Historical information
This is a set of 16 photograph of the Royal Australian Survey Corps’ Johnson Ground Elevation Meter (JGEM) Survey Vehicle taken at the Army Survey Regiment, Fortuna, Bendigo. The JGEM vehicle was extensively used by RA Svy within Australia from the late 1960s. A limited number of Ground Elevation Meter (GEM) station wagon type vehicles were manufactured by General Motors Corporation (GMC) in the USA for the United States Geological Survey, Canada’s mapping agencies, RA Svy and National Mapping (Natmap). The GEM was a four-wheel drive, four-wheel steer vehicle. Four-wheel steering was necessary to avoid systematic errors caused by non-tracking of front and rear wheels on conventionally steered vehicles. The manufacturer substituted the rear axle with a front axle and connected them to form the four-wheel steering mechanism. The two Australian GEM vehicles, referred to as Johnson GEMs (JGEMs) were converted into right-hand drive. After delivery in 1964, acceptance Natmap and RA Svy testing and operator training was undertaken at the Army's School of Military Survey located at Balcombe, Victoria. A small fifth wheel was mounted on a cantilever arm suspension midway between the front and rear wheels on the right side of the vehicle. It was lowered to and raised from its operating position by use of a constant pressure air cylinder. A telescopic bar, suspended between the front and rear axles, provided the reference datum for the angle measurement. The wheel provided the velocity or distance signal through a pulse generator system. A sensitive pendulum mounted on this bar provided the angle measurement for each minute distance traversed. The JGEM contained electromechanical instruments used to determine relative elevations, by trigonometric principles, along a traversed path. These relative elevations were obtained through apparatus which measures the instantaneous angle of inclination of the road and the instantaneous velocity of the meter along such a path. Road routes over which the JGEM operated were planned so that each started and ended as near as practicable to an existing point of known elevation (formally referred to as a level traverse bench mark). The difference in height from the bench mark and the road surface alongside the JGEM’s fifth wheel was measured with a level and staff. Along each route, mapping control photo reference points where new elevation values were required were identified on aerial photographs. Under favourable conditions it was possible to survey as much as 160km in an ordinary working day. The first of RA Svy’s JGEM operations was undertaken in 1:250,000 scale map areas of Queensland. CPL John Hook was the JGEM’s main operator in the early 1970s undertaking operations covering 1:250,000 scale map blocks over northern Victoria and central NSW, each requiring 36 points (9 runs of photography and 4 points across. SPR Lyn Thompson and SPR Bob McDonagh teamed with CPL Hook on some of these JGEM operations. When RA Svy was integrated into the Royal Australian Engineers in 1996, the JGEM vehicle with the Survey Corps collection was donated to its museum. It is believed to be the last of the original manufactured fleet in existence. The JGEM has undergone extensive refurbishment to achieve roadworthiness and is currently housed at The Australian Army Museum of Military Engineering, Hoslworthy Barracks, NSW. It can be viewed by making an appointment with the museum’s curator.
Physical description
This is a set of 16 photograph of the Royal Australian Survey Corps’ Johnson Ground Elevation Meter (JGEM) Survey Vehicle taken at the Army Survey Regiment, Fortuna, Bendigo. The photographs were on 35mm slide film and were scanned at 96 dpi. They are part of the Army Survey Regiment’s Collection.
.1) - Photo, colour, c1960s, Johnson Ground Elevation Meter (JGEM) Survey Vehicle
.2) - Photo, colour, c1960s, JGEM instrumentation, on-board computer.
.3) - Photo, colour, c1960s, JGEM instrumentation.
.4) - Photo, colour, c1960s, JGEM instrumentation, on-board computer.
.5) - Photo, colour, c1960s, JGEM tyre pressure controller
.6) - Photo, colour, c1960s, JGEM rear doors, SGT Geoff Briggs.
.7) - Photo, colour, c1960s, JGEM 5th wheel distance/angle measurement device in lowered position, SGT Geoff Briggs.
.8) - Photo, colour, c1960s, JGEM 5th wheel distance/angle measurement device in lowered position.
.9) & .10) - Photo, colour, c1960s, JGEM tyre pressure system, SGT Geoff Briggs.
.11) - Photo, colour, c1960s, JGEM tyre pressure system. SGT Geoff Briggs.
.12) - Photo, colour, c1960s, JGEM levelling scope, levelling staff, unidentified technicians.
.13) & .14) - Photo, colour, c1960s, JGEM levelling scope, unidentified technician.
.15) & .16) - Photo, colour, c1960s, probably survey operation adjusted height plotted on block base sheet.
Inscriptions & markings
.1P to .16P - Some of the equipment is annotated on the frame of the 35mm slides.
