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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