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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
e The process of photographing of the land surface is
continuous lasting for a period of 4 days. Owing to this
the most appropriate image was chosen.
e The formalities for aerial photography and flight
arrangement are avoided here.
e. The use of satellite images is considerably less expensive
than the aerial pictures.
Due to the above mentioned reasons GIS SOFIA Ltd. decided
to use satellite imagery for revision of the topographic map at
1:5000 scale for a part of the territory of the Capital Sofia.
4. MAIN CHARACTERISTICS OF QICKBIRD
SATELLITE IMAGERY
4.1. Geometric resolution
Because of the large scale of the map an image, obtained by
QuickBird is selected, which for the time being is with highest
resolution — 61 cm in nadir for panchromatic image.
4.2. Radiometric resolution
Data obtained by QuickBird, have 11-bit dynamic range, which
improves visualisation and makes them appropriate for use in
urban territories.
4.3. Spectral range
According to the spectral range, the delivered images are
grouped as follows:
e Panchromatic — with range of 450-900 nm.
° Multispectral — four channels with different spectral
range:
- Blue (450-520 nm).
- Green (520-600 nm).
- Red (630-690 nm).
- Near-Infrared (760-900 nm).
44. Main products
QuickBird offers satellite images in three basic products
depending on the level of processing of the rough image: Basic,
Standard and Orthorectified.
. Basic Imagery appears to be the product with least
additional processing. Just corrections for elimination of
the radiometric and sensor distortion are made, and no
other geometric corrections. It is delivered with additional
files with information for efemerides, sensor position, etc.
These data are needed for setting up of an rigorous sensor
model. Basic Imagery is considered as most appropriate
for further photogrammetric processing. Minimum order
is a full scene 270-300 km”.
. Standard Imagery is an image, which is corrected with
respect to the radiometric, sensor, and platform-induced
distortions and the topographic distortion, using the
GTOPO30 digital elevation model. The image is
georeferenced and is made in a pre-set cartographic
projection. It is possible to order “parts of the scene
(minimum area of 25 km”). A variety of this product is
Ortho Ready Standard Imagery. It is appropriate for
orthorectifying when using a ready digital elevation
model.
° Orthorectified Imagery is a product, corrected for
radiometric, sensor and topographic distortion. Usually
for its orthorectification, digital elevation model and
geodetic points are used, provided by the user. It is
georeferenced and transformed into pre-defined
cartographic projection.
4.5. Basic data about the selected image
The image is panchromatic Basic Imagery, which covers the
central part of Sofia. It is an archive image, taken on 27 March
2003 at 09:17 hours GMT. The period, when the image was
made, was very appropriate in view of vegetation of plants —
hardly any leaves, which could stand in the way of important
features. Although that early spring is characterized by unstable
weather conditions, in this case they are extremely appropriate
— 0 % clouds. The time interval between acquisition of the
image and the time of its purchase and start of further
processing for orthorectification and deriving the vector
information is within a period of four months, which is
completely satisfactory.
5. GEOMETRIC CORRECTIONS
5.1. Use of 3D polynomial transformation in the range
of entire image
The digital photogrammetric software PHOTOMOD 3.51 of
RACURS Co. is used for processing of the image. However, the
software package does not contain QuickBird sensor model.
Hence transformation of the image is performed by 3D -
polynomial transformations, derived on the basis of
geodetically defined points.
A special feature of such transformations is that the polynomial
coefficients have no physical meaning, in other words do not
correspond to any geometric model. They are empirically
obtained relations, which makes the result very much
influenced by the number, location and accuracy of the ground
control points in use.
Because of the nature of activity of GIS SOFIA Ltd. sufficient
number of geodetically determined points is provided. Specific
elements from the terrain, which are clearly visible on the
image and defined at earlier geodetic surveys, are used.
Despite of the large number of ground control points the
achieved accuracy is insufficient in the range of the entire
image. In some areas the deviations are up to 4 — 5 m.
5.2. Use of 3D polynomial transformation in the range
of a part of the image
The central part of the image, where the terrain is relatively flat
(high differences of less than 250 m) is selected. There are 16
control and 19 check points used. The obtained maximum and
RMS errors in position of the check points are 1.64 and 0.82 m
respectively. The residuals in X and Y directions, as well as the
location of points are given in Table 1 and Figure 1.
Orthorectification by use of DEM, which is generated from
contours at 5 m is performed. The obtained maximal
displacement and RMS error in position of the check points are
1.74 and 0.91 m respectively.
