9 - 
ee also [50] - [54]). 
d in [54] and [55]. 
eing abandoned in 
t been intended to 
Tectronic compu - 
Ling situation for 
it electronic compu- 
puters ( [55] ). Thus 
le computation with 
the theoretical ba- 
's are equivalent 
y will, for quite so- 
omputation is not yet 
~y with simple elec- 
[56] ). In view of 
mputers it must, ho- 
r ith slight modifica - 
riment a new analo- 
cross strips (after 
ent of heights with 
> yield weight coef- 
Dnic computation 
n on strip-adjust- 
f aerial triangula - 
putation. It seems 
ik-adjustment is an 
ical approach, prin- 
edures, program - 
the considerable 
e not yet led to prae 
ure ( [60] - [71] and 
ite where the practi- 
iow a matter of rou- 
Lth plate coordinates 
hmid [39] and De 
'ammetric applica - 
is been published as 
puters are used in 
idjustment procedu - 
; and yields simulta- 
y Proctor and Amer 
([56], [72] - [74]). Extensive experiments about the convergence of the iterations have been 
carried out. By application of overcorrection factors the originally encountered number of about 
80 iterations per block has been brought down to the order of 20 iterations for blocks of about 
100 - 200 models (see also Muller [70]). A high speed computer is needed in order to keep the 
procedure economical. 
ITC in Delft is using a block adjustment procedure developed by Van den Hout ( see 
[75] ). Up to now it is programmed only for planimetric adjustment. It is also working with in 
dependent stereo-models as basic units. The numerical solution of the normal equations is di 
rect, non-iterative. It is achieved with a computer (Stantec Zebra) ranging at the lower end of 
middle class computers. At present blocks up to about 500 models can be treated. 
From a third group of block-adjustment procedures, which use strips as adjustment 
units, the procedures of Schut and Bervoets are in routine application. These procedures ad 
just up to now planimetry and height separately. They correct the strips in a block by polyno - 
mial formulae of 2 n< ^ or 3 rc * degree which are determined together in the block connection. The 
numerical solution is iterative. Procedures and test results are described in [15], [76] - [8l] ; 
see also [82]. 
The majority of adjustment procedures which are at present either in practical use or 
in an experimental stage of development have been and are developed practically independent 
from each other .Up to now no comparative evaluation, neither theoretical nor experimental, has 
been attempted although a number of distinctions are obvious, and are generally known. In de 
veloping and evaluating block-adjustment procedures one is concerned with a number of pro - 
blems and considerations of different nature. Considering the items listed under 1. 2 as a basis 
of discussion may help to bring forward the most important points of view. 
3. 4 The theoretical approach to block adjustment has been classified (see [1] and [7 3] ) accor 
ding to the elementary units which have to be tied together by the adjustment. One has distin 
guished between procedures working with single photographs, stereomodels, sections, or strips. 
The list might be completed by mentioning triplets (3 photographs) and multiplets (i. e. 9 pho 
tographs). Generally speaking any form of subblock may be used. 
Regarding this search for the most suitable unit of adjustment a few general remarks 
may be made : The most general and hence the most detailed and most accurate approach to 
block-adjustment is at present given by the use of single photographs (bundles of rays) as basic 
adjustment units, with plate coordinates as uncorrelated observations. Given this system as a 
reference one can consider any other system as being derived from it, as any other unit can be 
derived from single bundles of rays. According to the rules of least squares adjustment any ad 
justment procedure working with derived elements remains completely rigorous and thus iden 
tical with the direct solution of the single plates approach in case the error properties (i. e. 
the correlations) of the derived elements are taken into account. It is even possible to achieve 
a rigorous solution when working with units which have been formed by allowing approximations 
and simplifications. In such a case the subsequent adjustment can still rectify these approxima 
tions. Thus as far as basic least squares theory is’concerned the search for various adjust 
ment units is not substantial. It is even not substantial - still referring to a completely rigo - 
rous solution - with regard to the problem of numerical treatment. The total amount of nume - 
rical work in an adjustment in phases is generally not less than that in the direct approach. 
It should be pointed out, however, that a sophisticated combination of simplified deri 
ved units and relaxation procedures might give advantages in the numerical treatment and still 
achieve results which are equivalent with a rigorous solution. It is conceivable that ultimately 
the most efficient numerical solution of the general task of block-adjustment would be such a 
" hybride system ", which would also incorporate the problem of obtaining approximate values 
of the unknowns. 
Regarding the present situation, the use of various adjustment units other than single 
plates becomes important, however, at a second stage of thought, namely when certain appro-