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Preparations for air photo survey.
Extension of
In October, 1924, a Colonial Survey Section consisting of 1 officer and 3 other ranks was sent out to do the necessary ground control for the air photographs. Previous to this the G.O.C.,Hong Kong was instructed to ask the civil authorities to cooperate by checking the P.W.D. triangulation and extending it and marking it in such a way that it would be visible from the air.
In September, 1924, a list of the
trigonometrical values of all the P.W.D. points in Hong Kong and the Leased Territories with a
triangulation diagram was obtained by the G.0.0. Hong Kong and forwarded to the War Office with an intimation that these were the best available data for the production of a map, and that triangulation errors varied between 2" and 9". These lists were merely coordinate lists in geographicals and rectangulars with no information as to the data on which they were based.
On the arrival of the Survey Section it was framework by the found that the P.W.D. were too much occupied withthett Colonial Survey otherwork to be able to give any assistance in checking or Seation.
extending the triangulation, but that they had marked some of the trigonometrical points in the field. It was also found that the triangulation points observed by King in the area of the 8" map had been lost and this triangulation was consequently of no value.
Discrepancies in
The Officer in charge of the Colonial Survey Section made efforts to get further information about the trigonometrical framework from the Survey Office but was told that no more information was available.
The Colonial Survey Section extended the P.W.D. triangulation to cover the whole Colony but their work was only of the order of accuracy required for the control of a topographical map on a scale of 1/20,000, and the points were only temporarily marked on the ground. It was arranged with the P.W.D. that they would subsequently erect permanent marks.
Some
On the return of the Colonial Survey Section the trigonometrical to England the compilation from the air photographs lists.
was started. It was found that there were certain discrepancies and deficiencies in the lists of trigonometrical points which had to be cleared up before the work of compilation could be started. of these had been already noted but it was considered advisable to ask again for further information on all points. Only a list of coordinates in geographicals and rectangulars was available. There was no information as to the method of computation, projection and figure of the Earth employed. In addition, the following points gave rise to special comment:-
(i)
Doubt as to the geographical values adopted for the origin.
(ii) Discrepancies between values given for
rectangular and geographical coordinates. These appear to be partly systematic and partly due to errors in copying.
(iii) False value for the zero at Victoria Peak.
Information/