a great deal closer to fulfilling such time requirements, especially 
when performed on digital images supplied by digital cameras. During 
developement of OLT, the need for quick algorithms for sequential least 
squares adjustment has become clear. 
ON-LINE PHOTOGRAMMETRIC TRIANGULATION 
The term On Line Triangulation (OLT) has been used in various contexts. 
With analog stereoplotters it involved the direct editing of data 
during measuring, making it easy to transfer data to the block 
adjustment program in a proper format, and with the most obvious errors 
removed. 
When the analytical stereoinstruments arrived OLT was further 
developed, e.g. with model transfer procedures. As a computer is an 
integral part of the instrument, it was natural to include more 
advanced error checking. This error checking has more and more become 
the important part of OLT, as the potential has become clearer. 
To perform more reliable blunder detection, it was desireable to 
involve more than the traditional two photos of a stereomodel in the 
same computed unit. Both triplets and quadruplets has been investigated 
for use in aerotriangulation. E.g. in /10/ triplets are recommended 
under certain conditions. 
For higher redundancy and better reliability, a greater number of 
photos should be treated as a unit, or sub-block. According to Grün in 
/4/ a sub-block of 3x3 photos will give optimal local reliability in 
aerotriangulation, if the overlap is 80 7. 
When choosing sub-block size, the following aspects of the response 
time also have to be considered: 
The OLT system must not cause unnecessary delay compared to a normal 
measuring procedure. This is an objective demand. By calculation there 
may be found acceptable waiting times in different stages of the work. 
The operator should not get the feeling of waiting for response. This 
is a subjective demand, but may become the dominant one. If the 
operator is repeatedly forced to wait a few seconds it will affect his 
patience and concentration and so the quality of the work. 
The conclusion is that whatever the OLT system computes between two 
consecutive point readings, it should be completed as the operator is 
ready to press the button for the next point. At certain stages some 
delay would be acceptable, e.g. for error checking after a number of 
point readings, or when changing photos. 
With the available hardware today it is beyond any expectation to 
handle an entire aerial block in one piece on-line. The scope for OLT 
in aerial triangulation is only to perform data collection and blunder 
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