2.2 PROCEDURE OF PSA 
2.2.1 PRE- AND POSTPROCESSING 
The program consists of seven principal parts, which can be called 
from the main menu. The first four options and the last one are 
callable for pre- and post-processing respectively of the data. 
From internal file input the program retrieves orientation and all 
model related data. The first step is to compute a theoretically- 
achievable height accuracy with standard photogrammetric formula 
from the given flying height, the distance between the perspective 
centres and an empirical value for the accuracy of parallaxes mea- 
surement. PSA proposes three times this value as the attainable 
height accuracy but the operator can also enter another value manu- 
ally. 
For data acquisition an area needs to be defined. So the second 
option asks for the definition of the boundaries which form the 
border of a convex quadrilateral. The nodes can be measured in the 
model or read from the control-pointfile, where they are stored 
later so that areas can be connected without gaps. The user can also 
check and plot the border definition. 
A basic capturing interval is derived from the harmonic analysis of 
test profiles (/10/). An arbitrary number of profiles can be mea- 
sured, after which the program always suggests the smallest of all 
derived intervals to guarantee the desired height accuracy. Again 
the user can enter another value manually. Several areas can be 
captured with different basic intervals. 
Option 4 allows any precompiled data input from MP-files such as 
scarps, ridges etc.. First the user is asked for the filenames where 
the breaklines are stored. After these data are read he has to enter 
the filenames for structure-lines. A11 other input on theses files 
is accepted. The data are automatically filtered with respect to 
the nearest-neighbour distance fixed for the quadtree database. A 
control plot of the prior information can be produced. 
After the data capturing loop the user can access the database for 
various tasks. The number of points registered up to that moment is 
displayed and the user may edit the points stored in the database. 
Heights of points which are suspect can be remeasured. A recording 
in the vicinity of the desired point allows the searching in the 
database for the nearest stored point, from which then x, y, z-co- 
ordinates are displayed on the screen. If the height was wrong this 
point can be deleted and remeasured. Other points can also be in- 
serted or deleted. This option also allows for the complete deletion 
of the database after an MP-file was generated. MP-files can be 
generated at any stage of the data capturing, so that different 
densification results can be compared. 
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