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satisfactory, with the equipment to be supplied by some of the larger
countries and the cost of the personnel to be contributed by the
countries actually engaged in the project.
Section IILevelling
1. The I.A.G., considering that, apart from any hypothesis about
the form of the geoid, the use of the gravitational potential is essential
for the study of several geodetic questions, notably for the adjustment
of extensive international levelling networks, and for the intercon
nection of Tidal Datums, considering that, in theory, the calculation of
the potential requires the knowledge and use of true values of gravity,
and moreover that measures of gravity are now easy and inexpensive,
but that in practise, in many circumstances, the use of a standard
theoreticalvalue of gravity gives a result which is amply accurate,
resolves
that the different countries should, at least for their principal
bench-marks, publish the gravitational potentials, and details of the
system by which they have been computed,
that studies be made, or continued, in order to distinguish between
situations where the true value of gravity is required in order to
determine the potential, and those where, some standard gravity for
mula may be used.
2. The I.A.G., considering that all the currently used methods of
converting gravitational potential, of dynamic heights, into "ortho-
metric" heights, give very similar values for the height, and especially
for the differences of height between nearby places, so that the dif
ferences are of no consequence in work of secondary accuracy
considering that any change in official or published results arising
from a change in the method of calculation, introduces a risk of
confusion, especially in the comparison of old and new levelling, made
for the study of vertical movements of the ground levelconsidering
that different systems for deducing approximate "orthometric" heights
from gravitational potential all take the form H (potential),
where gc is an estimated average value of g along the local vertical,
or an approximation to itexpresses the opinion that' each country is
free to choose its own method for calculating official altitudes, past,
present and future, which are intended for publication to the public
and that every country is similarly free to adopt any system it pre
fers for the estimation of gc.
3. The il.A.G., considering that there is great scientific value in
1studying the methods of determining the gravitational potential,
and
2) obtaining the most accurate and most convenient values of ortho-
metric altitudes,
resolves that researches be undertaken or continued
a) on the precision necessary in the values of gravity,
b) on the necessary spacing of the observed values of gravity,
