128 GRAPHICAL OR NUMERICAL PHOTOGRAMMETRY?, V D WEELE 
For these purposes a pre-reconnaissance of the terrain will be necessary. 
The numerical data can be obtained by reading off the appropriate dials of the in- 
strument, followed by reduction and calculation. An automatic registration has the ad- 
vantage of avoiding reading (and writing) mistakes but a full use of the technical devel- 
opments is made only if additional devices are used to obtain the results in the form of 
punched cards, punched tape, or magnetic tape. 
These can, if the corresponding programs are available, be fed in to electronic cal- 
culating machines which perform the necessary computations in a very short time. 
The procedure will give an optimum economy if the data-reduction by computer is 
carried-on to deliver all elements that may be of future use, such as: areas, distances 
etc. etc. 
A next step is to integrate the procedure in the cadastral administration for which 
several possibilities exist. 
It should be kept in mind that, although the numerical procedure is applied, a map 
will nearly always be required for purely practical purposes. This map, however, will 
then serve only as a graphical representation of the numerical data. 
Another point is the need to keep the numerical cadastre up to date. It seems as if 
this problem could be solved in the classical way. Numerical cadastral administrations 
existed long before photogrammetry developed into an accepted surveying method. The 
terrestrial measurements are automatically obtained in some digital numerical form so 
that they can easily be connected to the existing system if they are based on control-points 
of which photogrammetric coordinates have been previously determined. 
A secondary consideration for the choice between the graphical and numerical method 
might be that the material on which maps are produced (or reproduced) is not always 
completely stable, whereas the numerical data do not change their accuracy with time. 
This section should not be closed without mention of the possibility of using the air 
photographs themselves (or enlargements of rectifications of them) as a base for cadastral 
registration. 
This procedure, which is comparable to a graphical method, may have great advan- 
tages in countries where great areas, without undue complications, have to be treated for 
cadastral purposes in a short time. 
111. 3. 
A combination of numerical and graphical photogrammetry may be useful in some 
cases where the photogrammetric procedure is mainly used to determine coordinates of 
a number of pre-signalised points which are destined to serve as control for setting out 
procedures, for terrestrial measurements, or for the absolute orientation of models in a 
later stage. 
These cases occur e.g. in reallotment procedures where photogrammetry may be used 
in one or more of the following phases. 
1. Before the work starts to provide a good base map. 
2. After the setting out or construction of the main features of the new lay-out (land- 
and waterways) to serve as a base for the determination of the positions and sizes of 
the new lots. 
3. After the completion of the work to provide the new cadastral map. 
Especially in phases 1 and 2, a numerical procedure may be indicated. 
IV. Technical purposes. 
IV.a. 
In this case photogrammetry is used to provide maps and other data for the planning 
and execution of technical constructions such as roads, channels, bridges, dams etc.