10 
The approaches with inherent data rejection are applicable to samples with redundant 
points. Examples are digitised profiles, contour lines, multiple heights generated by 
electronic correlators,etc. From the original set only data pertinent for further pro 
cessing are extracted according to appropriate selection rules. The main shortcomings 
of the approaches are: 
- the sampled data are usually less accurate due to continuous (dynamic) operation; 
- the point density differs substantially along the lines traced and perpendicular to them 
Sampling is carried out in a single run. These approaches are objective. 
The progressive approaches implement two or more successive sampling runs, starting 
with coarse sampling and proceeding gradually towards finer sampling. Before initia 
ting each next sampling run, the data of the preceding runs are analysed and a list of 
further points to be sampled is set up. The progressive approaches are objective. 
' ; . - , s -... 
The basic approaches to sampling, outlined above, may also occur in different combi 
nations. 
The sampled points can be arranged in different patterns . These may be: 
1. Regular sequences or grids (i. e. coherent samples) which are convenient for 
automation; 
2. Irregular sequences or grids of characteristic points. Procurement and processing 
of such data is difficult to automate; 
3. Regular sequences or grids supplemented by morphologic lines and/or points. Pro 
curement and handling of the supplementary data is difficult to automate; 
4. Sets of regular sequences or grids having adapted densities (i .e. heterogenous 
square grids). The allotment of densities to different terrain areas can be subjec 
tive (by interpretation) or objective (by instrumental sampling and computer analy 
sis). 
The patterns listed under 4. may be supplemented by morphologic lines and/or points. 
Such D, T. M. representation appears to be very comprehensive. Their processing 
can to a great extent be automated. 
Amopg the different sampling approaches listed above, progressive sampling of 
heterogenous square grids [ 10 ] seems to be the most feasible. This requires, 
however, an on-line mini-computer for data analysis, for control of recording, and 
for locating the instrument tracking device (X, Y, Z). The principal merit of the pro 
gressive sampling method is the matched density of a locally regular point grid to 
variability in the terrain relief. This is achieved by alternative sampling and analysis