tuelle- 
iduelle 
multa- 
les re- 
les in- 
‘ochées 
re ces 
soudre. 
permis 
ui exi- 
s to be con- 
neval gives a 
d methods in 
ct of a block 
ngulation, es- 
gulation, but 
ormal slotted- 
ulation strips. 
ck adjustment 
. and the most 
lent, and pos- 
it gives only 
t be used for 
do not think 
meeting here. 
and analytical 
1sidered them 
tical air trian- 
according to 
stance, by Mr 
ime and some 
ording to ap- 
then basically 
rial triangula- 
angulation, we 
cal solutions, 
ent strips and 
nate obtained 
hod of cross- 
| of Professor 
yr third order 
e co-ordinates 
for the whole 
LA COMPENSATION DES BLOCS DE BANDES, DISCUSSION 61 
strip. Finally, there is the special method, such 
as the use of independent bases or the use of 
auxiliary data for the block adjustment. 
A second group of solutions will be all those 
methods which do not deal with the strip itself 
but divide the strip in sections and find certain 
corrections for these sections for the whole 
block together. These sections could be single 
models, of course, or they could be a certain 
number of single models together. 
Another block of adjustment methods would 
be mechanical methods, such as for instance the 
stereo template method which has been devel- 
oped to a high standard by the United States 
Geological Survey. Then there is the mechanical 
height adjustment method, as shown in the last 
Congress of the IGN. Finally, there is the anal- 
ogue computer adjustment method at Delft, 
International Training Centre. There are two 
different kinds of analogue computers, one for 
the height, which is basically similar to the IGN 
method, and a second one which is based on 
dividing the strips in sections. 
This is what I should like to mention out of 
the features of Mr Bonneval's report. I would 
like to add some personal remarks, namely on 
the problem of using rigid solutions for block 
adjustment on the one hand, or of using approx- 
imative solutions on the other. It is not thought 
that there are finally rigid solutions which can 
easily be obtained, even if it includes the solution 
of very large equation systems or including the 
finding of the very large number of unknowns. 
But I do think there will still always remain the 
fact that a big number of unknowns will always 
cost much more in computing time than a 
smaller number of unknowns, and so I would 
like again to stress the fact which has already 
been emphasised by Professor Thompson: that 
we do have to investigate very seriously whether 
a careful choice of unknowns — that is, reducing 
the number of unknowns in the block — will 
not essentially decrease the precision that we 
can obtain, but essentially decrease the work 
and the cost of such a block adjustment. 
With regard to the presented papers, I 
would only like to make some remarks to Mr 
Schut on the paper that was published in the 
National Report of Canada for the use of second 
order polynomes, and the results which have 
been shown. I do think that a second order 
polynome will never give a good correction for 
long strips. This was theoretically also shown by 
Professor Fórstners paper where he demon- 
strates that after a certain strip length a parabola 
cannot at all follow the real course of error in 
aerial triangulation. Of course, such methods as 
published there can meet the required specifica- 
tions, if we have only a small block and we just 
do not ask for higher accuracy. But as compared 
with the block adjustment method the errors are 
about four times as large. 
That is all I have to say here, and I would 
like to ask Dr Schmid or Professor Fórstner to 
say something. 
Discussion 
Prof A. BRANDENBERGER: I have to make 
only a few very short remarks. I would like to 
point out certain specific problems we might 
find in block triangulation. 
Basically, we have two approaches: the first 
one uses first stereo plotting instruments; and 
the second one is a complete analytical solution 
in one unit. The first approach, using stereo 
plotting instruments, is based on triangulation of 
individual models, and afterwards the discrep- 
ancies between individual strips will be eliminat- 
ed by block adjustment procedures. It is, of 
course, necessary that the discrepancy between 
adjacent strips is not too large. If the discrep- 
ancy is too large there is no sense in proceeding 
with block adjustment. Such a block adjustment 
of individual strips, which have been triangulat- 
ed by means of a first order stereo plotting 
instrument, has to be done in such a way that 
the individual models are not distorted too 
much. If, for instance, the discrepancies be- 
Archives 5 
tween adjacent strips are too large, it is possible 
that by the block adjustment you introduce 
deformations in an individual model, you will 
feel these deformations afterwards when you 
plot the model. This is a point to which we have 
to pay attention. 
In regard to a new approach, namely block 
adjustment using an analytical solution where 
we measure picture corners by means of stereo 
comparators and where we compute co-ordi- 
nates, there is also a special problem there: 
namely, that we have to adjust the block by 
means of a conformal system, or we have to use 
a non-conformal system, and this with regard to 
the fact that probably error propagation across 
the parallel strips might not be the same as the 
error propagation in the direction of the strip. 
These are some items which I have to men- 
tion, and actually I would like to point out, of 
course, that from the practical standpoint all 
these block adjustment procedures have to be