the standard error of the systematic errors can be determined 
from the special law of error propagation. In general the 
standard errors can be determined from the product of the 
standard error of unit weight and the square root of a 
weight number, which also can be directly determined from 
the normal equations. For a determination of the standard 
error of arbitrary linear functions of the parameters, for 
instance the combined corrections, the general law of error 
propagation generally has to be used. Attention must then 
be paid to possible algebraical correlation between the 
parameters, expressed by the correlation numbers. The 
weight and correlation numbers are usually summarized 
in a matrix, frequently called the matrix of weight coeffi 
cients. 
With the aid of the standard errors of the parameters 
and of functions thereof, it is possible to judge the signi 
ficance of the differences between the parameters and of 
functions thereof on one hand and the ideal values on the 
other. In this way tolerances for the systematic errors and 
functions thereof can be established. 
Because the standard error of unit weight in connection 
with calibrations can be used as an indication of the basic 
accuracy of the device under calibration, the magnitude 
of the standard error of unit weight can be used for a judge 
ment whether or not the obtained accuracy is in agreement 
with some prescribed value to be used as a characteristic 
of the device in question. According to the resolution No. 
II: 6 from the Lisbon Congress, quoted above, such values 
shall be furnished by the manufacturers in order that the 
instrument users shall be able to judge the quality of the 
actual device under calibration and in particular decide 
when repair or adjustment must be made. 
2.3. The Concept of Weight 
Above, all measurements have been assumed to be of 
equal weight, i.e. of the same basic quality. If, for some 
reason, different quality of the measurements in the same 
set is to be expected, weights can be introduced and used 
in the computations in a well known manner. When 
weights are introduced before adjustment computations 
they are generally called a priori weights. Weights can also 
be determined from the results of the adjustment and then 
have the character of a posteriori weights. 
Examples of a priori weights due to physical circum 
stances are weights of image coordinates due to the fact 
that the photographic image for evident reasons never is a 
mathematical plane. If such coordinates are measured 
orthogonally, the lacking flatness will have different 
influence in the center of the image in comparison with the 
edges. Practical experiments have also indicated that this 
assumption is justified and that there can be considerable 
differences in the geometrical quality of image coordinates 
depending on the location of the points. There is probably 
a weight function for each camera and photograph and the 
weights could be expressed as functions of the radii. This 
is a question that should in particular be noted in connec 
tion with calibrations of cameras and photographs under 
operational conditions. 
A posteriori weights are to be referred to the quality of 
results of adjustment procedures and can be determined 
with the laws of error propagation. 
The theoretical definition of weights of measurements 
usually refers to the quality of the measurements as deter 
mined from redundancies. Ordinarily weights are defined 
as inversely proportional to the variances. Because, in 
principle, variances can refer to the concepts of precision or 
accuracy respectively, weights can also be referred to either 
of the two concepts mentioned, depending upon how the 
variances were determined. 
In the case of precision, the variance of the mean can be 
arbitrarily decreased through repetitions only. 
In the case of accuracy, the residual variance cannot be 
decreased in the same simple manner because the variance 
will always be composed of another type of error popula 
tion, containing also systematic errors of such a magnitude, 
however, that they cannot be individually determined. 
Consequently there is reason to distinguish between 
precision weights on one hand and accuracy weights on the 
other. The precision weights refer to the squares of the 
standard deviations (of a single measurement or of func 
tions of such measurements, e.g. the mean of repeated mea 
surements), i.e. precision variances. The accuracy weights 
refer as a principle to the squares of the standard errors of 
unit weight of measurements after the adjustment or 
to the squares of standard errors of functions, i.e. accuracy 
variances. 
2.4. Summary 
The principles, summarized above, were in more detail 
sent to all national societies of photogrammetry, organiza 
tions, and individuals for critical examination. A number 
of answers were received, the majority of which agreed 
with the proposals. Only in a few cases some objections 
were noticed, mainly of formal nature. 
Those who did not answer were regarded as being in 
agreement, according to usual principles and especially 
emphasized in a letter, sent to all addresses in good time 
before the dead line, Oct. 1, 1966. 
Hence, the following may briefly be summarized as the 
opinion of the national societies of photogrammetry. 
1. Distinction should be made between the two basic 
concepts and terms of precision and accuracy according to 
the definitions in Ref. 2: 1, quoted from statistics. 
In brief the following definition, quoted from Eisenhart, 
Ref. 2: 2, can be used: 
Precision refers to closeness together,