GB2108347A

GB2108347A - Terrain surveying

Info

Publication number: GB2108347A
Application number: GB08224435A
Authority: GB
Inventors: Jurgen Schloss; Dietmar Stutzle
Original assignee: Diehl GmbH and Co
Current assignee: Diehl Verwaltungs Stiftung
Priority date: 1981-08-25
Filing date: 1982-08-25
Publication date: 1983-05-11
Also published as: FR2512197A1; FR2512197B1; DE3133570C2; DE3133570A1; GB2108347B

Abstract

Opto-electronic terrain surveying apparatus performs independently of the height (h) of a flying body 1 and at all times with the same digitisation resolution. From a detector output signal 6 during the covering in each case of a strip 7 of terrain a time span 27; 49 is singled out which fluctuates inversely proportionally to the instantaneous flying height (h), whilst the sequence frequency of control pulses 28 is varied proportionally to the height (h), so that irrespective of the instantaneous height (h) at all times an identical number of digitised image point signals 3 emerges in the case of terrain stops 7 of constant length, despite constant speed of rotation of a rotary mirror 9 carrying out the infra-red scanning. <IMAGE>

Description

SPECIFICATION A method of picking-up (receiving) and processing detector output signals in terrain surveying and apparatus for carrying out such a method This invention relates to a method of picking-up (receiving) and processing detector output signals in terrain surveying, for example, opto-electronically and more particularly, for example, using infra-red and to apparatus for carrying out such a method.

German Patent Specification No. 1798311 shows a multiplicity of detectors arranged on a flying body in the flight direction, and the speed of rotation of a rotary mirror is controlled by a height- and speed dependent signal in order to ensure that (despite high flying speed and also possible fluctuations in the speed and in the flying height) no gaps occur in the strips of terrain covered successively along the flight direction and extending on both sides transversely to the flight direction. In order to acquire this control signal, a particular terrain point, which is singular or unique with respect to the signal processing, is covered twice in succession by the detectors.

By way of an image evaluation, not indicated in more detail, it is ascertained in what direction this point shifts in the strip of terrain surveyed. This shift is, according to direction and size, a measure of a drive - too fast or too slow with respect to the instantaneous over-flight or overhead factors - of the rotary mirror for the optical terrain-strip scanning and is converted in a manner which is not shown into a correction signal for modifying the speed of rotation of the mirror.

It tends to be disadvantageous in such surveying that the speed of rotation of the rotary mirror is variable as a function of the insstantanous over flight or overhead factors which tends to lead to evaluation errors on account of the inertias of the electro-mechanical drive system, and which mirror tends also to be troublesome in operation. Furthermore, an image or picture comparison to ascertain the direction and amount of the shift of a particular point tends to be disadvantageous since it is not only complicated but also because an unequivocal comparative evaluation of this kind seems feasible only in the case of points on the terrain which are considerably pronounced relative to their surroundings.

According to the present invention there is provided a method of terrain or surface surveying comprising scanning the terrain or surface in a sequence of strips from a body moving relative to the terrain or surface, information from successive strips of the sequence being received by a detector and the detector producing an output signal indicative of the information received and in which, at a substantially constant scanning speed of the strips of terrain or surface received by the detector, only selected portions of the detector output signal are processed and digitized; respective selected portions decreasing in size with any increasing height differential between the body and the terrain or surface, and for said processing of the respective selected portions scanning or sequencing of the portions is effected with a scanning or sequence frequency which increases with increasing height differential.

Further according to the present invention there is provided a method of picking-up (receiving) and processing detector output signals, for example opto-electronically, and in particular, for example using infra-red, in terrain surveying in a moving (for example flying) - height dependent manner from a moving (for example flying) body by means of line-scanning, for example optical scanning, of respective ones of a sequence of strips of terrain, extending in their length transversely to the moving (for example flying) direction, and projecting or transmitting information therefrom onto a detector, in which, with a constant (for example optical) scanning speed of the strip of terrain from which information is projected or transmitted onto the detector, only a proportion -- shortening with increasing flying height of the detector output signal is quantised or processed and digitised, in which respect, for the quantising or processing, a scanning of the proportion is effected with a scanning frequency which rises proportionally with increasing height differential from the strip of terrain.

Still further according to the present invention there is provided apparatus for carrying out a method according to either one of the immediately preceding paragraphs.

Such apparatus may comprise a rotary mirror which is capable of optically scanning succesive strips of terrain along a flight or movement direction to project or transmit information from the strips onto a detector receiving surface in which a gate stage and a scan/hold or sample/hold circuit are connected subsequent to the detector, and in which a flight-data determination circuit is provided which (with the rotary mirror operated at a constant speed of rotation) by way of a gate-pulse length transmitter is capable of controlling the gate stage with gate controlling pulses, the length of which fluctuates inversely proportionally to the height differential, and which, by way of a scanning or sequencing-control-pulse sequence frequency transmitter, is capable of controlling the scan/hold or sample/hold circuit with scanning or sequencing control pulses, the pulse sequence frequency of which fluctuates proportionally with the flying height.

It is an advantage of the present invention that embodiments may provide a comparatively simple circuit arrangement which may be put together from standardised subassemblies of electronic information processing, and have a rotary mirror with a mode of operation which is less susceptible to trouble and which tends not to bring about addition ai processing errors. This may ensure that the flying body covers a series of strips of terrain over the terrain to be covered, which strips are at all times the same length and the picture or image information can be processed with constant resolution and can more particularly be digitised.

Thus embodiments may provide that interventions or corrections in the speed of rotation of the rotary mirror (which can tend to lead to a more complicated or expensive apparatus than necessary and which may tend to be susceptible to trouble) are disp@@@sed with and in order at all times to have constant strip lengths of the terrain covered in quanl::ifying, only a temporal (time and space) portion is signled or from the detector output signal in a menner which is dependent upon flying height, but in so doing at all times the same number of digitisation scanning points is distributed over the respective temporal portion just immediately signled out. With an increasing height and a constant aperture angle of the projection optical system, a wider strip of terrain is projected onto the detector in the course of the optical rotary-mirror scanning.

However, from the detector output signal which oc5uts with fluctuating amplitude over time, only a temporally shorter middle range is selected for furthe signal processing, which range or length corresponds to a specific terrain-strip length which extends symmetrically on both sides of the flight direction. Since, with a constantly predetermined scanning frequency for quantifying and digitisation of the lime-continuous detector output signal, in the case of tiliS shortened evaluation time span only correspondingly fewer instantaneous amplitude values would be covered or received.In other words the resolution of the quantified terrain information would fluctuate in a height-dependent manner but the scanning frequency is controlled in such a way that it incE--ases sikti3 increasing flying height in other words at all times the same number of instentaneous-value scannings fall into arbitranly abridged evaluation time-spans of the detector out pus signal.

Thus upon quantifying the detector output signal using electrical (electronic) measures the effect of a zoom o-Ljective (namely distance-dependent cover age of the same object region with constant resolu io can be achieved without the expenditure of providing an optical objective which is adjustable electro-mechanically and without accruing the aforementioned suseptibility for troublesome operation th er eo-F.

An 3aditl0nal low-pass filtering beyond the low pass behaviour of the detector may be necessary or desired in some embodiments for effective operation also in the event of severe fluctuations in height and accordingly severe fluctuations in scanning frequency.

In order to be able to cover or survey opto-electronically a contiguous sequence of strips of terrain in the flight direction in a height-independent manner and whilst achieving effective operation, and to be able to digitize this information for the further processing several detector elements are advantageously arranged - similarly as previously proposed in the publication explained at the beginning hereof-in the flight direction in the detector. More elements are connected together and to a low-pass member with decreasing flying height. Advantageously in each case, two detector elements situated symmetrically with respect to a central detector element are connected together securely, whereby a symmetrical variation in the pick-up characteristic as a function of the instantaneous flying height above ground can be realised.

To acquire the flying-height-dependent control variables to influence the interval of time over which the detector output sine is quantised (processed) with a constantiy-h'd number of instantaneous value scannings - as well as to switch between detector elements in the flight direction - recourse is advantageously made to a correlaticn evaluation.

In this respect a height-proportional signal emerges or results from cross-correlated sensor signals, which sensor signals are acquired with telescope optical systems orientated at different angles in the flight direction, weighted with a proportionality factor, into which the instantaneous flying speed over the terrain and the angle between the two telescope orientations is taken into account. The speed for its part emerges from the inverse value of the cross-correlation of two sensor output signals with telescope optical systems which are arranged with the same orientation (aligned) and mutually offset along the flight direction with a proportionality factor which is given by this mutual spacing.This signal processing is very much more reliable than the detection of image shift of specific but non-influencable points on the terrain, since the overflown terrain information enters or is processed in its entirety. In order to achieve a clear correlation result even in the case of small terrain irregularities it may be advantageous to connerst differentiating stages for the dynamic prominence or isolation of signal changes subsequent to the sensors.

The present invention may be realised for instance in radar scanning the terrain paths or in exploration of the bottom of the sea by means of towed probes which are equipped with appropriate sonar transmission and reception devie3s. In the case of the flying body mentioned at the beginning hereof the invention is not restricted to a projectile moving with its own drive or on a ballistic trajectory in the air space above the surface of the earth. Terrain includes any sort of land or surface feature and, for example, the bottom of the sea.

An embodiment of method and apparatus in accordance with the present invention will now be described, by way of example only, with reference to the accompanying simplified drawings in which: FIGURE 1 shows in schematic representation an optical system for terrain-strip scanning and a detector with subsequently-connected signal processing mounted in a flying body over flying a terrain path; FIGURE 2 shows an example of an output signal of the detector shown in Figure 1, in orderto explain the height-controlled dependence of a signal time span for height-independent length of a covered strip of terrain, but still not taking into account height-dependent constant digitisation resolution; and FIGURE 3 shows in block diagram a flight-data determination circuit in accordance with FIGURE 1 and the optical sensor system of the flying body.

The arrangement sketched in FIGURE 1 shows symbolically a flying body 1, which moves at an instantaneous height h and at an instantaneous speed vabove and along a terrain path 2. On board the flying body 1 a sequence of digital image point signals 3 corresponding to the peculiarities of the covered terrain path 2 is producible, for example by utilizing the two-dimensional distribution of terrain points 5 which radiate heat to a greater or lesser extent and which are covered or surveyed successively by means of an infra-red detector 4. Accordinglythis radiation is converted in a time-dependent manner into electrical detector output signals 6.

To this end, strips 7 of terrain are considered which extend transversely to the direction of the covered terrain path 2 and thus effectively transversely to the flight direction 8 of the flying body 1.

In fact the strips are inclined relative to the perpendicular line to the flight direction in accordance with the flight speed v but this is not shown in the drawings for reasons of clarity. The strips 7 are projected (or information therefrom is projected) in point or dashed-linewise manner by means of a segmented rotary mirror 9 and by a collecting optical system 10 onto a receiving or pick-up surface 11 of the detector 4, lying in the focal plane of system 10.

Provided in the beam path in front of the rotary mirror 9 there may be, in a manner known per se, a deflecting following mirror 12 which is controlled by, for example, a gyro-stabilised compensation circuit 13. Control of the mirror 12 by circuit 13 is for swivel motions of the mirror 12 about swivel axis 14 parallel to the strips 7 of terrain to be covered so that despite possible pitching motions of the flying body 1, by means of the rotary mirror 9 only the precise or immediate overflown strip 7 of land is received by the detector receiving surface 11.

The rotary mirror 9 for deflecting the terrain-point coverage beam 15 from along the precise overflown strip 7 of land is moved at a constant speed of rotation by an electric-motor drive mechanism 16.

From this it results that, with a low flying or recording height ha narrower terrain path 2, in other words a shorter strip 7 of terrain, is covered or received in a shorter time by the detector receiving surface 11, but on the contrary, at a much greater flying height h, a wider strip 7 of terrain is covered in a longer time. For simple data processing of repro ducibleterrain recordings or photographs it is desirable that at all times strips 7 of terrain of the same length with the same resolution are converted into a sequence of digital image point signals 3 irrespective of the height (variable from other criteria or made adjustable in approximately constant manner only with great additional expenditure over uneven terrain).

To this end, a flight-data determination circuit 17 (explained in more detail below) is provided which selectively controls, by means of a height-dependent output signal 18, a scanning or sequencing - control pulse sequence-frequency-transmitter 19 (for example a high-frequency-controlled frequency divider having an electrically variable divider ratio) and a gate-control-pulse length-transmitter 20 (for example a high frequency-controlled counter with interrogation of different counting positions symmetri callyto a mean counting position).

The detector output signal 6 is passed by way of a gate stage 21 to a scan/hold or sample/hold circuit 22. The temporal (time and space) course of the signal 6 corresponds to the detected image information of a continuous sequence of terrain points 5 along the precise immediate optically scanned strip 7 of terrain. This makes available to an analogue/ digital converter 24, in the rhythm of its (circuit 22) temporal control instantaneous values 23 (see Fi GURE 2) tapped from the constnt detector output signal 6 during the closing (on) time of the gate stage 21.The analogue/digital converter feeds a further processing circuit 25 with binary digital-coded image point signals 3, for instance for storing away the sequence of these image point signals 3 and/or for the comparison with preset terrain data for further control functions, to be derived therefrom, with respect to the operation of the flying body 1.

With a terrain-strip constant coverage or detecting angle 26 symmetrical to the perpendicular line from the flying body 1 to the terrain path 2 - in other words with a constant optical line scanning duration as a result of constant speed of rotation of the rotary mirror 9 -- different lengths of terrain strip 7 represented in FIGURE 2 symbolically by the temporal course of the detector output signal 6 would be covered (received) by the scan/hold circuit 22 as already mentioned, which lengths of strips 7 would be a function of the flying height h. In the case of an infra-red detector 4, the amplitude course or variation represents the intensities of the individual image points (terrain points 5) in the course of the time-proportional optical scanning of the precise immediate overflown strip 7 of terrain.The point in time tL indicates the covering or detection of the terrain point 5 perpendicularly underneath the flying body 1 and thus the scanning point in time of the angle bisector of the coverage angle 26. In order to process only image point signals 3 of terrain strips 7 of constant length which are independent of height (symmetrically disposed to the perpendicular line under the flying body 1), in other words to cover a terrain path 2 of constant width, the transmission time span 27 of the gate stage 21 is increased with low flying height or altitude h, compared with surveying from a greater height h, as is illustrated in FIGURE 2 by the corresponding parameter arrow.

As shown in FIGURE 2 with a constant sequence frequency of scanning or sequencing control pulses 28 for acquiring the sequence to be quantised (processed) of instantaneous values 23 from the amplitude course of the detector output signal 6 and depending on the instantaneous height ha different number of digitised image point signals 3 would accrue in respect of the precise immediate covered strip 7 of terrain corresponding to a different resolution of the scanned image information and accordingly this would be detrimental to the further processing and all the more so to a comparison with a preset or stored information pattern. Therefore, by means of the flight-data ascertainment circuit 17 the sequence frequency of the control pulses 28 is increased in inversely proportional manner to the fluctuation of the flying-body height h above the strip 7 of terrain; i.e., with a shorter gate transmission time-span 27 (see FIGURE 2) as a result of greater height h instantaneous values 23 are supplied to the converter 24 by the scan/hold circuit 22 in a closer sequence than with longer gate transmission time-spans 27, so that within each gate transmission time-span 27 - in accordance with the information to be evaluated regarding the terrain strip 7 - (contrary to the representation in FIGURE 2) the same number of digitised image point signals 3 is acquired from a corresponding number of scanned instantaneous values 23 (see below).

In order, by way of the frequency variation width of the scanning-control-pulse sequence frequency, to ensure the fulfilment of the scanning principle, it may be necessary or desirable in some embodiments to connect-in a low-pass member or filter 30 behind the detector output in addition to the anyway afforded low-pass behaviour of the detector 4.

To acquire the height output signal 18 two sensors 31,32 are provided on the flying body 1 (see FIGURE 3) which corresponds to the detector 4 and which are directed to receive information by way of optical telescope systems 33 or 34 respectively appropriately aligned at a mutual expansion or rotation angle 35 towards the terrain path 2 which is overflown by the flying body 1. Preferably, the optical system 33 is orientated perpendicularly, so that the length of its beam pick-up path corresponds to the instantaneous flying height h; whereas the further optical system 34 is preferably orientated obliquely forwardly.By virtue of geometrical arrangement it can be shown that the instantaneous height his proportional to the flying time for overflying the terrain-path length 36, which is represented as the side of the triangle lying opposite the expansion angle 35 (in which respect it is supposed that terrain unevennesses results only in a negliglible evaluation error or leads to such variations in evaluation that the corresponding eva luations can readily be eliminated as incorrect by virtue of inplausibility considerations). The proportionality constant entering into this evaluation is the product of the instantaneous flying speed vof the flying body 1 and the cotangent of the expansion angle 35.

The interval of time (proportional to the desired height h) between coverage of an artibrary terrain point 5' by the beam paths of the two mutually-inclined optical systems 33 and 34 is, preferably, carried out by means of a cross-correlation arrangement 37 which is known per se and which is fed with the two output signals of the sensors 31 and 32 respectively.

As is known, the maximum value (ascertained by means of a maximum filter 38) of the cross-correlation represents a measure of the time shift between the two output signals -- which is to this extent in agreement with respect to the radiation picked up from the terrain point 5' when disregarding the different angles of incidence -- of the sensors 31 and 32, in other words a measure of the desired or required interval of time proportional to the height h, during which the length 35 has been overflown.

In orderto ascertain the instantaneous flying speed voftheflying body 1 which is contained in the said proportionality factor, preferably recourse is also made to cross-correlation signal processing.

This is because a lower signal-processing circuit expenditure may be called for than the Doppler evaluation offering itself 'per se' in the case of an inclined pick-up characteristic. Therefore, there is provided, in the flying body 1, a further sensor 39 with a telescope optical system 40 which is arranged optically parallel to one of the two other optical systems 32 or 33 respectively and this is, preferably, the perpendicularly-orientated optical system 33.

Since it can be shown that the instantaneous speed v is inversely proportional to the interval of time during which the flying body 1 traverses the distance 41 (see FIGURE 3), advantageously a cross-correlation evaluation may again be effected. For this, the output signals of the two sensors 31,39, utilised to determine the flying speed are fed into a further cross-correlation arrangement 42 in order to acquire a measure of the interval of time of interest, in other words of the instantaneous flying speed v, by means of a further maximum filter 43 and by way of the maximum value of the cross-correlation function emerlinl in this respect.

The height output signal 18 which is time-dependent in accordance with the overflow terrain factors then emerges as the product of the two output signals of the maximum filters 38 and 43 which are switched to a multiplier 44, taking into account the output information of two constant transmitters 45 or 46 respectively which act upon. a multiplier 47 according to the expansion angle 35 or respectively (as dividend) on a divider 48 according to the effective optical system spacing distance 41.

Advantageously, differentiating stages 55 are connected subsequent to the sensors 31,32 and 39 in order to make clear or isolate terrain-surface changes by their high-pass behaviour; in other words to have available actually significantly - time variable sensor signals for the cross-correlation, even in the case of a terrain surface which varies little.

Since limited accuracies are adequate for the height-dependent control of the gate stage 21 and of the scan/hold circuit 22, it is sufficient to provide in the interests of saving on expenditure, eith . or in the sequence frequency transmitter 19 or in the length transmitter 20 (see FIGURE 1), tables of values stored away to switch over accordingly the sequence frequency of the control pulses 28 (and the length of the possibly realised gate control pulses 49) in dependence only upon exceeding of specific tolerances ranges with respect to the instantaneous flying height (h).

Such ROM-prepared tables are advantageously realised within the framework of a data processing device which is designed to carry out also the other data-processing steps, which steps are explained within the scope of this description for simplification with the aid of discrete block diagrams. The steps are explained more particularly with respect to the flight-data determination circuit and the function elements controlled by this, as well as with respect to the further-processing circuit 25 and analogue/ digital converter 24, insofar as this analogue/digital conversion is not in accordance with the concept of modern data-processing equipment already realised as a peripheral constituent part of a microcomputer.

More particularly upon realisation of the described (and still to be described) functions by an approp example flying) - height dependent manner from a moving (for example flying) body by means of line-scanning, for example optics; scanning, of respective ones of a sequence of strips of terrain, extending in their length transversely to the moving (for example flying) direction, and projecting or transmitting information therefrom onto a detector, in which, with a constant (for example optical) scanning speed of the strip of terrain from which information is projected or transmitted onto the detector, only a proportion-shortening with increasing flying height-of the detector output signal is quantised or processed and digitised, in which respect, for the quantising or processing, a scanning of the proportion is effected with a scanning frequency which rises proportionally with increasing height differential from the strip of terrain.

3. A method as claimed in Claim 1 or Claim 2 in which a low-pass influencing is effected prior to the scanning or sequencing of instantaneous amplitude values of the detector output signal.

4. A method as claimed in any one of the preceding claims in which the information length received in the moving direction by the detector of the strip of terrain projected or transmitted to the detector is reduced proportionally to an increase in height differential.

5. A method as claimed in Claim 4, in which a summing of output signals of several detector elements of the detector is effected, which elements can be connected up individually in a height-controlled manner, with subsequent low-pass filtering.

6. A method as claimed in Claim 5, in which height-controlled switching-on or switching-off respectively of detector elements is effected symmetrically on both sides of a central detector element.

7. A method as claimed in any one of the preceding claims, in which a height-dependent signal is acquired from a cross-correlation of two sensor signals having different optical orientation in the moving or flight direction, multiplied bythe inverse value of the cross-correlation of two sensor signals having the same optical orientation but with mutual shift in the flight direction.

8. Apparatus for carrying out a method according to any one of the preceding claims.

9. Apparatus as claimed in Claim 8 comprising a rotary mirror which is capable of optically scanning successive strips of terrain along a flight or movement direction to project or transmit information from the strips onto a detector receiving surface, in which a gate stage and a scanthold or sample/hold circuit are connected subsequent to the detector, and in that a flight-data determination circuit is provided which (with the rotary mirror operated at a constant speed of rotation) by way of a gate-pulse length transmitter is capable of controlling the gate stage with gate controlling pulses, the length of which fluctuates inversely proportionally to the height differential, and which, by way of a scanning or sequencing contorol-pulse sequence frequency transmitter, is capable of controlling the scan/hold or sample/hold circuit with scanning or sequencing control pulses, the pulse sequence frequency of which fluctuates proportionally with the flying height.

10. Apparatus as claimed in Claim 9 in which the detector has several detector elements which are arranged one after the other in the movement or flight direction and, in use, the number which are connected through the flight-data determination circuit onto the detector output varies inversely proportionally with the flying height.

11. Apparatus as claimed in claim 10 in which a low-pass member is connected subsequent to the detector elements.

12. Apparatus as claimed in Claim 9 or Claim 10 in which some of the detector elements are connected together in pairs and each one of a respective side of a central detector element.

13. Apparatus as claimed in any one of Claims 8 to 12, in which the flight-data determination circuit has a first cross-correlation arrangement which is adapted to be acted upon by two sensors having optical systems which, in use, are differently orientated in the flight direction, and the flight determination circuit having a second cross-correlation arrangement which, in use, is acted upon by two sensors arranged mutually offset in the flight direction with identically orientated optical systems, and the outputs of which cross-correlation circuits are connected by way of maximum filters and by way of a constant multiplier or by way of a constant divider respectively onto an output multiplier for the emission of a height output signal.

14. Apparatus as claimed in Claim 13 in which differentiating stages are connected subsequent to the sensors.

15. Apparatus as claimed in anyofClaims8to 14, in which the gate stage and the gate-pulse length transmitter are replaced by a gate-length counter which, released periodically by the rotary mirror, allows a fixedly presettable number of scanning control pulses to become effective.

16. A method as claimed in any one of Claims 1 to 7 and substantially as herein described with reference to the accompanying drawings.

17. Apparatus for use in terrain or surface sur- veying substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. example flying) - height dependent manner from a moving (for example flying) body by means of line-scanning, for example optics; scanning, of respective ones of a sequence of strips of terrain, extending in their length transversely to the moving (for example flying) direction, and projecting or transmitting information therefrom onto a detector, in which, with a constant (for example optical) scanning speed of the strip of terrain from which information is projected or transmitted onto the detector, only a proportion-shortening with increasing flying height-of the detector output signal is quantised or processed and digitised, in which respect, for the quantising or processing, a scanning of the proportion is effected with a scanning frequency which rises proportionally with increasing height differential from the strip of terrain. 3. A method as claimed in Claim 1 or Claim 2 in which a low-pass influencing is effected prior to the scanning or sequencing of instantaneous amplitude values of the detector output signal. 4. A method as claimed in any one of the preceding claims in which the information length received in the moving direction by the detector of the strip of terrain projected or transmitted to the detector is reduced proportionally to an increase in height differential. 5. A method as claimed in Claim 4, in which a summing of output signals of several detector elements of the detector is effected, which elements can be connected up individually in a height-controlled manner, with subsequent low-pass filtering. 6. A method as claimed in Claim 5, in which height-controlled switching-on or switching-off respectively of detector elements is effected symmetrically on both sides of a central detector element. 7. A method as claimed in any one of the preceding claims, in which a height-dependent signal is acquired from a cross-correlation of two sensor signals having different optical orientation in the moving or flight direction, multiplied bythe inverse value of the cross-correlation of two sensor signals having the same optical orientation but with mutual shift in the flight direction. 8. Apparatus for carrying out a method according to any one of the preceding claims. 9. Apparatus as claimed in Claim 8 comprising a rotary mirror which is capable of optically scanning successive strips of terrain along a flight or movement direction to project or transmit information from the strips onto a detector receiving surface, in which a gate stage and a scanthold or sample/hold circuit are connected subsequent to the detector, and in that a flight-data determination circuit is provided which (with the rotary mirror operated at a constant speed of rotation) by way of a gate-pulse length transmitter is capable of controlling the gate stage with gate controlling pulses, the length of which fluctuates inversely proportionally to the height differential, and which, by way of a scanning or sequencing contorol-pulse sequence frequency transmitter, is capable of controlling the scan/hold or sample/hold circuit with scanning or sequencing control pulses, the pulse sequence frequency of which fluctuates proportionally with the flying height. 10. Apparatus as claimed in Claim 9 in which the detector has several detector elements which are arranged one after the other in the movement or flight direction and, in use, the number which are connected through the flight-data determination circuit onto the detector output varies inversely proportionally with the flying height. 11. Apparatus as claimed in claim 10 in which a low-pass member is connected subsequent to the detector elements. 12. Apparatus as claimed in Claim 9 or Claim 10 in which some of the detector elements are connected together in pairs and each one of a respective side of a central detector element. 13. Apparatus as claimed in any one of Claims 8 to 12, in which the flight-data determination circuit has a first cross-correlation arrangement which is adapted to be acted upon by two sensors having optical systems which, in use, are differently orientated in the flight direction, and the flight determination circuit having a second cross-correlation arrangement which, in use, is acted upon by two sensors arranged mutually offset in the flight direction with identically orientated optical systems, and the outputs of which cross-correlation circuits are connected by way of maximum filters and by way of a constant multiplier or by way of a constant divider respectively onto an output multiplier for the emission of a height output signal. 14. Apparatus as claimed in Claim 13 in which differentiating stages are connected subsequent to the sensors. 15. Apparatus as claimed in anyofClaims8to 14, in which the gate stage and the gate-pulse length transmitter are replaced by a gate-length counter which, released periodically by the rotary mirror, allows a fixedly presettable number of scanning control pulses to become effective. 16. A method as claimed in any one of Claims 1 to 7 and substantially as herein described with reference to the accompanying drawings. 17. Apparatus for use in terrain or surface sur- veying substantially as herein described with reference to the accompanying drawings. riately adapted computer it can be advantageous to replace the function of the gate stage 21 and of the length transmitter 20 because per covered strip 7 of terrain, in other words per optical scanning cycle of the terrain-point coverage beam 15, only one specific preset number of quantisations of the detector output signal 6 is effected. This is because, as explained above, it is required that by way of the sequence frequency of the control pulses 28fluctuat- ing in a height-dependent manner with opposingly fluctuating quantising time span per optical scanning cycle (effective segment of the rotary mirror 9) the succession of the covered strips 7 of land is at all times the same number of discrete instantaneous amplitude values which are digitised.This corresponds, however, per scanning cycle to a constant number of control pulses 28 effective for the scanning or sequencing and analogue/digital conversion; i.e., instead of the function blocks bordered in dotted manner in FIGURE, 1 a -for example adjustable gate-length counter 56 may be provided. The numberzof control pulses 28 (height-controlled in their sequence frequency), effective per scanning cycle, for the quantisation function of the scan/hold circuit 24 or respectively of the analogue/digital converter 24 corresponds to the covered length of the respective strip 7 of terrain, in other words to the covered width of the terrain path 2.For this, the gate-length counter 56, periodically released by a control mechanism on the rotary mirror 9, can act directly on the height-controlled sequence frequency transmitter 19; or connected subsequent to this (as taken into account in the drawing in the interests of clarity) is an AND-gate 57 for transmission of the respective z scanning control pulses 28.When this numberzis reached, the counter 56 is reset, until it is started afresh by the rotary mirror 9, in order to count up the control pulses, emitted by the sequence frequency transmitter 19, again up to z. In order to avoid a mutual displacement of the consecutively covered strips 7 of 1 and in the event of fluctuating flying height h, the triggering mechanism 58 on the rotary mirror9-taken into account only symbolically in the drawing -- can, on account of the constant speed of rotation thereof, readily be set up for allowing, by appropriate advancing of the point of release for the counter 56, the distribution of the pulse count z, effective per scanning cycle, to become effective as to each half on both sides of the perpendicularly overflown terrain point 5 (in which the instantaneously effective sector of the rotary mirror 9 stands parallel to the receiving surface 11 or respectively stimulates the central detector element 50'). The discontinuous signal processing of the detector output signals 6 and feeding of the analogue/ digital converter 24 by way of the scan/hold circuit 22 is (in view of the scanning principle to be observed) not critical with respect to the information sequence along a particular optically scanned strip 7 of terrain because - as has already been mentionedthe detector 4 for converting the input signal (for example infra-red signal) into an electrical output signal 6 has a low-pass behaviour which may possibly be strengthened by a subsequently-connected low-pass member 30 still readily to the requisite degree.On the other hand, for discrete signal processing it is critical that at the end of an optically-scanned strip 7 of terrain with the transition to the start of the next-following strip 7 of terrain in the flight direction 8 that an information jump occurs instead of a constant course of the detector output signal. The band limitation effect of the detector 4 lacking in the coverage direction prevents the scanning principle being realised and thus results further non-steady digital signal processing. In order to overcome this restriction, provision is made to arrange on the flying body 1, in the flying direction 8, a number, variable in height-dependent manner, of individual detector elements 50 in the detector 4 and to switch through thereof, inversely proportional to the instantaneous flying height h, more or less of them onto the detector output 29 by way of a low-pass member 51.In this way, the width, orientated in the flight direction, of the strips 7 of terrain covered successively by the optically-scanning rotary mirror 9 can be varied in a flying-height-dependent manner, in order to ensure mutual overlaps and thus a continuous detector output signal 6. The arrangement and connecting possibility of the individual detector element 50 is provided symmetrically of the centrally-situated one of the detector elements 50' as shown in FIGURE 1. The detector element 50' is at all times connected through by way of a summing amplifier 52 onto the low-pass member 51. The height output signal 18 controls a switch control stage 53 for connection first of the two next-adjacent ones of the detector elements 50 as shown, on a decrease in flying height h, and with further decreasing height h signal 18 controls also switching-through of the two outer ones of the detector elements. In order to make do with the least possible number of switches 54, the simultaneously switched detector elements 50 are directly connected together, in other words not put by way of separate switches onto further summing inputs of the summing amplifier 52. CLAIMS

1. A method of terrain or surface surveying comprising scanning the terrain or surface in a sequence of strips from a body moving relative to the terrain or surface, information from successive strips of the sequence being received by a detector and the detector producing an output signal indicative ofthe information received and in which, at a substantially constant scanning speed of the strips of terrain or surface received by the detector, only selected portions of the detector output signal are processed and digitized; respective selected portions decreasing in size with any increasing height differential between the body and the terrain or surface, and for said processing of the respective selected portions scanning or sequencing of the portions is effected with a scanning or sequency frequency which increases with increasing height differential.

2. A method of picking-up (receiving) and processing detector output signals, for example, opto-electronically and in particular, for example using infra-red, in terrain surveying in a moving (for