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rigorous, this "screw" may be turned to the bottom which means that 
the problem is unsolvable. Additional vertical controls must than be 
planned between the original ones; that is to break down the strip in 
shorter parts. The terrain must be crossed with a levelling-line or simi- 
lar and than, of course, many extra z-points could be determined with 
out extra costs, so that you get vertical controls in almost every pl 
model. 
[f the elevation accuracy specifications are not so rigorous, but still 
not of the type "reconnaissance map" and if the contour line interval is 
of the magnitude >—10 meters the modern highresolving lenses place 
you in a dilemma. You can get sufficant elevation accuracy in separa- 
tely horizontalised plotting models even if the photographs are taken 
from the maximum height of the airplane (e. g. 6—9 000 meters) and 
you get a tremendous area in each plotting model (netto 30—65 sqkm), 
If you, on the other hand plan to use real z-bridgings — and I do not 
call a "bridging" over one or two models a real z-bridging — the prob- 
able heighterrors in the triangulation may be an absolute obstacle for 
using the very economic high-altitude photography. 
Still without speaking about "reconnaissance mapping we must ob- 
serve 
1:0 that a net of terrestrial elevation fixes is needed for many activi- 
ties soon after or even before the mapping is concluded and 
2:0 that levelling of a sparse network of elevationtixes normally is 
not a very complicated or expensive task compared with the determina- 
tion of a geodetic terrestrial network sufficiant for scaling and orienta- 
tion of each plotting model. Of course there are also areas inaccessible 
for levelling e. g. in high-mountains, the tropics or the artics but than 
the accuracy specifications are not so rigorous and permit high altitude 
photography and photogrammetric bridging which could be substituted 
with radar profiling, barometric levelling or similar measurements. 
As a consequence of the foregoing it might be emphasized that — 
excluding for small scale mapping and reconnaisance mapping — photo- 
grammetric bridging for determination of z-controls in many cases 
could be and as a matter of fact are substituted with terrestrial levelling 
needed also for other purposes. If the maps shall be used for planning 
of irrigation, reservoirs or water power projects a terrestrial levelling 
of vertical passpoints 1s especially advisable. 
When discussing photogrammetric control point densening there is 
another problem of common interest — how the horizontal accuracy is 
to be defined. The classical definition, an inheritance from the geodesy, 
is the mean square coordinate-error in relation to geodetic points 0! 
higher order. So far as geodetic terrestrial surveys are concerned, this 
is quite natural and the only possible way for practical purposes. In 
photogrammetric triangulation and mapping and especially in block- 
otting