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Full text

Title
International cooperation and technology transfer
Author
Fras, Mojca Kosmatin

121
An additional difficulty is that manufacturers of laser
equipment specify measurement accuracy not as an
extreme but statistic value due to which big number of
control measurement is required for the correct
assessment. Given that the resulting accuracy
obviously depends also on surveyed object type
(actually on the condition of its surface), the task of true
accuracy verification turns into quite a labor consuming
one. The results of thorough investigation in this field
are available in (Murtagh, 1999 and Piwowar,1999).
An important factor influencing resulting accuracy of
laser locator measurement, besides, obviously,
technical features of laser locator itself, is a quality of
GPS signal used to register an aircraft trajectory. In this
regard, it is worth mentioning that:
1) There are areas overall unfavorable in terms of GPS
measurements quality. This may be both caused by
unfavorable satellites configuration, and the number of
other factors as well. The worst situation (among the
countries the author had experience in) is in Italy. The
favorable on number of satellites and working
constellation periods fall in the night time, as a rule,
when aerial survey is impossible. Mountainous relief
greatly shields the satellites. But the major problem is a
big number of radio repeaters complicating GPS signal
receipt up to its full loss.
2) Accuracy of trajectory data obtained with cinematic
GPS measurements is directly tied with piloting quality.
The issue is that required accuracy of 10-15 cm may be
provided with only special double-frequency carrier
phase GPS receivers, working in regime of continuous
tracking of active satellites. Any abrupt evolution of
aircraft or appearance of relief obstacle in the upper
hemisphere may lead to loss of the working
constellation and failure of tracking as a result.
Resumed coverage and tracking may take up to several
minutes as they require a sophisticated procedure of
infinity resolution on whole wavelengths. Up to that
moment the registered data shall be absolutely
unsatisfactory on accuracy.
Moreover, even for trajectory portions with quality GPS
receipt, the correct results are only possible with
sufficient statistics available. Practically this means
necessary continuous quality GPS signal receipt of no
less 40-50 minutes long. This requirement meeting may
grow into a serious problem.
Possible solution may be making up some integral
aircraft trajectory based upon joint analysis of GPS and
IMU data (Lithopoulos, 1999).
It is also no doubt that a major advantage of DTM
production with laser locator is that its production is very
technologically convenient. In such case DTM is a
result of direct measurement having an original 3D
nature. All the processes of technologic cycle are fully
automatic, excluding therefore any subjective element.
The latter circumstance has a decisive meaning, and it
is directly followed by a possibility to ensure speed of
processing and production of final DTM equal to aerial
survey data collection.
♦ Informative capacity of DTM made by laser location
appears much better. The most interesting is locator's
ability to register more than one return per each
scanning. So, the first return may refer to tree canopy,
grass level, wires or towers of power line, as well as
manmade facilities over the ground level. Return from
true relief level will always be the last of returns.
In its essence the laser locator image obtained upon
aerial survey is a complex topologic matter that may be
in the very first sight deemed a mix of DTM showing the
true relief, and returns of other ground objects. A
deeper approach allows, upon analysis of laser returns'
spatial distribution, to get various specific features of
scene topology. The most developed by the present
time are the algorithms of automatic detection of power
transmission lines for database creation and automatic
designing (Medvedev, 1999). The works on laser
locator data application in forestry topology are under
way, algorithms of automatic detection of roads and
railroads are developed. Obviously, the applications of
automatic detection and positioning upon laser locator
data will actively grow.
However, from purely photogrammetric point of view,
high informative capacity of laser locator data has its
negative side as well, that is required DTM is not
directly presented in the laser image. To separate it
clearly, application of a number of sophisticated
algorithms is needed, the current versions of which are
not perfect yet.
2.2. Aerial photography
Laser survey can be naturally integrated with digital
photo survey. Indeed, digital cameras contrary to
classic ones, are lighter, more compact and economic.
Mounting thereof into an aircraft along with laser locator
assumes no serious technical problem. GPS receiver
included in any laser locator unit, may be as well used
for registration of principal point coordinates, providing
additionally full synchronization of laser and photo data.
By the way, it may be mentioned that the idea of such
integration is as well applicable to other digital remote
sensing facilities like IR sensor, or multispectral
scanner.
However, the matter is not limited by only technological
comfort. As it has been pointed, the integrated analysis
of laser locator and digital photo data allows principally
new final results.
In addition to above, such integrated aerial survey is
always provided with quite accurate data on camera's
optical axis angular orientation, as the laser locator
always utilizes such information for registration of