COMMONWEALTH SURVEY OFFICERS CONFERENCE 1971
APPLICATION OF AIRBORNE PROFILE RECORDING TO 1:100,000 SCALE MAPPING
In 1965, the Australian Commonwealth Government initiated a programme of 1:100,000 scale topographic mapping of continental Australia and Tasmania, with a then projected time frame of 10 years for completion. Although this programme has recently been extended to 12 years due to economic pressures, it is an ambitious programme for a large country with a relatively small population.
This programme overlapped the completion of a national 1:250,000 scale map coverage which was completed in 1967. Maps at this scale were all compiled from vertical aerial photography, but the great majority were planimetric editions with the hilly country portrayed by hill shading techniques. A few were contoured at 250 feet intervals and even some of these were based on approximate methods.
In retrospect, for the 1:100,000 programme, we were virtually starting from scratch in the accurate depiction of contours, as less than 1% of the country had been contoured to standards meeting the specifications for map accuracy specified for the 1:100,000 map series, which inter alia, require a standard contour interval of 20 metres. In some mountainous areas, this requirement has perforce been extended to 40 metre intervals.
Terrain profile recording is being used extensively by the Division of National Mapping of the Department of National Development for the production of individual vertical model control for its share of the programme, which is approximately 2000 map sheets, and involving about 65,000 models of super-wide angle photography at approximately 1:80,000 scale.
WHY TERRAIN PROFILING RECORDING
The reader may well ask "why terrain profile recording?", when more favoured and perhaps conventional methods were in wide use.
The reasons lie fundamentally in the basic compilation technique originally adopted after an extensive testing programme some time ago. These tests showed that horizontal accuracy to meet accuracy specifications could be obtained with what we call "precise templates". This is a carefully quality controlled application of the slotted template principle, using mainly radial templates with nadir templates, stereo templates and azimuth templates introduced as circumstances dictate. The principle virtues of this method are :-
. it is economical
. it meets accuracy requirements for 1:100,000 mapping
. it offers mass production possibilities
. it does not need large scale involvement with computers
. it is within the competence of a large number of people.
The Division is gradually acquiring experience in analytical block adjustment techniques, and some use has recently been made of this technique. It will gradually be infused into the programme as confidence is gained in the "sure-fire" solution of very large simultaneous adjustments, which offer very real economies in the density of survey control. This is a separate subject and will not be pursued here.
The adoption of the slotted template was made with the knowledge that the third dimension could be obtained by airborne terrain profiling techniques to provide the height control for individual photogrammetric models, generally without recourse to any other form of photogrammetric activity. Other techniques were available such as helicopter-borne barometric heighting, but as these relied on a homogeneous levelling network for stabilization, and these were not initially available over wide areas, it was decided to stay with the APR technique. Other factors swayed this decision, the most important being speed of operation, a small field party effort and a preference for the productivity of airborne surveying in remote areas. It should here be noted that the Division has never had more than 100 personnel available for all the topographic surveying and photogrammetric (map compilation) activities needed in this programme, and this has engendered a predeliction for airborne surveying techniques; perhaps resulting from an innate taste for remote sensing from a reasonable distance above the earth's crust!!
To be fair, it should also be stated that at present, about $0.9M per annum is expended on goods and services in support of all the activities that contribute to the ultimate map compilations.
In initiating terrain profiling, a radar-type equipment developed by Canadian Applied Research Ltd, the CARL Mark V, was operated on contract to the Division by a private contractor. It is still being used, but is now being supplemented by a laser profiling equipment operated by the Division.
Radar type equipment can only be operated in a rather large aeroplane because of the physical size of the antenna reflector, which is 44 ins. in diameter. The Australian equipment is operated in a converted Lochheed Hudson ex-bomber aircraft, where the antenna is housed in the bomb bay section covered by a fibreglass screen. This does not detract from the aerodynamic performance of the aircraft, as it would if fastened externally to the hull of an aircraft.
The Canadian equipment is essentially an accurate radar altimeter operating in the 3 cm band, which measures the separation between the aircraft and the ground below, or the terrain clearances. Additionally, an hypsometer, locked to the initial flight altitude, measures deviations of the aircraft from this pressure altitude, which represents the plane of the isobaric surface.
In combination, these results provide the relative terrain profile recorded on a paper chart by the "red" pen, and the measured terrain clearance is shown by the "blue" pen. Maintenance of pen sensitivity is most important.
To relate the profile to the mapping photography, a 35 mm frame camera is mounted in, and aligned with, the antenna reflector. Each camera exposure is event recorded on the profile chart by a fiducial mark. Correlation is maintained by exposure numbers recorded on the film in the camera, and close clerical attention by the equipment operator.
The radar system is pulsed at a rate of about 2000 pulses per second, and the echo delay time is referred to a quartz crystal-controlled oscillator for maintaining precision of measurement. The terrain profile drawn on the chart is the result of a sampling of statistically derived groups of measurements, which tends to smooth out excessive "noise" in the profile, and provide a better and more usable trace.
The size of the antenna and reflector in this system provides a 1° cone of the transmitted signals at the point of emission, and herein lies one of the disadvantages of the system, as at 3000 m operating altitude, the signals are sampling a circular area of some 50 m in diameter. To obtain better "pointing accuracy" a much larger transmitter reflector would be required and this is not considered practicable.
So in areas of steep slopes and fairly dense and tall vegetation cover, is not possible to obtain the degree of precision necessary for 1:100,000 mapping.
Where this occurs, it is necessary to supplement the usable portions of profiles by photogrammetric height bridging of the profile gaps.
Altogether, the necessary profiles have been completed for map sheets covering some 2,000,000 km2 of Australia and an equal, if not larger area remains. With the limitations of radar type equipment in mind, and the need for increased production of this type of work, the Division of National Mapping initiated a project for the design and development of an operational laser terrain profiling system, through the Weapons Research Establishment of the Australian Department of Supply.
The emphasis on the design and operation of the equipment was placed on the production of a "surveying system" suitable for operation in a light twin-engined aircraft. After examination of existing systems, it was felt that care must be exercised to reduce support staff in the field to a minimum, and orient the whole operation to a surveying rather than an engineering operation.
Some of the criteria specified in the conceptual initial stages were :-
. power and weight restrictions compatible with the capabilities of light twin engined aircraft
. compact, convenient layout with controls grouped to permit 2 man equipment operation
. operating altitudes at 2000 - 3000 m to escape atmosphere turbulence, and remain in "no-oxygen" altitudes
. ease of operation and maintenance in the field by a party primarily constituted for field surveying and supported by expertise in electronic maintenance; this implied freedom from frequent return to headquarters to attend to unserviceabilities.
. inclusion of a strip camera; fool-proof correlation of profile data with positioning photography; a sampling rate which provided for practical purposes, continuity of profile; improved barometric reference unit; "bore sighting" of laser optics with positioning camera and drift sight
. safe operating parameters.
The officers of the scientific service not only accepted the challenge, but brought forward many innovations in design and concept which has resulted in the production and subsequent operation by the Division, of a sound, practical surveying system. It is being operated in a Grand Commander aircraft.
More details of the equipment are giver in references (1) and (2).
The 70 mm strip camera and the ultra-violet 7 channel chart recorder output, with the timing code superimposed on both records for correlation of data. The modulation frequencies are so arranged that full scale deflection on the chart is 50 m, with coarse height indicated in multiples of 300 metres with the coarse height read-out, ambiguity of profile heights is avoided, provided at least one height along the profile is known to better than 150 m.
The chart also illustrates the sensitivity of the equipment and the relatively low level of signal / noise ratio. One of the main advantages of the laser equipment is the narrow pencil beam of the transmission, which has an angular field 10-4radians. This means, in fact, an illuminated spot on the ground of 30 cm from an altitude of 2000 m. At a transmission rate of 50 c/s, this provides ample profile data for practical purposes.
This small spot also gives very good vegetation penetration as it needs only quite small gaps to penetrate timber to provide reliable ground heights. With radar APR, one never quite knows where the reflected signal comes from in heavy vegetation, but you may be sure it is not the ground.
The strip camera incorporates automatic exposure control, and a dichroic mirror which takes off a portion of the light bundle which is imaged on a screen and monitored by the operator through a rotating disc engraved with a spiral. This disc is driven by the film transport motor and can be regulated to synchronize film speed with aircraft ground speed and ensure equal longitudinal and transverse film scales. Drift angle settings can also be viewed and compensated by the operator.
The laser used is an argon ion gas laser and has operated successfully close to 4000 m. Operations at this height above terrain are not necessary for the mode in which it is being used, particularly when it is realized that the transmission power required varies approximately as the square of the altitude.
With the addition of a recently completed new type barometer reference unit based on a transducer, a gyro-controlled drift meter for navigation, an aircraft roll indicator and an automatic pilot, the system is highly productive in line miles per week (average about 1000).
Fairey Australasia Pty Ltd, Elizabeth, South Australia, have recently been licensed to manufacture and market the Mark 1 equipment.
PLANNING TERRAIN PROFILES
All the new compilation work in the 1:100,000 programme is being prepared from super-wide angle photography taken from an altitude of 25,000 feet (about 7500 m), giving an empirical photo scale of 1:84,000. The unit for aerial photography is the standard 1:250,000 map sheet and for some time now, this photography has conformed to standard flight line formats where east-west flight strips are made at specified geographic parallels with a permissible deviation of 800 m. The most common standard flight line format contains 8 flight strips, but this is increased in mountainous areas.
This system not only builds up a uniform pattern of photography over large areas, but enables proper planning of control survey operations and terrain profiling to take place whether the super-wide angle photography is available or not, but it is always more convenient to have recent photography.
Terrain profiling takes place along the centre of the common side laps of the super-wide angle photography, and again with a permissible deviation of 800 m from the planned flight path. Additionally, north/south tie lines are flown at 30 minute intervals of longitude, but may be varied to suit the position of existing control points.
Normally, horizontal and vertical control points can be expected close to the corners of 1 degree quadrilaterals over most of Australia, and the tie lines are generally flown to pick these up.
Each mission is designed to commence at operating altitude over an initial datum surface (usually an aerodrome) and similarly terminate. This practice is primarily to enable field checks to be made for unacceptable results.
Now that the National Levelling Survey has come to fruition, increasing use is being made of height checks obtained over levelling bench marks, and this pre-supposes they have previously been identified on aerial photography.
With more effort in this direction, it will enable a more rigid control of uncertainties of corrections calculated for the gradient of the isobaric surface. There are not many areas now which are situated more than 80 km from a bench mark on the National Levelling Survey.
In the central portions of Australia in the southern winter months, reference to the meteorological synoptic charts will show there is a very shallow isobaric gradient indeed over very large areas. In these areas, and in these months, there is not such a pressing need for frequent height checks on profiles. However, in the more atmospherically turbulent areas closer to the coast, these checks assume much more importance.
Experienced barometric heighting observers will know only too well the vagaries of not only the diurnal conditions, but the influences of local micro-climates in areas reasonably close to the coast.
Work flow has been developed from the procedures used in acceptance and subsequent processing of contract radar-type APR. In this type of data acquisition, there is a contractual requirement to formally accept or reject the delivered navigation reports, profile charts and positioning photography etc. within a specified time.
As field work is generally planned and executed well before the office requirement, both for reasons of orderly supply of material and the restrictions of suitable climatic conditions for successful APR work, a thorough processing of the material to produce reduced height values for specific photo points is not possible in the permissible period available before payment is due. In these conditions the acceptance checks are confined mainly to :-
. computation of the Henry correction and misclosure of closed profile traverses between datum surfaces
. examination of profile records for obvious anomalies in the red and blue pen records
. "stickiness" or lack of sensitivity in red pen particularly
. examination of positioning photography for quality and correlation with profile data
. some check levelling in photogrammetric plotters at "cross‑over" profiles to verify derived profile reduced heights.
With the laser profiling equipment data, much of this checking can be avoided due to the increased sensitivity and internal profile accuracy; the non-mechanical drawing of the profile; the automatic exposure control in the strip camera and the superior correlation technique between the various records.
In the office procedures for the isolation of specific heights for individual model control, a much more intensive procedure is adopted. In essence, firstly the cross-over profiles are compared to determine datum differences. This is achieved by transferring some profile heights obtained only from red pen readings to a photogrammetric model from two adjacent E-W profiles and the N-S tie line, setting up the model in a stereo-plotter and recording the discrepancies. These are generally of the order of 0-5 m when the previously deduced datum values for the red pen readings are error-free. In the event of non-agreement of profile heights within a profile as disclosed in the stereo-model, short sections of the APR profile as reproduced as a machine plot and compared with the recorded profile to resolve differences. Where vertical control points established by ground survey are available within these cross-over checks, datum corrections are made to the various profile lines.
Secondly, other photogrammetric models are similarly set up for comparing two adjacent E-W profiles with vertical control points, which are fairly widely scattered, and provide a density of 1-2 per 1:100,000 map sheet.
From a synthesis of all this recorded data, priority is given to establishing corrections to the N-S tie lines which are weighted towards the available ground height data. From this data, correction graphs are prepared for the red pen readings of the E-W profiles, between tie lines and any intervening ground control points.
This procedure is somewhat expensive in labour requirements, but it makes best use of available ground control surveys and has evolved as the method giving the highest level of confidence in the accuracy of model control values. More importantly it confines the unknown variables in the plane of the isobaric surface to relatively short sections of profile.
Experience has shown that the average profile height accuracy when processed in this manner is ±3 m with an upper limit of ± 5 m. Due to the uncertainties of the isobaric surface, it is not expected that the laser equipment will provide any more precision in deducing model control heights, but many of the operations included in the processing of data will be handled with more despatch.
In using this form of vertical model control over some 3500 – 4000 models, the residuals in the model corners after orientation are of the order of 1-2 m. Occasional large reported errors have been entirely due to transcription errors.
Field survey accuracy tests have so far indicated that contours derived in this way meet the required accuracy specification for 20 in contouring, ie. an average error not greater than ±5 m and a 90% confidence limit of ±10 m.
ADDITIONAL COMMENT ON LASER PROFILER
There exists the potential for acquisition of data for other purposes, particularly for preliminary engineering investigations and for some forestry applications. Already one project has been completed for investigation of a long new railway proposal, and foresters are interested in relation to tree height determination.
Terrain profiling for the provision of heights for mapping where the standard contour interval is 20 metres has proved successful in Australia, particularly in the inland areas. Radar-type equipment has decided limitations in rugged, heavily timbered country. The laser- type equipment has better potential to handle steep gradients and has much superior timber penetration. However, both equipments require a denser control pattern when working in the adverse influences of coastal areas. Terrain profiling seems more suited to areas where the climate is temperate to cold. In tropical areas it is suspected terrain profiling would not provide sufficient accuracy for 20 metre contouring without a great deal of control.
(1) M.P. Penny, Technical Note OSD 116, Australian Defence Scientific Service, Department of Supply.
(2) B.P. Lambert - The Australian Laser Terrain Profiler - Experience in support of the 1:100,000 mapping programme - To be presented at F.I.G. Congress, Wiesbaden 1971.