1:3,000 to 1:5,600. The remaining two tests 
employed much smaller scale photographs taken at 
approximately 1:20,000 and 1:40, 000 scale 
respectively. Maps were available for all areas and 
were used to provide control data. 
Table 1 gives a summary of the main characteristics 
of the various test models. 
4. USE OF DIFFERENT INSTRUMENTS 
installed in 
Topographic 
instruments 
In addition to the APY instrument 
the Department of Geography and 
Science, University of Glasgow, other 
were used for the tests. These were: 
— the APY of the Economic Forestry 
Group, Moffat; 
— the APY at the Dublin College of 
Technology, Dublin, Eire; 
— the Kern DSR 11 at the University of 
York; 
— the Wild BC-2 of Mason Land Surveys 
in Dunfermline. 
The other two APY instruments were used to allow 
a comparison of the results achieved from 
measuring the same models in three different APY 
instruments. 
The two mainstream analytical plotters were 
used to measure the Llandudno and Rorbas models 
to give terrain coordinates (E, N and H) of the 
control points and to provide comparative data 
that could be used in the analysis of the measured 
APY data. 
5. ORIENTATION AND ACCURACY TEST RESULTS 
5.1. Test of the APY Digitizing Tablet 
This test aimed to establish the accuracy of 
the tablet digitizer of the APY. The quoted 
resolution is 0.025mm., and the quoted accuracy 
+  O.1mm. Since the map which is being revised 
is placed on and measured by this digitizer, the 
accuracy of its output is very important, as the 
tablet digitizer generates the X, .Y coordinates 
which are the input to the analytical 
photogrammetric solution based on ob ject 
coordinates primary. 
The grid intersections of a stable gridded 
plastic sheet were digitized. The test was 
carried out twice, once for the lowest 
magnification (20mm spacing; 134 points) and again 
for the greatest magnification available in the 
map viewing channel (10mm spacing; 42 points). The 
positions of the grid intersections were measured 
three times. 
The standard deviations of a 
observation (stdev) 
following results:- 
single 
were computed with the 
For 194 points 
stdevx- +0.067mm. 
stdevy- +0.069mm. 
For 42 points 
stdevx- +0.038mm. 
stdevy- +0, 053mm. 
These are measures of the pointing accuracy in the 
X and Y directions for the lowest and greatest 
magnifications. 
The standard deviation values found for the 
greatest magnification are greater than the quoted 
value of +0.025 but part of the difference might 
be due to the errors in the observations made by 
  
FLAT TERRAIN 
  
  
  
  
  
  
Area | P.D. [Height |B:H|Ph.Scale [Map Scale 
Kelvingrove|152.57| 840 10.611: 5,400 |1: 2,500 
Llandudno  |304.77|1,510 710.31: 5,000 |1: 2,500 
Greystoke 19S2.05]2,950 10.611:18,000 l1:10,000 
HILLY TERAIN 
Area | P.D. |Height |B:H|Ph.Scale |Map Scale 
Greenock 152.40] S585 10.611: 3,700 l1: 2,500 
Rorbas 152.35 900 0.631: 5,600 |1: 4,000 
  
  
Table 1. Model Summary Table. 
the operator. 
The residual error at each grid intersection was 
then computed. The root mean square error values 
for each of the three sets of measurements at the 
two viewing magnifications were as follows:- 
Highest Magnification Lowest Magnification 
+ 0.08mm. t O.14mm. 
+ 0.12mm. + O.11mm. 
t O.15mm. + 0.18mm. 
AV. R.M.S.E. Value 
+ 0.12mm. + 0. 14mm. 
The quoted accuracy of the digitizing tablet 
incorporated in the APY instrument is O.1mm. 
which is rather better than the actual results 
found from the test, but these results include both 
observation errors and any errors present in 
the stable gridded plastic sheet. Obviously 
these two factors could have affected the results 
of this test. 
In production, it is likely that control points, 
even pre-marked or artificial points, will be more 
difficult to measure than the grid intersections 
used in this test. The rmse value of  t0.12mm. 
must therefore be regarded as the best that 
could be achieved and, for planning purposes, a 
value of *0.15mm, or even +0.20mm, might be more 
realistic. 
9.2. Orientation Tests 
Testing the orientation of the APY continued 
using eerial photographs, starting with the 
Kelvingrove model. The control points were 
digitized on the map during the measurement of 
the model. Numerous orientations were carried out 
at different times. The root mean square error 
values for planimetric and height and the values 
of the orientation elements (XO, YO , ZO , omega 
and phi) for each photograph are shown in Table 2. 
The maximum r.m.s.e. value in planimetry (mpl) 
obtained is +1.00m, the minimum value is  tO.29m. 
The mean r.m.s.e. value in planimetry (mpl) for 
the 12 orientations listed is +0.48m, equivalent 
to -0.2mm at the map scale of 1:2,500 and is 
twice the quoted value of the accuracy of the 
tablet digitizer (0.1mm), which is equivalent 
to +0.25m at this particular map scale. 
Turning next to the height errors, the r.m.s.e. 
value (mz) is *1.02m for a flying height of 
840m which is equivalent to +1.21 per mil of the 
flying height (H). This figure seems surprisingly 
486