network with equal distances between the points in x and y directions has 
proved to be the best and most preferable. 
The operation of the floating mark in y-direction and x-steps can be done — if 
no better facility is available — by hand but should be done much better by 
application of special registration and measuring units which are now available 
for instance for the Zeiss - and Wild-Instruments. It is very important to 
measure each point's z with the highest precision. Only this leads to the best 
possible approximation of the surface's roughness and to a determination also 
of very small differences in height. 
Using an incremental-registration unit a half automatic movement of the float 
ing mark has proved to be the best kind of movement. In this case the floating 
mark runs from one point to the next by an order of the operator and stops 
there. In this way the operator can measure without any time limit and trouble 
He also can rest if it is necessary or convenient for him and of course he can 
repeat a measurement if he feels he was wrong. 
The first digital surface models of forest stands have been measured with a 
Kern PG 2. In this preparatory phase we still have used hand operation. The 
operator mapped the boundaries of the stands which should be measured. Then 
he overlaid a square-net on this map and orientated it so that the lines of 
the grid became parallels to the x and y coordinate-axes of the stereomodel. 
After this the operator put the drawing pencil one after another on each point 
of the grid. By this the floating mark within the stereomodel has been located 
for each measurement to the correct x and y position. On each point the oper 
ator brought the floating mark with highest precision to the surface and there 
by he measured "z". Of course, using this handmade method one only can set up 
a relatively coarse network. We mention this hard and provisional method, 
because incremental registration and measuring units still often are not avail 
able. 
Operational use and more precise measurement require incremental registration 
instruments. We have used the following instruments-set from Zeiss-Oberkochen 
(Schwebel 1971): 
Stereoplotter: Planimat D 2 
Ecomat 11 
Incremental-Registration Unit 1 
Measuring Unit DTM 1 
paper-tape printer Facit 4070 
aut. typewriter IBM 073 
Computer: Univac 
Aerial photographs with scales between 1:5000 and 1:8000 (Model scale 1:5000) 
have been used to measure the detailed surface roughness of various forest 
stands.