i8i
industrial purposes. Besides these instruments this company makes
another machine with a drawing area of 65 x 150 cm, which is
more suitable for profile drawing etc. This machine, the model
6515, does not need to be installed on the level. Although the speed
does not exceed that of the former model, they give a drawing
accuracy of only 0.2 mm.
The firm of Zuse (Germany) manufactures, in co-operation with
Hagen Systems (Holland) the Graphomat G4 electronic draughting
table. The drawing area is 120 x 140 cm. Drawing speed: about
11 mm/sec. The maker gives a drawing accuracy of about 0.15 mm.
If one wishes to purchase either an electronic drawing table or
an electronic co-ordinatograph, then not only the size of the
drawing area, the drawing speed and drawing accuracy play a role.
Other considerations that might be as important in certain cases
are for exampleIs the instrument capable of drawing straight lines
and curves, as well as point pricking What type of input-data is
needed? What control unit? What peripheral equipment can
be used? By what ancillary means (pen, ballpoint, engraving
stylus, light beam) can draughting be done For all these aspects
in their most varied combinations, there are more than twenty
firms offering a far greater number of electronic draughting tables
and co-ordinatographs more or less suitable for use in geodesy.
One might wonder how it can be commercially attractive to
develop so many instrument types in a price range comparable
with that of first order photogrammetric restitution instruments
and of stereo-comparators, since there surely exists a certain
amount of competition in this field. This has been possible because
only a small portion of the customers for co-ordinatographs and
drafting tables are from the geodetic sector. Surveyors have had
the opportunity of profiting from the demands of, among others,
the ship-building, automobile, aircraft and electronic industries.
Fortunately, many surveyors have done so. It is unlikely that an
electronic drawing table would have been developed exclusively
for geodetic work.
Section 5automation of area measurement
A confusion of terms often exists with regard to the accuracy
of area measurement. The ideas of "tolerance" and "error limit"
are more often put forward than the basic concept, namely the
standard deviation of the determined quantity, which naturally
depends on the method used for area measurement. If such a
standard deviation is too large in comparison with the value of the
lot of land, it only means that a more accurate method (i.e. instru
ment) should be used. So, in principle, any area (even that of the
most expensive property) could be measured graphically (i.e.
mechanically), if the scale of map and line thickness on the map
are correctly chosen.
