11 
of the data In several runs. After sampling of the heights of a coarse homogenous 
grid is accomplished, the data are analysed by the computer. As a result, locations 
of the points to be sampled by the next run are generated. These points are arranged 
in an incomplete grid, which has the double density (in each direction) of the initial 
coarse grid. Then the computer locates the tracking device of the instrument ac 
cordingly to perform a new sampling run. This procedure is repeated for all suc 
cessive higher order sampling runs, which results in a local densification of the grid 
until a specified accuracy is met. 
Acceptable criteria are essential to progressive sampling in data analysis. These 
criteria are decisive both for the selection of new points and the rejection of gross 
errors. For both purposes the second differences of heights between the adjacent 
grid points can be used. 
The necessary storage capacity of an on-line computer can be reduced substantially 
if sampling is carried out in small patches successively. After completing the sampling 
of each patch, data can be transferred to a mass-storage. 
In the computer's main storage only boundary data of patches to be sampled later, 
need be kept, A well designed sampling and analysis technique provides for smooth 
transition across the boundaries between adjacent patches. 
The equipment suitable for progressive sampling may be: 
-a conventional instrument supplemented by a mini-computer, recording device, and 
interfaces for both directions; 
^hybrid systems; 
-fully digital type systems. 
The recording of patch data on a magnetic tape seems most appropriate. The pro 
gressive approach reduces sampling effort, it increases the speed, and it may imply 
an pn-line detection of gross errors. Moreover, it optimises the relationship between 
a specified accuracy, the sampling density, and the terrain characteristics. The 
D. T. M, data can be used for a wide spectrum of applications. Our further consider- 
. ations will be limited to purely photogramme trie operations: automatic orthophoto 
printing and digital mono-plotting. 
IV. AUTOMATIC OFF-LINE ORTHOPHOTO PRINTERS 
Computer-controlled orthophoto equipment may employ either optical or elctronic 
printers [11] .In either case a computer can locate the tracking devices of the 
input photograph and the printer. In optical versions it may control in addition 
optical units (e. g, a zoom-system and a Dove prism) which transform perspectively 
distorted images into ortho-correct ones. This transformation is, however, only a