database contains information about variety of spatial elements. Often, it becomes very useful for the user to generate a hardcopy map containing spatial data elements such as roads, water bodies, administrative boundaries, important landmarks, features of special significance etc. with satellite image as a background. Such map plays an important role when the user wants to visit a particular site on the ground for the purpose of making ground truth. Such maps are also very useful to give a total picture of a study area. The Image-Map generation procedure should be developed to generate hardcopy maps for an area of interest with satellite image in the background over which other spatial database features are overlaid.
The Image-Map making should have the following steps.
- Selection of the area of interest from the entire spatial database.
- Extraction of the area of interest as a function of map-scale and paper size.
- Examination of the extracted area on screen by superimposing different spatial database layers on the selected image.
- Making map composition once the contents are finalized.
- Annotation of the map using map elements like text, box, circle, line etc.
- Creation of a plot-file for obtaining hard copy of the map.
Selection of the Area of Interest
The user should select the area of interest by clicking the point on the displayed image. Before selecting the area of interest, user should select appropriate image from the image catalogue. The selected image will appear in the background of the Image-Map.
Extraction of the Area of Interest
The size of the area of interest to be plotted depends upon the map scale and the paper size. After the map scale and paper size are selected, the system should determine the size of the area to be extracted around clicked point as a function of map scale and paper size and display only the extracted area on the screen in place of entire spatial database layer. The user should be able to change the map scale and paper size if required until the entire area is covered properly.
Examination of the Area of Interest
During this stage, the user should be able to select different options for superimposing various spatial elements from the spatial database on the background image. Facility should be provided to execute this step repeatedly until the user finally decides that which features should be drawn on the final Image-Map.
Making Map Composition
The extracted and examined area of interest should undergo map composing process in which all the image and spatial database map elements should be entered and finally marks showing distance in kilometers should be drawn on all four sides of the map. This map composition could be annotated and printed later on.
Annotation of Image-Map
This Image-Map should be annotated to increase its usefulness. The process of annotation may include writing the names of the places, putting the date of the background satellite image, writing the name of the area of interest, writing scale of the map etc. Facility should also be provided to put text strings of different size and colors at desired location on the Image-Map. Facilities for drawing circles, boxes, irregular polygons etc. with different colors and sizes are also provided.
Generation of Hard-copy
This step should ask the user about the format in which the plot-file is to be generated. The choice could be generating the plot-file in HPGL or Encapsulated Post-Script (EPS). The plot-file could be plotted using an operating system command.
The process of digital image integration into the spatial databases is described along with the two of its utilities namely spatial database layer updation using the images integrated with the database and preparation of the Image-Map containing spatial database features with image as background. Such utilities will be very much useful when NRIS spatial databases will be operationally used for the management of natural resources. Towards these, these concepts are being used for developing GIS based menu driven and easy to use command procedures called ‘Shells’. A prototype has already been developed and it will be offered for use after proper Test and Evaluation.
The authors are thankful to Shri A.K.S. Gopalan, Director, SAC and Shri A.R. Dasgupta, Deputy Director, SIIPA for providing us the opportunity to work in this field and constant guidance and encouragement provided by them. The authors are also thankful to Shri I.C. Matieda, Shri Satishkumar, Shri Gogoi and Ms Sanjukta for their valuable contribution made for image downloading procedures.
- NRIS Node Design and Standards, Technical Note, SAC/SIIPA/NRIS-SIP/SD-02/98, July 1998.