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15. 
persion of the subject edge location of the 4 observers was very large 
in relation to the standard error of a single observation. 
Decutance is derived from the density spread function as the ratio 
of the amplitude and the width of the spread function. Thompson found 
that his measure of image quality correlated well with Decutance. 
Image quality and hence reliability of edge pointing improved, as the 
spread function decreased. That is the measure of image quality is 
inversely proportioned to the spread function width. 
A firm estimate of the magnitude of the systematic errors is not 
available as they vary so greatly, but Thompson believes that it may 
be possible to determine the personal bias and then compensate for this 
error during the observations. Since the errors vary with spread 
function and contrast it appears this proportion would present some 
difficulties in practical situations where contrasts and spread 
functions would be variable over an image. 
Welch and Halliday performed an extensive series of pointing 
observations to the edges of bar targets, and isolated square objects 
photographed at a scale of 1:24,000. Measurementsof the sizes of the 
objects were made on a stereocomparator at an optical magnification of 
35X to 40X. Errors in the measurements of the widths of the bars and 
the actual edge pointing locations of the squares were presented in 
graphical form. From the plot of pointing positions to the edges of . 
the squares, average errors were derived and an attempt made to relate 
these errors to imaging quality of the system, for this paper. A 
summary of these mean errors is given in Table 3. 
The systematic errors increased with target contrast. Indeed the 
error of pointing to high contrast targets was approximately twice 
(7.6um or 250 secs of arc) that for medium contrast targets (3.5um or 
100 secs of arc). There is no significant overall change in the 
systematic errors for measurements in the "with flight direction 
compared with measurements in the 'Against flight'direction. Welch and 
Halliday also found that for bar target width measurements, higher 
width measurement errors occurred for the high contrast bar targets 
than for the medium contrast (2.5 to 1 to 6:1), for which errors of 3-7 
um regularly occurred. (The precision of pointing was 2-3um). For 
both low and high contrasts systematic errors deteriorated rapidly. 
Martucci (1972) found a similar relationship and claimed that medium 
contrasts resulted in the highest quality of bar width measurement. 
In order to relate the results of Table 3 to a measure of imaging 
quality of each photogrammetric system, MIF curves were derived from 
each of the square wave functions given by Welch and Halliday. An 
estimate was then made of the spread function width, assuming each MIF 
was approximately Gaussian. The approximations in general were 
acceptable, but in some cases they were poor. The spread function 
widths however were used only as a guide. A plot was then made of the 
reciprocal of the spread function width against the systematic errors 
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