(b) Concerning composite sampling: 
As far as we can judge at present, CS allows efficient aquisi- 
tion of comprehensive and precise DEM data. An example of the 
point pattern obtained by CS is shown in fig.2. It refers to one 
‚of the test areas of ISPRS working group III/3. 
SS has a substantial impact on the time efficiency of CS and the 
comprehensiveness of the DEM data. Knowledge about the proper-: 
ties of subsequent PS is a prerequisite for appropriately col- 
lecting S-data. SS challenges the operator. PS is a relatively 
monotonous operation, but also from the human engineering point 
of view it is superior to complete grid sampling. A fully manual 
Z-control in PS was preferred by some operators to approximate Z 
positioning by the machine and manual differential control. As 
can be expected, imprecise measuring (e.g., in forest areas, as 
in lower right region of fig.2) leads to undesired densifica- 
tion. 
In the present program version of PS, gridded S-data are inserted 
into the patch matrix. Occupied grid points are not available for 
measuring. This can cause deficiencies in PS, which depend on ter- 
rain slope and finest grid spacing. An alternative incorporation of 
the gridded S-data is still being investigated. Another point of 
further study is the refinement of the criteria for data analysis 
and synthesis. For obvious reasons, error detection should be reali- 
zed on-line during sampling. 
IMPROVEMENTS AND ALTERNATIVES 
Controller and computer 
The prototype system was developed before microprocessors were on 
the market. A sytem with the same capabilities could now be designed 
with a microprocessor. The micro could be used in two ways: first as 
a recorder, transmitter, receiver and buffer device and second for 
the functions mentioned and additional tasks such as data analysis 
and synthesis, data conversion, etc. The host computer PDP 11/45 
would become obsolete. The main advantages would be stand-alone, 
high flexibility because of software control of many functions, and 
no long distances for data transmission. A floppy disk unit would be 
sufficient for intermediate data storage. 
Analogue stereoinstrument and motors 
The Planimat has the disadvantage that the moving parts are heavy. 
Hence powerful motors are required and tracking speed must be rela- 
tively low. The X,Y tracking speed,which substantially influences 
the time efficiency of PS, can be increased when using an instrument 
with light carriages and a small model space. Moreover, fitting DC 
motors instead of step motors should be considered. Analytical 
plotters are most suitable for CS. 
Terminal 
A graphic display terminal (e.g., VT125) would allow immediate in- 
spection of data and make operation more interesting and convenient 
(graphics, menu facilities). All hardware components needed for 
adapting an analogue stereoplotter for CS can be bought today. 
258