procedure is to be carried out at server level as an
input data base creation task.
Updation of each map layer (one for each feature, viz,
road network, elevation, annotations, and other
cultural features) is done at the photogrammetric
client, once all the required information is available.
DEM generation is the first step to be performed, in
case it is not available or the available DEM is not a
recent one. DEM is generated, using the IRS-1C PAN
stereo pair and a few GCPs, with the help of an
orbit/attitude model, automatic conjugate point finding
algorithms and space intersection [IRS-1C Stereo Data
Products Team, 1993.]. Then the DEM editing is done
in real time using stereo display, image draping and
image processing concepts. Hence a powerful CPU at
the client end is a minimum requirement. The DEM
also can be obtained by the external sources like by
digitising map, or previously derived from stereo pairs
etc. After having DEM an orthoimage is generated
using the image data and the corresponding ground to
image transformation model. The generated
orthoimage will form as the base information for the
map sheet to be updated. Now using the DEM, model
(in back ground), orthoimage, the desired map layers
can be freshly obtained using 3D feature coding and
online editing. This involves stereo display, image
processing concepts like automatic topographic
feature identification, edge detection, image
classification and several other functions like vector to
raster conversion, vector raster overlay, contouring
etc. All the map layers created at client will be stored
in a local database.
Verification (automatic and manual, some times
ground verification of obtained features), validation,
editing and labeling is performed for all the derived
layers before updating the database. A number of
tools are required here finally. These include
annotation generation, symbolisation, data conversion
routines etc. Standard data formats like DVD1 and
DVD2 are to be used throughout, so that the inputs
and outputs are compatible with those of Survey Of
India (SOI) standards. Finally all the map verified
layers can be transferred to the server and then the
main database will be updated with these layers. A
separate map compilation, map drawing software is
used to make the paper prints of final updated maps.
As an integrated version, this system has a multi-user
environment, where at a given time 2 to 3 users
simultaniously can work on different map sheets or on
different layers of the same map sheet. Each client
has its own database. There is no client to client
communication planned, which may hamper the
system performance. All the final layers of the map
are compiled at server for map making/ updation.
3. FUNCTIONALITIES OF THE SYSTEM
For the above described procedure the various
important functionalities/ software elements required
are as follows:
482
3.1. Data Ingest
This is done through cartridges, CD-ROMs or from
DAT devices. Different input data sets can be (a)
stereo pairs/triplets , (D) DEM derived from external
source or digitized from a scanned map (c) GCPs (d)
map/image database (e) required layers of map
information and (f) other ancillary information for
image rectification/orientation. Data conversion
routines, digitiser/scanner software, database
extraction/ updation are the main software elements.
3.2. Models
Models for DEM generation and image correction
with or without GCPS are required in the system.
Two different models are available currently. The first
one uses the space resection for updating the
satellite orientation and space intersection technique
to generate 3D ground co-ordinate of a given
conjugate point [IRS-1C Stereo Data Products Team,
1993. ,Rebanta Mitra etal, 1994.]. Modified
collinearity conditions are the basic equations in this
model. As a second model (for refining both satellite
orientation and the derived DEM) a bundle adjustment
software developed jointly by Space Applications
Centre and German Aerospace Research
Establishment (DLR), Germany is used [Gopala
Krishna, B., 1994.]. In this approach DEM is
computed in a combined solution for GCP
coordinates, conjugate point coordinates and
orientation parameters of the stereo pair imageries. In
addition to these two models image-to-ground and
ground-to-image transformations and image
resampling are part of this software for generating
the final corrected images. Several height
interpolation algorithms are inbuilt in the system for
generating regular grid of DEM from a set of irregular
DEM points.
3.3 Stereo Display Related Functions
Stereo display related functions are required at
several places like (a) GCP identification (b) conjugate
point verification, (c) DEM editing and (d) 3D feature
coding. The related software elements are (i) stereo
display, (ii) epipolar image creation, (iii) floating
cursor and its positioning, (iv) DEM editing and
feature coding, (v) height interpolation, (vi) point
height measurements etc. These functions are
implemented on R-4000 based workstation and are
tested with several data sets of SPOT PLA and IRS-
1C PAN data.
3.4. Conjugate Point Finding
This is an important task in automatic/semi-
automatic way of DEM generation on a digital
photogrammetry workstation. This has several
components like interest operator, digital correlation,
local mapping and blunder detection. In this system
a hierarchical automatic point matching technique
based on an interest operator followed by an area
based correlation is implemented. Hierarchical
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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