GB2191859A - Navigation aids - Google Patents

Abstract

An electronic cursor (12) is movable over a digitising table (10) on which a chart (16) has been spread. The cursor can give an indication of which way it should be moved to reach a defined position on the chart. The cursor can also convert positions in terms of x, y coordinates of the table into positions in terms of latitude and longitude co-ordinates of the chart, and can display the chart positions. The cursor can also determine bearings and distances between its own position and a defined position. Several other facilities are also available. Thus, the cursor can be used to facilitate the plotting of positions and courses on the chart and in reading off positions and bearings from the chart. <IMAGE>

Description

SPECIFICATION Navigation aids This invention is concerned with facilitating the plotting of points on a map or navigational chart and with taking data off a chart.

Traditionally, positions are plotted on a chart by drawing intersecting positions lines determined from observations, measurements and/or calculations, and further positions can be plotted using the dead-reckoning or estimated position methods involving calculations of speed, course, set and drift. The further positions and bearings can then be read-off the chart using instruments and the charts scale and compass markings. Such methods are time consuming and laborious, and requires a degree of expertise with navigational drawing instruments such as a parallel-ruler and dividers or other manual mechanical chart plotting and reading aids.

A very expensive partial replacement for the traditional plotting method is provided by a plotting table which receives an input or inputs from one or more external navigation aids and projects a light beam onto or draws a line at the corresponding location on the chart. In addition to being expensive, such equipment takes up a substantial amount of space and is heavy, and is therefore not particularly suited to anything but large ships.

Electronic chart systems are also being developed to assist in navigation. These have the disadvantages of high cost, the requirement of a cathode ray tube as the chart display, a lack of high resolution in the displayed output, and the lack of a permanent paper record of the output.

The present invention aims to provide a useful and not particularly expensive aid to chart plotting, and the preferred features of the invention described below are aimed at providing a number of useful further facilities which willwreduce the time taken and likelihood of error in plotting points or courses, or determining positions as compared with the traditional methods.

In accordance with one aspect of the present invention, there is provided a navigational aid comprising: a charting surface (which may be, for example, the surface of a chart table or chart board, or a chart itself), a cursor device movable over the surface; means to provide a signal indicative of the current position of the cursor device on the surface; and directing means responsive to the current position signal to determine and provide an indication of a direction of movement for the cursor device towards a defined position.

The advantage provided by this aspect of the invention is that the cursor device can be moved to the defined position quickly using the indication provided by the directing means, and the cursor device can then be used to plot the defined position on a chart disposed on or forming the charting surface.

Conveniently, the directing means may provide the direction indication on the cursor device, and in one arrangement the direction indication is provided by indicators of the required movement in two directions at right angles to each other, such as North-South and East-West.

Preferably, the direction indication is provided substantially instantaneously and is repeatedly updated.

Preferably, the aid further comprises means to provide a signal indicative of the defined position, the signal and the defined position being selectably variable, and the directing means being responsive to the defined and current position signals in determining the required direction of movement. Thus, once a defined position has been selected, the cursor can quickly be moved to that position.

Conveniently, the aid is designed for use with or incorporates a chart having a co-ordinate system, such as longitude and latitude, and further comprises means to establish a relationship between the chart co-ordinate system and a co-ordinate system of the charting surface, such as orthogonal Cartesian co-ordinates, and means to convert positions between the two co-ordinate systems. With such a relationship established and such a conversion possible, positions, bearings, distances, and speeds can be determined in terms of the chart co-ordinate system.

Preferably, means are provided to determine and display the current position of the cursor device and/or the defined position in terms of the chart co-ordinate system, e.g. as latitude and longitude. With this feature, it is no longer necessary to use rulers, dividers or the chart's scale-markings to determine the positions.

Preferably, the aid also comprises means to determine and display the distance between the current and defined positions in units derivable from the chart co-ordinate system, such as nautical miles, miles or kilometres or other appropriate units, and preferably the current position is displayed substantially instantaneously and is repeatedly updated. Thus, it is not necessary to use dividers and the chart's scale-markings to read off distances from the chart. The aid may further comprise means to determine and display the bearing of the defined position from the current position, and/or vice versa, in terms of the chart coordinate system, such as in degrees clockwise from True North. Thus, it is not necessary to use a parallel-ruler and the chart's compass marking in working out bearings.Conveniently, the aid may further comprise means to store a bearing offset, the bearing display means displaying the sum or difference of the determined bearing and the offset. The stored bearing offset may take into account the local magnetic variation or the local grid convergence, if it is desired to work in terms of Magnetic North or Grid North, rather than True North, and can also take into account compass deviation (or error).

An advantage of the features described in the preceding paragraph is that the defined position may be set off the chart, and yet the distance to and/or bearing of the defined position can still be determined.

The aid may include manual data entry means to enter a position in terms of the chart co-ordinate system and to set the defined position as that entered position.

Furthermore, the aid may include means to receive an input data signal indicative of a position in terms of the chart co-ordinate system (or some other co-ordinate system, in which case a conversion is made) and to set the defined position as that input position.

Such a data signal may be provided by a receiver responsive to a radio navigation system such as Decca Mainchain or Loran or a satellite navigation system such as GPS or Transit.

The aid may further include a clock means, and means responsive to (i) the clock means, (ii) to speed and course signals, and (iii) to the defined position signal, and which is operable to determine and display an updated position.

Thus, the aid can give a dead-reckoning position. Manual data entry means may be provided to enter the speed and course, or alternatively or additionally means may be provided to receive signals indicative of the speed and course,for example from an electrical compass and speed sensor. Manual data entry means may be provided to enter corrections to the speed and course, such as set and drift, the updated position taking into account the corrections. Thus, the aid can display an estimated position in addition to the raw dead-reckoning position. Preferably, the directing means is selectively responsive to the updated position rather than the defined position. Thus, the cursor device can be quickly positioned on the chart using the directing means at the dead-reckoning or estimated position.

The aid may include means to store and recali a plurality of such defined positions.

Thus, the stored positions may be used as way-points in plotting a desired course. Preferably, the defined positions, or way-points, are stored in terms of the chart co-ordinate system. This enables the way-points to be transferred from a first chart to a second chart having the same co-ordinate system once the relationship between the co-ordinate system of the second chart and that of the charting surface has been established. For example, way-points on a long voyage may be stored using a small-scale chart, and then re called and plotted on a series of large-scale charts each of which covers only part of the voyage.

The aid may further include means to determine and display in units derivable from the chart co-ordinate system the distance of the updated position along a course between two such defined positions and the distance from the updated position to that course. Thus the distance along a course from a way-point and the distance off-course can be displayed.

The display means mentioned above are conveniently provided on the cursor device and in the case where at least two display means are specified, at least some of them may have a common display. Furthermore, the manual data entry means mentioned above are conveniently provided on or by the cursor device, and in the case where at least two manual data entry means are specified, they may have a common key pad. Furthermore, a facility may be provided to vary data to be entered prior to entry in dependence upon movement of the cursor device with respect to the charting surface.

In accordance with a second aspect of the present invention, there is provided an electronic mouse having means to sense its position in terms of a first co-ordinate system, means to convert a sensed position to being in terms of a second co-ordinate system, and means to indicate the position in terms of the second co-ordinate system. Preferably, the first co-ordinate system is a planar co-ordinate system, such as x, y or r, theta and the second co-ordinate system is longitude and latitude related.

In accordance with a third aspect of the present invention, there is provided an electronic mouse having means to input a position in terms of a first coordinate system, means to convert such an input position to being in terms of a second co-ordinate system with which the mouse can relate, and means to indicate where the mouse should go to reach the position. In this case, the first co-ordinate system is preferably longitude and latitude related and the second co-ordinate system is a planar co-ordinate system such as x, y or r, theta.

In accordance with a fourth aspect of the present invention, there is provided a chart reading apparatus of the type having a charting surface, a mouse movable over the surface and means to sense the position of the mouse on the surface, characterised by means selectable to vary a stored value unrelated to such position in response to movement of the mouse with respect to the surface.

The mice according to any of the second, third and fourth aspects of the invention may further have the features of any other of the aspects of the invention and also any of the preferred features described above of the first aspect of the invention.

There follows a description by way of example of a specific embodiment of the present invention, reference being made to the accompanying drawings, in which: Figure 1 is a schematic perspective view of a navigation aid; Figure 2 is a plan view of the cursor of the navigation aid; Figure 3 is a block diagram of the circuitry of the aid; Figure 4 is a flow diagram showing how the chart is referenced; Figure 5 is a flow diagram concerning the current cursor position display; Figure 6 is a flow diagram concerning direction indication; and Figure 7 shows the distances along and off the course between two way-points.

Construction Referring to Figure 1, the navigation aid comprises a digitising table 10 and a cursor or mouse 12 connected to the table 10 via a signal line 14. By means of a sensor coil in the cursor and a grid of wires beneath the surface of the table, it is possible to determine the position in terms of x and y coordinates of the cursor on the table. A chart 16 is spread across the table 10 approximately squarely on the table, and the cursor is freely movable by hand across the chart.

Referring to Figure 2, the cursor 12 has an elongate housing 18 formed with a throughhole 20 at one end. A transparent disc 22 is fitted across the base of the through-hole and is marked with cross-hair lines 24, 26. A hole 28 is formed through the disc 22 at the intersection of the cross-hair lines 24, 26 so that a pencil can be used to mark the chart through the hole 28 at the intersection of the lines 24, 26. The sensor coil is mounted in the housing around the hole 20 so that the determined position of the cursor 12 corresponds to the positon of the hole 28.

Four LED indicators 30 to 36 designated "North", "East", "South" and "West", respectively, are arranged around the hole 20, and a further LED indicator 40 designated "On Target" and a "fine"push button 42 are disposed adjacent the hole 20.

At the other end of the housing 18, there is a keypad 44, and in between the keypad and hole 20, there are four LCD displays 46 to 52 designated "Cursor Position", "Fixed Point", "Other Data", and "Data Entry", respectively.

Referring to Figure 3, the cursor 12 contains within its housing a central processor unit (CPU) 54 with associated read only memory (ROM) 56 storing the operating program of the cursor and random access memory (RAM) 58 for storing data. The CPU 54 drives the LED displays 30-40 and the LCD displays 4652 via a display interface 60. The CPU 54 also receives signals from the keyboard 42 and push-button 44 via a keyboard interface 62. The sensor coil 64 in the cursor and the digitising table 10 supply signals to a position interface 66, which can then supply position signals in terms of x, y co-ordinates of the position of the cursor 12 on the table 10 to the CPU 54. The CPU also receives a clock signal from a clock circuit 67. Lastly, the CPU can receive external information signals via an external interface 68.

There now follows a description of the operation of the navigation aid.

Referencing Before the aid can be used for plotting and reading off points on the chart, the chart 16 must firstly be referenced with respect to the digitising table. The detaii of the referencing operation is shown in the flow diagram of Figure 4 from which it can be seen that three points of known latitude and longitude, as shown in Figure 1, are chosen, and for each the cursor 12 is positioned at that point and the position is entered via the keyboard, for example in the form (degrees latitude), N or S, (degrees longitude), E or W, REF. As the data is entered, it is displayed on the data entry display 52. The entered data for each point is stored, as too is the sensed position of the cursor 12 on the table 10. Once all three reference points have been entered, the CPU 54 sets up a mathematical mapping system of cursor position onto latitude and longitude.

Current Cursor Position Display Once the mapping system has been set up, the current cursor position can be displayed on the display 46 by performing the steps shown in Figure 5. The CPU 54 regularly and frequently performs these steps and thus the display 46 is continually updated. The display may take a form such as CP: (degrees latitude) N or S: (degrees longitude) E or W Setting a Fixed Point A fixed point may be set by entering its position via the keyboard in the form (degrees latitude), N or S, (degrees longitude), E or W, FIXED POINT. As the position is entered it is displayed on the data entry display 52 and once it has been entered it is displayed on the fixed point display 48 in a similar form to the current position display and is also stored in the RAM 58.

Another way to enter a fixed point is to position the cursor on the required point on the chart and then enter FIXED POINT on the keyboard 44 without any prior data. The fixed point position is then set equal to the current cursor position and is stored in the RAM 58.

Moving to the Fixed Point Once the fixed point has been set, the MPU 54 performs the routine set out in Figure 6 in controlling the LED displays 30 to 40. This routine is performed regularly and frequently to provide a continually updated display by the LED displays 30 to 40. It will be appreciated from Figure 6 that the LED's indicate whether the cursor 12 should be moved North or South and East or West to reach the fixed position, and once that position is reached, the On Target LED 40 is lit.

In a modification of the routine of Figure 6, the LED's 30 to 36 are flashed at a rate which increases as the distance of the cursor 12 from the fixed point decreases, and once the cursor is nearly on target the fine button 42 can be depressed to decrease the flashing rate by a predetermined factor.

Display of Other Data The Other Data display 50 is capable of displaying the following forms of other data when operating in different modes and can be advanced from one mode to the next cyclicly using the CONTROL key of the keyboard: 1. Blank Display 2. (Distance in nautical miles) NM: (Bearing to fixed point in degrees) DEG TO 3. (Distance in nautical miles) NM: (Bearing from fixed point in degrees) DEG FROM 4. (Bearing in degrees) CRS: (Speed in knots) KN 5. DR:(latitude in degrees) N or S: (longitude in degrees) E or W 6. SET (bearing in degrees) DEG: DR (rate in knots) KN 7. EP: (iatitude in degrees) N or S: (longitude in degrees) E or W 8. TIME: (hours): (minutes): (seconds) 9. WPI: (latitude in degrees) N or S: (longitude in degrees) E or W 10.WP2:(latitude in degrees) N or S: (longitude in degrees) E or W 11. 1 to 2: (distance nautical miles) ALONG: (distance in nautical miles) OFF 12. WP3: (latitude in degrees) N or S: (longitude in degrees) E or W 13. 2 to 3 (distance in nautical miles) ALONG: (distance in nautical miles OFF) 14. WP4: (latitude in degrees) N or S: (longitude in degrees) E or W 15. 3 to 4 (distance in nautical miles) ALONG: (distance in nautical miles) OFF Mode 2: Distance and Bearing to Fixed Point The CPU 5 is programmed to be able to calculate, from the latitude and longitude of the current cursor position and the fixed point, the distance between them and the bearing from True North of the fixed point from the cursor position. The distance and bearing are displayed on the Other Data display 50 in the form set out above and are continually up dated as the cursor is moved.

Thus, if bearings are taken from a boat of two landmarks of known position, they can successively be used as fixed points and the cursor can be used to plot on the chart two further points with appropriate bearings to the respective fixed points. The intersection of lines connecting each fixed point and the re spective further point is then a plot of the current position of the boat.

Mode 3: Distance and Bearing from Fixed Point This mode is similar to Mode 2, except that the calculated bearing is from, rather than to, the fixed point and thus differs from the bearing of Mode 2 by 180 degrees.

Mode 4: Current Course and Speed The cursor may be connected to a digital compass and speed-sensor, via the external interface, and in Mode 4 a display is made of the course and speed in the form (bearing in degrees) CRS, (speed in knots). KN. The course and speed signals are used in calculations of positions as described below with reference to Modes 5 and 7.

In this mode the signals from the external interface may be overridden by manually entering a course and speed via the keyboard in the form (course in degrees), COURSE, (speed in knots), SPEED; and such override can be released by entering nuli data in the form COURSE, SPEED.

Mode 5: Dead-Reckoning Position In this mode, or mode 7 described below, a datum dead-reckoning or estimated position may be entered in the same way as for entering a fixed point, but by presseding D.R. & BR< E.P. RESET rather than FIXED POINT. Thereafter from the data provided in Mode 4, and with reference to the clock 67, the CPU 54 continually calculates a current dead-reckoning position which is displayed in Mode 5 in terms of latitude and longitutude on the Other Data display 50. Furthermore, when in Mode 5, the routine of Figure 6 for controlling the LED's 30 to 40 is responsive to the latitude and longitude of the calculated dead-reckoning position, rather than of the fixed point, and thus the cursor 12 can be quickly located at the dead-reckoning position on the chart and that position can be plotted.

Mode 6: Set and Drift of Current In this mode, a display is made of a set and drift which has been entered via the keyboard in the form (set in degrees), SET, (drift in knots), DRIFT. When this data is entered, it is stored for use in calculating an estimated position.

Mode 7: Estimated Position In addition to calculating a current dead reckoning position as described above under Mode 5, the CPU 54 similarly calculates a cur rent estimated position which takes into account the set and drift entered as described above under Mode 6, and in Mode 7, the estimated position is displayed. When in Mode 7, the routine of Figure 6 is responsive to the latitude and longitude of the estimated position, rather than of the fixed point.

Mode 8; Time In this mode, the current time is displayed on the Other Data display 50. The time can be set when in this mode by entering a time via the keyboard 44 in the form (hours). (minutes). (seconds).

Modes 9, 10, 12 and 14: Way-Points Entry and Display In each of these modes the latitude and longitude of a respective way-point 1, 2, 3 or 4 can be entered via the keypad in the form (latitude of way-point), N or S, (longitude of way-point), E or W, WAY POINT; and once entered the position of the way-point is stored in the RAM 58 in terms of latitude and longitude and is displayed on the Other Data display 50. Alternatively, a way-point can be set by positioning the cursor 12 at the desired point on the chart and depressing the WAY POINT key without any prior data entry.

When the cursor is operating in mode 9, 10, 12 or 14, the routine of Figure 6 is responsive to the latitude and longitude of the respective way-point WP1, WP2, WP3 or WP4, rather than of the fixed point, and thus the LED's 30 to 40 can be used quickly to position the cursor 12 at the way-point on the chart.

The way-points remain in memory even if the cursor is re-referenced. Therefore, the latitudes and longitudes of a series of way-points can be set using a small scale chart, and the cursor can then be re-referenced a number of times for a series of large scale chart and the way-points can be plotted on the large scale charts.

Modes 11, 13 and 15: Distance Along and Off Course In modes 11, 13 and 15, the CPU 54 calculates the distances (as represented in Figure 7) in nautical miles of the current cursor position along and off a course between a respective pair of way-points WP1 and WP2, WP2 and WP3, and WP3 and WP4, and two distances are displayed on the Other Data display 50.

The way-points need not necessarily be within the boundary of the chart 16 on the table 10, and the boundary may be as represented by the dot-dash line 70 in Figure 7.

Whilst the facility to store only four waypoints has been described above for the sake of brevity, it will be appreciated that a greater number than that may be provided.

Entering Data by Cursor Movement Data entry via the keyboard 44, especially when several digits are to be entered, can be tedious and can possibly introduce errors due to mis-keying. As an alternative to data entry in this manner, the CPU 54 is also responsive to movement of the cursor in the x direction of the table 10 as a means of varying data prior to entry. Thus, once the chart 16 has been referenced on the table 10, initial latitude and longitude values are taken to be equal to the latitude and longitude of the current cursor position and initial values of other sorts are taken to be zero. Then, the values can, in turn, be ramped up and down by movement of the cursor 12 in the left and right directions, respectively, across the table.As an example, in order to set way-point 1 off the chart: (i) the cursor is placed on the table; (ii) Mode 9 is selected and the Data Entry display 52 displays the current cursor position; (iii) the displayed latitude is then increased or decreased by moving the cursor 12 to the right or left; (iv) if it is desired to increase/decrease the value above/below that obtained when the cursor has been moved to the right/left edge of the table, then the cursor is lifted clear of the table and replaced at the left/right edge; (v) once the displayed latitude value is approximately equal to that which it is desired to enter, the FINE key 42 is depressed to make the change in displayed value less sensitive to cursor movement, and the value is finely adjusted; (vi) the N or S key is then depressed to enter the selected latitude value; (vii) steps (iii) to (v) are then repeated for the longitude value; and lastly (viii) the E or W key is depressed to enter the selected longitude value.

Despite the length of this paragraph, after a small amount or practice by the user, this method of data entry can be surprisingly quick except when large changes need to be made to the values.

Bearings and speeds can be entered similarly.

Calculator It will be noted from Figure 2 that the keyboard 44 includes the basic function keys of a calculator, i.e. plus, minus, times, divided by and equals. These keys, in combination with the digit and decimal keys can be used for simple calculations during manual data entry and also for calculations unrelated to the other uses of the cursor.

Bearing Datum Bearings may be entered and displayed with reference to True North. However, if it is desired to enter and display bearings with respect to some other datum, the difference between True North and the datum may be entered in the following form (bearing offset in degrees), E or W, BEARING OFFSET, and is then stored in the RAM 58. The keypad of the cursor then expects to receive entered bearings with reference to that other datum and converts such entered bearings to being True North referenced before storing them or using them in calculations.Any bearing which is to be displayed is converted to being with reference to that other datum rather than True North before it is displayed, Thus, the local variation of Magnetic North from True North, or the local convergence of Grid North and True North, together with any compass deviation (or error) may be entered as the bearing offset, and then bearings with reference to Magnetic North or Grid North, as the case may be, can be entered and will be displayed.

Modifications In a development of the referencing system, each chart is provided with a code; three reference points are marked on the chart; and the navigation aid has in memory the latitudes and longitudes of the reference points for each coded chart. Thus, the code can be entered, manually or by means of a bar code and reader, and the cursor placed on the three reference points, whereupon the CPU can set up the mapping system.

The cursor may be adapted for operation with other forms of digitising table for example employing acoustic or optical sensors. Alternatively, the cursor may be connected to a chart table using a pantographtype pivoted linkage having a pair of potentiometers operably connected at a pair of the linkage pivots such that the position of the cursor can be determined from the potentiometers.

As a modification to the type of digitising table having a grid of wires beneath its surface, the wires may be provided in a flexible sheet which can be spread over a surface, with the chart being spread over or under the flexible sheet. Thus the flexible sheet can be rolled up and stored when not in use. Furthermore, specially adapted charts may be used which have the digitising wires embedded in the thickness of the chart.

The cursor may be adapted to be able to display latitudes and longitudes selectably in (i) degrees and decimal degrees, (ii) degrees, minutes and decimal minutes, and (iii) degrees, minutes and seconds, or indeed in units of other chart co-ordinate systems.

The LED direction displays 30-36 and the transparent disc 22 may be formed as an assembly which is mounted for limited rotation of, for example, 30 degrees either way from the position shown in Figure 2 so that the cursor may be used at a slight angle to that shown in Figure 1 in order to suit right or lefthanded use.

A different number of LED direction indicators may be used, for example sixteen, or alternatively a completely different form of direction indication may be provided as, for example, by a rotatable needle.

Instead of it being necessary to enter a local magnetic variation in order that the cursor can receive and display bearings with reference to Magnetic North, the cursor may be adapted to store magnetic variations for a series of positions at different latitudes and longitudes and to use the current cursor position to interpolate between the stored values in producing an estimated current magnetic variation which is used automatically in converting between True North and Magnetic North referenced bearings.

In addition to being able to calculate deadreckoning and estimate positions, the cursor may also be adpated to as to be able to calculate, from given values for any six of the following seven parameters, the value of the remaining parameter: (i) Start Position; (ii) Finish Position; (iii) Course; (iv) Speed; (v) Elapsed time; (vi) Set; and (vii) Drift.

In a further modification the cursor may be adapted to store tidal stream tables for at least part of the Earth's waters, and the cursor then interpolates from those tables in producing estimated values of local set and drift.

The cursor may also be adapted to store the Almanac of astronomical information so that information can be provided by the cursor to assist the user in astronomical observations and calculations.

The cursor may be provided with releasable suckers or some other means of releasably holding the cursor in position on the chart so that when used on, for example, a vessel, the cursor can be fixed to the chart during bad weather to prevent it falling off the table.

Furthermore, the cursor may be linked to the table by a mechanism such as that used for parallel-motion drawing boards so that the cursor is more easily movable in the x and y directions of the table but can be moved with more resistance in other directions across the table.

The external interface 6 described above may be bi-directional so that the cursor can output data, for example stored way-point positions, to other equipment.

It will be apparent that many other modifications and developments may be made to the arrangement described above whilst still adopting the inventive concepts disclosed in this specification.

Claims (36)

1. A navigational aid comprising: a charting surface; a cursor device movable over the surface; means to provide a signal indicative of the current positon of the cursor device on the surface; and directing means responsive to the current position signal to determine and provide an indication of a direction of movement for the cursor device towards a defined position.

2. An aid as claimed in claim 1, wherein the directing means provides the direction indication on the cursor device.

3. An aid as claimed in claim 1 or 2, wherein the direction indication is provided substantially instantaneously and is repeatedly updated.

4. An aid as claimed in claim 2 or 3, wherein the direction indication is provided by indicators of the required movement in two directions at right angles to each other.

5. An aid as claimed in any preceding claim, further comprising means to provide a signal indicative of the defined position, the signal and the defined position being selectably variable, and the directing means being responsive to the defined and current signals in determining the required direction of movement.

6. A aid as claimed in any preceding claim, and further including means to set the defined position as the current position.

7. An aid as claimed in any preceding claim for use with or incorporating a chart having a co-ordinate system, and further comprising means to establish a relationship between the chart co-ordinate system and a co-ordinate system of the charting surface, and means to convert positions between the two co-ordinate systems.

8. An aid as claimed in claim 7, and further comprising means to determine and display the current position of the cursor device in terms of the chart co-ordinate system.

9. An aid as claimed in claim 8, wherein the current position is displayed substantially instantaneously and is repeatedly updated.

10. An aid as claimed in any of claims 7 to 9, and further comprising means to determine and display the defined position in terms of the chart co-ordinate system.

11. An aid as claimed in any of claims 7 to 10, and further comprising means to determine and display the distance between the current and defined positions in units derivable from the chart co-ordinate system.

12. An aid as claimed in any of claims 7 to 11, and further comprising means to determine and display the bearing of the defined position from the current position, and/or vice versa, in terms of the chart co-ordinate system.

13. An aid as claimed in claim 12, and further comprising means to store a bearing offset, the bearing display means displaying the sum or difference of the determined bearing and the offset.

14. An aid as claimed in any of claims 7 to 13, and further comprising manual data entry means to enter a position in terms of the chart co-ordinate system and to set the defined position as that entered position.

15. An aid as claimed in any of claims 7 to 14, and further comprising means to receive an input data signal indicative of a position in terms of the chart co-ordinate system and to set the defined position as that input position.

16. An aid as claimed in any of claims 7 to 15, and further including a clock means, and means responsive to the clock means, to speed and course signals, and to the defined position signal to determine and display an updated position.

17. An aid as claimed in claim 16, and further comprising manual data entry means to enter the speed and course.

18. An aid as claimed in claim 16 or 17, and comprising input means to receive signals indicative of the speed and course.

19. An aid as claimed in any of claims 16 to 18, further comprising manual data entry means to enter corrections to the speed and course, the updated position taking into account the corrections.

20. An aid as claimed in any of claims 16 to 19, when appendent to claim 5, wherein the directing means is selectably responsive to the updated position rather than the defined position.

21. An aid as claimed in any of claims 18 to 20 and including means to store and recall a plurality of such defined positions.

22. An aid as claimed in claim 21, wherein the defined positions are stored in terms of the chart coordinate system.

23. An aid as claimed in claim 21 or 22, when appendent to any of claims 16 to 20, and further including means to determine and display in units derivable from the chart coordinate system the distance of the updated position along a course between two such defined positions and the distance from the updated position to that course.

24. An aid as claimed in any of the preceding claims which specifies a display means, wherein the display means is provided on the cursor device.

25. An aid as claimed in 24 and for which at least two display means are specified, wherein at least some of the display means have a common display.

26. An aid as claimed in any of the preceding claims which specifies a manual data entry means, wherein the data entry means is provided on or by the cursor device.

27. An aid as claimed in claim 26, wherein data to be entered is variable prior to entry in dependence upon movement of the cursor device with respect to the charting surface.

28. An aid as claimed in claim 26 or 27, and for which at least two manual data entry means are specified, wherein the data entry means have a common key pad.

29. An navigation aid substantially as described in the description with reference to the drawings.

30. An electronic mouse having means to sense its position in terms of a first co-ordinate system, means to convert a sensed position to being in terms of a second co-ordinate system, and means to indicate the position in terms of the second co-ordinate system.

31. A mouse as claimed in claim 30, wherein the first co-ordinate system is a planar co-ordinate system, and the second coordinate system is longitude and latitude related.

32. An electronic mouse having means to input a position in terms of a first co-ordinate system, means to convert such an input position to being in terms of a second co-ordinate system with which the mouse can relate, and means to indicate where the mouse should go to reach the position.

33. A mouse as claimed in claim 32, wherein the co-ordinate system is longitude and latitude related and the second co-ordinate system is a planar co-ordinate system.

34. A chart reading apparatus of the type having a charting surface, a mouse movable over the surface and means to sense the position of the mouse on the surface, characterised by means selectable to vary a stored value unrelated to such position in response to movement of the mouse with respect to the surface.

35. A mouse as claimed in any of claims 30 to 34 and further having any of the features specified in any of claims 1 to 28.

36. An electronic mouse substantially as described in the description with reference to the drawings.