131 Bty Reports, Survey/Sound Ranging
August 68

17. No Sound Ranging Equipment has arrived in country.
18. During Op PLATYPUS, common grid survey was carried out to the FSPBs. Because of the nature of the terrain, aerial survey was the method used with an accuracy of 50 metres being obtained. A number of factors contributed to this low order of accuracy:
a. The base established for the aerial intersection had a total length of approx 5000 metres and as the furtherest FSPB was approx 15,000 metres distant from the base, ill conditioned triangles resulted.
b. Screening close to the base forced the helicopter to hover between 1000 – 2500 feet over the different FSPBs before it could be observed from all the stations. Additional runs were made to counter the inaccuracies of hovering over a given point from such a height.
c. Climatic conditions were poor, squalls and visibility hampered observations.
19. This survey was carried out whilst the occupation of the FSPB was in progress and data was available with minimum delay. During this operation, map reading was difficult and individual map spots differed by up to 1500 metres, due in part to inaccurate marking of fire trails on maps and lack of detail on the maps of recent land clearing operations. Although the results achieved were not normal Artillery Survey Standards, they provided the Artillery Commander with an acceptable answer under the circumstances. The base stations were established on theatre grid from a control point recovered by the section near an ARVN outpost.
20. This control point served as the centre station of the base with the ARVN forces providing the necessary protection for the survey party. The other bases had to be sited in insecure areas where considerable enemy activity had been recently reported. One of the survey parties were given protection by a troop of tanks from FSPB AVENGER whilst the other party were escorted by armoured ammunition carriers and a platoon of Infantry. Base stations were established at : 745570 1184560 / 745302 1183913 / 746403 1181026.
21. Further problems were added to the scheme by the head high grass covering the area. To achieve visibility between stations, platforms of logs had to be erected. A further hazard to survey was discovered when the survey vehicle collapsed the entrance to a VC tunnel system within the FSPBs.
22. A separate scheme to fix the calibration point for charge 3 (YS411 697) was attempted using aerial survey, but as the are was not secured from directed small arms fire, the helicopter was forced to hover to high for calibration accuracy. A strong wind blowing at the time caused the helicopter to drift, making it impossible for the observer to sight their instruments. This was aggravated by the relatively short distance from each station to the point. The scheme was then altered and the provision of a protection party made available to enable us to occupy a nearby feature and carry survey into the area by normal bearing and distance methods. This method was not planned at first because of the problems associated with protection parties and helicopter movement onto the feature.
23. The section was also tasked to provide met corrections for the calibrations of 12Fd Regt guns. This was done by obtaining raw met from the US Met Section located at Nui Dat; then deducing calibration met corrections. It was necessary for our section to be tasked as the US Met Team are not trained to provide any data other than straight met gun data and are inflexible to a point.

Conclusion
Due to Operational requirements the Det Svy Offr has been carrying out full time shift duties in Arty Tac at the request of the CO Fd Regt.

Capt PM Perrin, OC 131 Div Loc Bty