Abstract:
An apparatus to enable one&#39;s position to be easily found on a traditional map having its own coordinate system distinct from a wide area coordinate system, data is associated with the map from which the relationship—between the map coordinate system and the wide area coordinate system can be determined. The apparatus has a reader for reading the data associated with the map, a memory for storing the read data, a GPS receiver for receiving transmissions from GPS satellites and determining the position of the GPS receiver in terms of the wide area coordinate system, a processor for processing the determined wide area coordinate position and the stored data to determine the position of the GPS receiver in terms of the map coordinate system, and a display or the like for annunciating to a user the determined map coordinate position. The data associated with the map preferably also includes information on the area covered by the map. The map may be one of a series of maps or part of an atlas, the data relating to all of the maps in the series or atlas.

Description:
This is a divisional of U.S. patent application Ser. No. 09/077,118 filed Jul. 20, 1998, now U.S. Pat. No. 6,304,819 which is a national phase entry from PCT/GB96/02849 filed Nov. 19, 1996. 
    
    
     This invention relates to the location of positions on maps. 
     Various systems are on the market for facilitating the location of one&#39;s position on a map. One example, called the Silva GPS Compass, marketed by Silva Sweden AB, of Sollentuna, Sweden, is a portable device which includes a global positioning system (“GPS”) receiver and can display the current position of the receiver in terms of longitude and latitude with an accuracy of about 100 m under the current level of “selective availability” applied by the U.S. Department of Defense who operate GPS. When used with a map having longitude and latitude grid lines, one&#39;s position on the map can be located. However, there are two main disadvantages with this system. First, many maps (particularly road and street atlases and steeet plans) do not have any longitude and latitude markings whatsoever. Many other maps (for example British Ordnance Survey (“OSGB”) maps), although having longitude and latitude markings around their border and 5 minute graticule markings on the map, do not have any longitude and latitude grid lines drawn across the map. Thus it can be difficult to locate one&#39;s position on the map. Second, because longitude and latitude is a global co-ordinate system, much more information is provided by full longitude and latitude co-ordinates than is actually required to locate one&#39;s position on a map covering only a minuscule part of the surface area of the planet, and this can lead to confusion. For example, the old UK Patent Office building in Southampton Buildings, off Chancery Lane, London is situated at (51°30′59″N, −0°06′34″E), but it is difficult to locate that position on the relevant map in the OSGB 1:50,000 Landranger Second Series, Sheet 176, which covers latitudes 51°19′N to 51°41′N approximately and longitudes −0°03′E to −0°38′E approximately, but without any longitude and latitude grid lines. Many GPS receivers also produce co-ordinates in various map-grid systems relevant to different countries&#39; mapping systems. The conversion between latitude and longitude and such map-grid systems is made using known and documented algorithms. Throughout this specification, latitude and longitude is therefore used as an example of any wide-area co-ordinate system as hereinafter defined. 
     A system which deals with some of the above problems is foreshadowed in patent document WO-A-87/07013 and is marketed by Yeoman Marine Limited, of Lymington, Hampshire, United Kingdom, under the name “Navigator&#39;s Yeoman”. Also, an accessory is available for the Silva GPS compass, called the “Silva Yeoman Navimap”, which has a similar effect. These systems use (a) a digitising table on which a traditional map can be placed, (b) a cursor which is movable across the map on the table and has indicators to indicate a required direction of movement, (c) a GPS receiver (or an input to receive a signal from a separate GPS receiver) from which one&#39;s current position can be determined, and (d) a processor which determines, from the cursor position on the digitising table and the current GPS position, which way the cursor should be moved so that it is over the current position and activates the indicator lights on the cursor accordingly. Whilst this system has the advantages that it can be used with any traditional map to any scale and of any part of the world, and one&#39;s current position can be located by following the indicators rather than thinking in terms of latitude and longitude, it does suffer from two disadvantages. First, before it can be used, the map must be manually referenced with respect to the digitising table so that the processor can translate between the latitude and longitude co-ordinates obtained from the GPS receiver and the co-ordinates of the digitiser table. Referencing can be performed by placing the cursor at two (and preferably three) positions on the chart, and at each position instructing the processor with the latitude and longitude co-ordinates of that position. Alternatively, when used for relative, rather than absolute, positioning when one&#39;s current position on the map is known, referencing can be carried out by registering the cursor on the current known position on map and then indicating to the apparatus the north direction of the map and its scale. Second, it requires the use of a digitising table which is as large as the map, or at least as large as a folded portion of the map which is to be viewed at one time without re-referencing. 
     Another apparatus which deals with this problem is described in patent document WO-A-95/22742, in which a paper map is placed behind a transparent liquid crystal display which can cover the whole area of the map. The map has a mark, for example in the form of a bar code, which can be read by the apparatus and used to correlate positions on the map with latitude and longitude positions. The apparatus also includes a GPS receiver, and the current position obtained from the receiver is converted into a map position, which is then indicated on the LCD by a graphic image, such as crosshairs, to show the current position on the map. However, this apparatus suffers from a number of disadvantages. First, a liquid crystal graphic display is required which is as large as the map, which is thus expensive, cumbersome and liable to be damaged. Second, it is necessary for the map to be used opened out, and the apparatus cannot apparently deal with folded maps. Third, the apparatus can apparently be used only with single sheet maps. 
     One solution to these problems is to “computerise” the map. In the SkyMap system marketed by Skyforce Avionics Limited of Ramsgate, Kent, United Kingdom, which is primarily designed for aircraft navigation, a representation of the map is held in computer memory. The current position is determined using a GPS receiver, and a relevant part of the map, together with the current position, is displayed on a liquid crystal graphic display. The disadvantages of this sort of system are that (a) it would be extremely expensive if a large, high-resolution, map and colour display were to be used; and (b) many people prefer to use a traditional map. The SkyMap system also has the ability to display the full OSGB grid co-ordinates of the current position so that the position can be located on an OSGB map. However, OSGB maps are not indexed according to the grid reference covered, only according to sheet numbers, and the sheet numbers depend on the scale and series of the maps. Therefore it is not straightforward to select the appropriate map. Also, for a resolution of 100 m, the full eight digit (or two letter and six digit) OSGB grid reference is not needed and not normally used when referring to a particular 1:50,000 sheet, and therefore providing all eight digits can lead to unnecessary confusion. 
     The present invention is concerned with dealing with the problems mentioned above of the known systems. 
     In this specification, the following terms are intended to have the following meanings: “Traditional map” means a map which is printed on paper, linen, plastics sheet or the like: “Map co-ordinate system” means a system of co-ordinates which are normally used to define a position on a particular map, and differs from a “Wide-area co-ordinate system” which includes the longitude and latitude system for defining positions on the earth&#39;s surface, but also includes other co-ordinate systems. For example, when considering a single map in the OSGB 1:50,000 series, for instance Sheet  176  mentioned above, the co-ordinates ( 312 ,  815 ) are co-ordinates according to the map co-ordinate system for that map, and differ not only from the co-ordinates (51.5164°N, −0.1095°E) according to the longitude and latitude co-ordinate system but also from the full OSGB co-ordinates ( 5312 ,  1815 ) or (TQ,  312 ,  815 ) according to the complete wide-area OSGB coordinate system; and “Wide-area position transmitting system” means a system which transmits signals which can be received over a large area and which can be processed so as to determine the receiver&#39;s position in terms of a wide-area co-ordinate system. An example of a wide-area position transmitting system is “GPS”, which is operated by the U.S. Departnent of Defense and comprises a couple of dozen or so transmitters which orbit the earth and transmit signals. When the signals from three or more transmitters are being received at a single site, they can be processed so as to determine the position of that site in terms of longitude and latitude. However, the term “wide-area position transmitting system” is intended to include not only other satellite positioning systems, but also terrestrial positioning systems which rely on transmissions from land-based transmitters, and from which one&#39;s position in terms of a wide-area co-ordinate system can be determined. 
     SUMMARY OF THE INVENTION 
     First and second aspects of the invention are concerned in particular with a position locating apparatus, for use with a traditional map having its own co-ordinate system distinct from a wide-area co-ordinate system and for use with conversion data associated with the map (e.g. in the form of a bar code, magnetic stripe, smart card or encoded text) from which the relationship between the map co-ordinate system and the wide-area co-ordinate system can be determined, the apparatus comprising: means for receiving the conversion data; means for receiving transmissions from a wide-area position transmitting system and determining therefrom the current position of the transmission receiving means in terms of the wide-area coordinate system; means for processing the current wide-area co-ordinate position and the conversion data to determine the current position in terms of the map co-ordinate system: and means (e.g. a display or speech synthesiser) for annunciating to a user the current map co-ordinate position. Such a system is disclosed in WO-95/22742 mentioned above. 
     The first aspect of the invention is characterised in that: the processing means is operable to determine the current map co-ordinate position as a coarse position relative to the map and a fine position relative to the coarse position; and the annunciating means comprises means for annunciating the coarse position and means for annunciating the fine position. 
     The aspect of the invention therefore provides the advantage that a form of annunciation which is suitable for coarse positioning can be used for that purpose, and another form of annunciation which is suitable for fine positioning can be used for that purpose. 
     In one embodiment for use with such a map which is divided up into an array of blocks, the coarse position annunciating means is preferably operable to annunciate an identity of that one of the blocks containing the current position, as in the above example “Block M 42”. 
     For annunciating the fine position, each block could, for example, be notionally divided in ten in each of the x and y directions, and the location of the old Patent Office building could be annunciated as “Page 62, Block M 42, Position (1, 7)”. However, this may cause confusion, for example as to whether it means 1/10 along and 7/10 up, or 1/10 up and 7/10 along. To deal with this, and the fine position annunciating means preferably comprises a graphical display and means for activating the display to display the fine position relative to a datum point. Preferably, the display comprises a see-through display which can be registered over the identified block on the map and which is operated to indicate the current position within that block. Accordingly, the display need only be as large as a block on the map, and need not be as large as the whole map. 
     The apparatus may be used with such conversion data from which the area covered by the map can be determined, and in this case the processing means is preferably operable to determine whether the current position is covered by the map. In this way, invalid or inappropriate annunciations, which might confuse the user, can be avoided. 
     The apparatus may be used with a series of such maps and with such conversion data from which the relationship between the co-ordinate system(s) of the maps and the wide-area co-ordinate system can be determined and the area covered by each map can be determined. In this case, preferably the processing means is operable to determine (a) the identity of that map, or at least one of those maps, which cover the current position, and (b) the current position on that map in terms of the co-ordinate system of that map; and the annunciating means is operable to annunciate to the user (a) the identity of that map and (b) the current position in terms of the co-ordinate system of that map. Thus, the user can be assisted in selecting the appropriate map. Thus, for example, in the case of the OSGB 1:50,000 Landranger Second Series, the data for all two hundred and four sheets in the series may be stored, and in the case of the position mentioned above, the annunciation may be in the form “Sheet 176, grid block (31, 81).” 
     This latter feature may be provided in an apparatus which does not possess the other features of the first aspect of the invention. Accordingly, the second aspect of the invention is characterised in that: the apparatus is for use with a series of such maps and for use with such conversion data from which the relationship between the co-ordinate system(s) of the maps and the wide-area co-ordinate system can be determined and the area covered by each map can be determined; the processing means is operable to determine (a) the identity of that map, or at least one of those maps, which cover the current position, and (b) the current position on that map in terms of the co-ordinate system of that map; and the annunciating means is operable to annunciate to the user (a) the identity of that map and (b) the current position in terms of the co-ordinate position of that map. 
     The apparatus may be used with such maps having some portions which overlap, and in this case preferably the processing mans is operable, in the case of a position falling on more than one of the maps, to determine (a) the identity of two or more of the maps which cover the current position, and (b) the current position(s) on those maps in terms of the co-ordinate system(s) of those maps; and the annunciating means is operable to annunciate to the user (a) the identities of those maps and (b) each current position in terms of the co-ordinate system of the respective map. Thus, when used with an atlas, for example, the apparatus does not force the user to turn a page when that may not be necessary. 
     The apparatus may be used with such maps which are not all to the same scale and with such conversion data which takes into account the different scales of the maps, and in this case preferably the processing means is operable to take into account the scale of the maps in determining the current position in terms of the map co-ordinate system(s). For example, the Geographers&#39; A-Z Master Atlas of Greater London, Edition 4(B), has a first set of maps on pages 2 to 160 covering Greater London to a scale of 3″ to 1 mile (1:21,120), a second set of maps on pages 162 to 183 covering central London to a larger scale of 9″ to 1 mile (1:7,040) and a third set of maps covering London and its environs to a smaller scale of 2½ miles to 1″ (1:158,400). The site of the old UK Patent Office building off Chancery Lane is covered by all three sets of maps, on pages 62, 173 and 186, as shown in FIGS. 1,  2  and  3 , respectively, of the accompanying drawings. In accordance with this feature of the invention, the position of that building could be annunciated as “Page 62, Block M 42”, “Page 173, Block G 2” and “Page 186, Block (30, 80)”, simultaneously, or one after another. 
     The apparatus may be used with such conversion data from which the relationship between the map co-ordinate system and at least one datum point of the medium on which the map is formed can he determined, and in this case one embodiment of the apparatus tier includes: a digitising surface having its own co-ordinate system means for referencing the map with respect to the digitising surface; a cursor which is movable with respect to the referenced map and the digitising surface; means for determining the position of the cursor with respect to the digitising surface in terms of the digitising surface&#39;s co-ordinate system; means for processing the current wide-area co-ordinate position and the conversion data to determine the current position in terms of the digitising surface&#39;s co-ordinate system and/or for processing the cursor position and the conversion data to determine the cursor position in terms of the wide-area co-ordinate system; and means for annunciating to a user the current position in terms of the digitising surface co-ordinate system and/or the cursor position in terms of the wide-area co-ordinate system; wherein the referencing means comprises means for registering the or each datum point of the map medium with respect to the digitising surface. 
     These latter features may be provided in an apparatus which does not possess the other features of the first and second aspects of the invention. Accordingly, a third aspect of the present invention is concerned with a position locating apparatus, for use with a traditional map having its own co-ordinate system distinct from a wide-area co-ordinate system and for use with conversion data associated with the map from which the relationship between the map co-ordinate system and the wide-area co-ordinate system can be determined, the apparatus comprising: a digitising surface having its own co-ornate system; means for referencing the map with respect to the digitising surface; a cursor which is movable with respect to the referenced map and the digitising surface; means for determining the position of the cursor with respect to the digitising surface in terms of the digitising surface&#39;s co-ordinate system; means for receiving the conversion data; means for receiving transmissions from a wide-area position transmitting system and determining therefrom the current position of the transmission receiving means in terms of the wide-area co-ordinate system; means for processing the current wide-area co-ordinate position and the conversion data to determine the current position in terms of the digitising surface&#39;s co-ordinate system and/or for processing the cursor position and the conversion data to determine the cursor position in terms of the wide-area co-ordinate system; and means for annunciating to a user the current position in terms of the digitising surface co-ordinate system and/or the cursor position in terms of the wide-area co-ordinate system. Such an apparatus is known from WO-A-87/07013 mentioned above. The third aspect of the invention is characterised in that: the apparatus is for use with such conversion data from which the relationship been the map co-ordinate system and at least one datum point of the medium on which the map is formed can be determined; and the referencing means comprises means for registering the or each datum point of the map medium with respect to the digitising surface. Thus, by making use of the conversion data and the registering means, referencing of the map with respect to the digitising surface can be greatly facilitated. 
     Preferably, the annunciating means is operable to annunciate the current position by indicating a required direction of movement of the cursor towards that position. Also, preferably, storing means is provided for storing a target position; the processing means is also operable to calculate the distance between the stored position and the current position and/or the bearing of the stored and current positions relative to each other; and the annunciating means is also operable to annunciate the calculated distance and/or bearing. 
     A fourth aspect of the present invention is concerned with a map having: its own co-ordinate system distinct from a wide-area co-ordinate system; and conversion data associated therewith from which the relationship between the co-ordinate system of the map and a wide-area co-ordinate system can be determined. As will be appreciated from the above, such a map is known from WO-A-95/22742. The fourth aspect of the invention is characterised in that: the map is one of a series of such maps; and the conversion data is collectively provided for all of the maps in the series. Accordingly the maps can be used in a system which can indicate to the user which of the maps to use. 
     Preferably, the area covered by each map can be determined from the conversion data. Some portions of the maps may overlap. Not all of the maps need be to the same scale, and in this case the conversion data preferably takes into account the different scales of the maps. The maps in the series may be bound together in the form of an atlas. When used with the apparatus described above, the apparatus can then tell the user to which page of the atlas to turn, and where on that page to look. The conversion data may be provided on or in the maps or atlas, for example in the form of a bar code or magnetic stripe printed on the map or inside cover of the atlas, or a smart card forming an additional page to the atlas, or which is removably inserted in a pocket or the like on the map or in the atlas. The conversion data includes at least one parameter related to the cartographic projection(s) of the maps, so that the system can accurately locate positions on maps employing different cartographic projections. In one embodiment, the conversion data is machine-readable, and the data receiving means of the apparatus may then comprise means for reading the machine-readable data. Alternatively, the conversion data may be provided as user-readable encoded text, and the data receiving means of the apparatus may then comprise means (such as a keypad) to enable a user to enter the encoded text and means for decoding the entered text. 
     A fifth aspect of the present invention provides a position locating system, comprising: an apparatus according to any of the first to third aspects of the invention; a traditional map having its own co-ordinate system distinct from a wide-area co-ordinate system; and conversion data associated with the map from which the relationship between the map co-ordinate system and the wide-area co-ordinate system can be determined. 
     A sixth aspect of the present invention provides a position locating system, comprising: an apparatus according to any of the first to third aspects of the invention; and a series of maps and associated data according to the fourth aspect of the invention. 
     In the fifth or sixth aspect of the invention, when the fine position annunciating means comprises a graphical display, the display is preferably substantially smaller in area than the or each map. 
     In accordance with a seventh aspect of the present invention, there is provided a data carrier per se associated with a series of maps and containing data from which the relationship between the co-ordinate system(s) of the map and the wide-area co-ordinate system can be determined, the data being machine-readable or being encoded user-readable text. 
     It may be appreciated from the above that at least certain embodiments of the invention provide the advantages over the known systems described above that; (1) there is no need for a digitising surface or the like, although in one version of the invention a digitising surface may be employed; (2) there is no need for a graphics display which is as large as the map; (3) traditional maps can be used, the only requirement being the additional data associated with the map, this being possible at low cost; (4) the user does not have any difficulty in selecting the appropriate map; (5) the user does not need to concern themself with latitude and longitude, but instead coordinate data is provided which is more relevant to the map being used. Using the example given above, instead of (or in addition to) being presented with the latitude and longitude co-ordinates (51°30′59″N, −0°06′34″E), the user might instead be presented with the abbreviated OSGB grid block for the position on Sheet  176 , i.e. “(31, 81)” or with the abbreviated higher resolution grid reference “(312, 815)”; and (6) referencing is carried out simply by entering the data associated with the map. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A specific embodiment of the invention and some modifications and developments thereto will now be described by way of example with reference to the accompanying drawings, in which: 
     FIGS. 1-3 show portions of pages 62, 173 and 186, respectively, of the Master Atlas of Greater London, Edition 4(B), published by Geographers&#39; A-Z Map Company Limited, Sevenoaks, Kent; 
     FIG. 4 is a block diagram showing functional elements of one embodiment of apparatus for use in performing the invention; 
     FIG. 5 is top view of the apparatus of FIG. 4; and 
     FIG. 6 is a top view of another embodiment of the apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Conversion of Wide-area Co-ordinates to Map Co-ordinates 
     In the remainder of this description: θ and φ denote the wide area co-ordinates of a position on the surface of the earth in terms of longitude (θ) and latitude (φ) with the sign convention that east of the Greenwich meridian is positive and north of the equator is positive; ψ denotes the product of the longitude and the cosine of the latitude of a position on the surface of the earth, i.e.: 
     
       
         ψ=θ.cos φ  (1) 
       
     
     and x and y denote coordinates (in the usual directions) of a position on a map in terms of the map&#39;s own co-ordinate system. 
     In the apparatus of the embodiments, is necessary to convert from (θ, φ) to (x, y). In a simplistic “flat-earth” model for maps covering a small part of the earth&#39;s surface, it can be assumed that, for a particular map, x and y are each linear functions of ψ (=θ.cos φ) and φ, i.e.:                (         x       y         )     =       (         ψ       φ       1         )     ·     (         a       b           c       d           e       f         )               (   2   )                                
     Assuming that the x direction of the map is approximately parallel to the lines of latitude, then in the conversion matrix above, the elements a and d relate predominantly to the scale of the map in the x and y directions, the elements e and f relate predominantly to the position of the map, and the elements b and c account for any skew between the x direction of the map and the lines of latitude. By taking the wide-area co-ordinates (θ 1 , φ 1 ), (θ 2 φ 2 ), (θ 3 , φ 3 ) of three positions on the surface of the earth and by taking the map co-ordinates (x 1 , y 1), (x   2 , y 2 ), (x 3 , y 3 ) of the corresponding three positions on a particular map, it is possible to formulate six simultaneous equations derived from Formula 2 which can be represented by:                (           x   1           y   1               x   2           y   2               x   3           y   3           )     =       (           ψ   1           φ   1         1             ψ   2           φ   2         1             ψ   3           φ   3         1         )     ·     (         a       b           c       d           e       f         )               (   3   )                                
     These simultaneous equations can then be solved to obtain the unknown values a to f of the conversion matrix by multiplying both sides by the inverse of the matrix containing the wide-area co-ordinates:                (         a       b           c       d           e       f         )     =         (           ψ   1           φ   1         1             ψ   2           φ   2         1             ψ   3           φ   3         1         )       -   1       ·     (           x   1           y   1               x   2           y   2               x   3           y   3           )               (   4   )                 ∴     (         a       b           c       d           e       f         )       =         (             φ   2     -     φ   3               φ   3     -     φ   1               φ   1     -     φ   2                   ψ   3     -     ψ   2               ψ   1     -     ψ   3               ψ   2     -     ψ   1                     ψ   2                     φ   3       -       ψ   3                     φ   2                   ψ   3                     φ   1       -       ψ   1                     φ   3                   ψ   1                     φ   2       -       ψ   2                     φ   1               )     ·     (                      x   1           y   1               x   2           y   2               x   3           y   3                      )           (           ψ   1           ψ   2           ψ   3           )     ·     (             φ   2     -     φ   3                   φ   3     -     φ   1                   φ   1     -     φ   2             )                 (   5   )                                
     As an example, considering the above-mentioned OSGB map sheet  176 , and taking the three positions as the bottom-left comer, bottom right corner and the midpoint along the top edge of the map, this gives: 
     
       
         ( x   1   , y   1 )=(495 km, 160 km) (θ 1 , φ 1 )=(−0.6367°, 51.3308°)∴ψ 1 =−0.39782° 
       
     
     
       
         ( x   2   , y   2 )=(535 km, 160 km) (θ 2 , φ 2 )=(−0.0628°, 51.3225°)∴ψ 2 =−0.03925° 
       
     
     
       
         ( x   3   , y   3 )=(515 km, 200 km) (θ 3 , φ 3 )=(−0.3358°, 51.6860°)∴ψ 3 =−0.20819° 
       
     
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           a 
                         
                         
                           b 
                         
                       
                       
                         
                           c 
                         
                         
                           d 
                         
                       
                       
                         
                           e 
                         
                         
                           f 
                         
                       
                     
                     ) 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             111.47994 
                              
                             
                                 
                             
                              
                             km 
                              
                             
                               / 
                             
                              
                             ° 
                           
                         
                         
                           
                             2.574881 
                              
                             
                                 
                             
                              
                             km 
                              
                             
                               / 
                             
                              
                             ° 
                           
                         
                       
                       
                         
                           
                             
                               - 
                               3.2092944 
                             
                              
                             
                                 
                             
                              
                             km 
                              
                             
                               / 
                             
                              
                             ° 
                           
                         
                         
                           
                             111.23796 
                              
                             
                                 
                             
                              
                             km 
                              
                             
                               / 
                             
                              
                             ° 
                           
                         
                       
                       
                         
                           
                             704.10065 
                              
                             
                                 
                             
                              
                             km 
                              
                             
                               / 
                             
                              
                             ° 
                           
                         
                         
                           
                             
                               - 
                               5548.9032 
                             
                              
                             
                                 
                             
                              
                             km 
                              
                             
                               / 
                             
                              
                             ° 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
                 
         
             
         
      
     
     To evaluate the accuracy of conversion, the wide-area co-ordinates for the other two corners of sheet  176  are (θ 4 ,φ 4 )=(−0.6250°, 51.6903°) and therefore ψ 4 =−0.38744° at the top-left corner and (θ 5 , φ 5 )=(−0.0464°, 51.6817°) and therefore ψ 5 =−0.02877° at the top-right corner. Applying the conversion formula 2 using the conversion matrix of formula 6 for sheet  176  gives map co-ordinates (x 4 , y 4 )=(495.019 km, 200.023 km) and (x 5 , y 5 )=(535.032 km, 199.990 km). On the map, the co-ordinates of these two corners are, in fact, (495 km, 200 km) and (535 km, 200 km), and thus there are discrepancies of: 
     
       
         {square root over ( )}((495.019−495) 2 +(200.023−200) 2 ) km=30 m; 
       
     
     and 
     
       
         {square root over ( )}((535.032−535) 2 +(199.990−200) 2 ) km=34 m 
       
     
     which are less than the inccuracy provided by GPS and therefore not a limiting factor. On the paper of the map, the discrepancies are 1/50,000 of these amounts, that is 0.60 mm and 0.68 mm, respectively. 
     Testing whether a Position is within the Boundary of a Map 
     When the invention is applied to a single map, it is preferable that a determination is made as to whether the current position is within the boundary of the map, so as to avoid attempting to calculate and annunciate map coordinates which are not covered by the map. Also, when the invention is applied to a series of maps, it is preferable that a determination is made as to whether the current position is within the boundary of any of the maps, and if so which one(s). Due to the nature of the longitude and latitude wide-area co-ordinate system, relatively few maps have boundaries which extend along lines of constant longitude or constant latitude. It would be possible to develop a formula defining each boundary line in terms of both longitude and latitude. However, the invention preferably takes advantage of the fact that the majority of maps are square or rectangular and their boundaries run parallel and at right angles to the map&#39;s co-ordinate system. Accordingly, it will be appreciated, from formula 2, that if the co-ordinates of the bottom-left and top-right corners of a map are (x L , y B ) and (x R , y T ), respectively, then a position having wide-area co-ordinals (θ, φ) and accordingly a value ψ=θ.cos φ will fall on the map if it satisfies both of the relationships: 
     
       
           x   L   ≦aψ+cφ+e≦x   R   (7) 
       
     
     
       
           y   B   ≦bψ+dφ+f≦y   T   (8) 
       
     
     Thus, given the wide-area co-ordinates (θ, φ) of a position, the values a to f of the conversion matrix for a particular map, and the co-ordinates (x L , y B ) and (x R , y T ) in terms of the map&#39;s co-ordinate system of two, diagonally-opposite corners of the map, it is simple to test whether that position falls on that map. 
     The mapped region of some maps is not a simple rectangle or square. For example, maps often have a small legend rectangle at one comer. In this case, the mapped area can be divided up into, say, two rectangular sub-tnaps, each having the same conversion matrix, but with different values of x L , x R , y T  and y B . 
     Different Co-ordinate Styles 
     It will be appreciated that different maps have different styles for representing their co-ordinates. For example, the co-ordinates of the grid lines on most OSGB sheets, in each of the x and y directions, consist in full of a three digit number, e.g. the old Patent Office site mentioned above is in OSGB grid block ( 31 ,  81 ), but the most significant digits are not normally used and are usually marked only at the corners of the map. In the first set of maps in the A-Z Master Atlas of Greater London mentioned above, the y co-ordinates of the map blocks are numerical but increasing in the downwards direction, as seen in FIG. 1, and the x co-ordinates are alphabetical with repeats, but the same letter is never repeated on two adjacent pages of the atlas. As shown in FIG. 2, in the second set of maps of that atlas, the x co-ordinates of the map blocks are alphabetical, but re-starting at “A” on each double page. As shown in FIG. 3, the third set of maps are marked with OSGB grid lines at 10 km intervals. In the embodiment of the invention, all map co-ordinates (x, y) are stored and processed numerically until they are displayed, when they are converted into their proper form (X, Y) in dependence upon co-ordinate style codes S x  and S y  for the map or set of maps in question. 
     The Data Associated with a Map or Maps 
     In the case of a single map, the data which is provided may comprise: a “single map” flag set at  1  indicating that the data relates to a single map; the six values a to f of the conversion matrix; the four values x L , x R , y T  and y B  defining the map&#39;s boundaries; two codes S x , S y  representing the x and y co-ordinate styles of the map; and the name of the map. 
     In the example given above, the data night consist of: 111.47994; 2.574881; −3.2092944; 111.23796; 704.10065; −5548.9032; 495; 535; 160; 200; S x ; S y ; “OSGB Landranger Series 2 Sheet 176”. 
     The eleven data items may be printed in the form of a bar code or magnetic stripe on the map. 
     In the case of a series of maps, the data which is provided may comprise: 
     the single map flag, but set to 0 indicating that the data relates to more than one map; 
     the name of the series of maps or the atlas (e.g. “A-Z Master Atlas Greater London”); 
     the number S of sets of maps in the series or atlas (e.g. “3”); 
     {for each set of the S sets:-} 
     two codes representing the x and y co-ordinate styles of the maps in that set; 
     the number M of maps in that set; 
     {for each map in that set:-} 
     the name of that map (e.g. “Page 62”); 
     the six values a to f of the conversion matrix for that map; and 
     the four values x L , x R , y T  and y B  defining that map&#39;s boundaries. 
     {next map} 
     {next set} 
     Construction of the Apparatus 
     Referring to FIGS. 4 and 5, the apparatus  10  is portable, of so called “palm-top” size, and comprises a casing  12  having a GPS antenna  14  mounted on its top edge and a slot  16  providing access to a smart card reader  18  in its bottom edge. The smart card may form part of a special page of an atlas, or may be removably inserted in a pocket inside the cover of the atlas. Alternatively, the smart card which is insertable into the slot  16  may be a completely separate article, for example containing the data of all 204 sheets in the OSGB Landranger Second Series. The top face of the casing  12  has: an on/off switch  20 ; a mode switch  22  having standby, display and setup positions; a liquid crystal text display  24 ; and a liquid crystal graphical display  26 . Alternatively, these two displays could be combined into one. The graphical display  24  comprises an 11×11 array of pixels, any one  38  of which can be activated to display a position within a grid block. 
     Referring in particular to FIG. 4, the casing also contains: a microprocessor unit  28 ; a GPS receiver  30 ; ROM  32 ; RAM  34 ; and a battery  36 . An external power socket may also be provided to receive power from a mains adaptor to charge the battery or from a vehicle&#39;s cigar lighter socket. 
     Operation of the Apparatus 
     When the on/off switch  20  is “on”, the MPU  28  operates in accordance with a program stored in the ROM  32 . When the mode switch  22  is at “setup”, the smart card reader  18  is activated, and it can read the data mentioned above associated with the map, series of maps or atlas, and supply it to the MPU  28  for processing and storage in the RAM  34 . The GPS receiver  30  is connected to the GPS antenna  14 , and when the mode switch  22  is at “display”, the GPS receiver  30  is activated, and when it receives signals from three or more GPS satellites it supplies the current wide-area co-ordinates (θ, φ) to the MPU. When the mode switch  22  is at “display” or “setup”, the LCDs  24 ,  26  are activated. When the mode switch  22  is at “standby”, the smart-card reader  18 , GPS receiver  30  and LCDs  24 ,  26  are inactive, but the data which has been read continues to be stored in the RAM  34 . 
     The program stored in the ROM  32  causes the apparatus to operate according to the following routines of steps while the on/off switch is “on”: 
     Main Routine 
     R 1  Set to 0 a “data present” flag F indicating whether data associated with a map has been read. 
     R 2  If the mode switch  22  is at “display” jump to step R 5 , or if at “setup” call the Setup subroutine, otherwise if the mode switch  22  is at “standby” continue. 
     R 3  If the displays  24 ,  26  are on, or the GPS receiver  30  is on, switch them off. 
     R 4  Loop back to step R 2 . 
     R 5  If the data present flag F is 0, activate the text display  24  to invite the user to switch to “setup” and loop back to step R 2 , otherwise continue. 
     R 6  If the GPS receiver  30  is off, switch it on. 
     R 7  If the GPS receiver  30  is not producing wide-area co-ordinate signals, activate the text display  24  to inform the user that the GPS position is awaited and loop back to step R 2 , otherwise continue. 
     R 8  Store the wide-area co-ordinates (θ, φ) from the GPS receiver  30  in the RAM  34 . 
     R 9  From the stored wide-area co-ordinates (θ, φ) and Formula 1, calculate the value of ψ and store it in the RAM  34 . 
     R 10  If the single map flag in the read data is 1, call the Single Calculation subroutine, otherwise call the Multi Calculation subroutine. 
     R 11  Loop back to step R 2 . 
     Setup Subroutine 
     S 1  Switch on the smart card reader. 
     S 2  Activate the text display  24  to invite the user to insert the smart card associated with the map, series of maps, or atlas to be used. 
     S 3  Loop on this step until data has successfully been read, and then continue. 
     S 4  Store the read data in the RAM  34 . 
     S 5  Switch off the smart card reader. 
     S 6  Set the data present flag to 1. 
     S 7  Activate the text display  24  to inform the user of the name of the map, series of maps or atlas, and that the data has been successfully read. 
     S 8  If the mode switch is at “setup”, loop on this step, otherwise continue. 
     S 9  Return to the main routine. 
     Single Calculation Subroutine 
     C 1  If, from Formulae 7 and 8 and the stored data, it is determined that the current position is not covered by the map, activate the text display  24  to inform the user that the current position is off the map and return to the main routine, otherwise continue. 
     C 2  From Formula 2 and the stored data, calculate the map position (x, y) and store it in the RAM  34 . 
     C 3  From the stored data, convert the style of the map position to (X, Y) and store it in the RAM  34 . 
     C 4  Calculate a fractional part (f X , f Y ) of the map position within the grid block (X, Y) and store it in the RAM  34 . 
     C 5  Activate the text display  24  to inform the user of the name of the map and the stored map position (X, Y). 
     C 6  Activate that pixel  38  of the graphical display  26  corresponding to the stored fractional part (f x , f y ) of the map position. 
     C 7  Return to the main routine. 
     Multi Calculation Subroutine 
     M 1  Set a counter C to 0. 
     M 2  For each set s of the S sets of maps:- 
     M 3  For each map m in the M maps of set s:- 
     M 4  If, from Formulae 7 and 8 and the stored data, it is determined that the current position is not covered by that map m in that set s, jump to step M 10 . 
     M 5  Increment the counter C. 
     M 6  Store the name of map m in set s as N C . 
     M 7  From Formula 2 and the stored data, calculate the position (x C , y C ) and store it in the RAM  34 . 
     M 8  From the stored data, convert the style of the map position to (X C , Y C ) and store it in the RAM  34 . 
     M 9  If C=1, calculate the fractional part (f X , f Y ) of the map position within the grid block (X 1 , Y 1 ) and store it in the RAM  34 . 
     M 10  Next m 
     M 11  Next s 
     M 12  Activate the text display  24  to inform the user of the name of the series of maps or atlas. 
     M 13  For each map I from 1 to C:- 
     M 14  Activate the text display  24  to inform the user of the name N i  of the map. 
     M 15  Activate the text display  24  to inform the user of the stored map position (X i , Y i ). 
     M 16  Next I. 
     M 17  Activate that pixel  38  of the graphical display  26  corresponding to the stored fractional part (f x , f y ) of the map position on map N 1 . 
     M 18  Return to the main routine. 
     Example Display 
     In the case of the position of the old UK Patent Office Building on the A-Z Master Atlas of Greater London, as mentioned above, an example of the display which might he produced is shown in FIG.  5 . The top part of the text display  24  shows the name of the atlas. This is followed by the names of those maps (page numbers) which show that position and, for each page number, the grid block which contains that position. The example display assumes that the data which is read from the atlas contains the data for the large scale set of maps, then for the medium scale set of maps, and then for the small scale set of maps. The graphical display  26  shows the relative position within the grid block of the first map which is listed, i.e. within Block G  2  on Page 173. 
     Second Embodiment 
     FIG. 6 shows a second embodiment of the invention. A digitising tablet  40  has ledges  42 ,  44  along its bottom and left edges against which the bottom and left edges of a map or atlas  46  can be registered. A cursor  48  is movable over the atlas and incorporates many or all of the features of the apparatus  10  of FIG.  5 . The cursor  48  and digitising tablet  40  co-operate in a known way so that the MPU  28  can determine the position of a reference point  50  of the cursor  48  with respect to the co-ordinate system of the digitising tablet  40 , as denoted by dashed lines  51  in the drawing. In this embodiment, the data which is associated with the atlas also includes, for each map, information relating the co-ordinate system of that map with the co-ordinate system of the digitising tablet  40  when the atlas is registered with the ledges  42 ,  44  and the atlas is open at the map in question. For example, in addition to giving the co-ordinates (x L , y B ) and (x R , y T ) of two diagonally opposite corners of the map in terms of the map&#39;s co-ordinate system for boundary testing purposes, as described above, the data may also include the co-ordinates (u L , v B ) and (u R , v T ), as shown in FIG. 6, of those two corners in terms of the digitising tablet&#39;s co-ordinate system. Once the relationships between the three co-ordinate systems have been established, the MPU  28  can, for example, determine the direction in which the cursor should be moved so as to lie over a particular point, and indicate that direction by illuminating one or two of four lights  52  on the cursor. (For further information about this technique and modifications thereto, reference is directed to patent document WO-A-87/07013.) The particular point may be the current position, or it may be a target position which has been entered and stored in the RAM  34  by placing the cursor over that position on the map and pressing a push-button  54 . Also, the MPU  28  can calculate the distance between the current position and the stored position, and the bearing of one with respect to the other, and display this information on the display  24 . 
     Modifications and Developments 
     It will be appreciated that many modifications and developments may be made to the embodiments described above. For example, the apparatus may be simplified so that it deals with only one map, or only one page of an atlas, at a time, the data for that map or page being read from, for example, a bar code or magnetic stripe printed on the map or page. 
     In the arrangements described above, the data associated with the map(s) is machine-readable. Alternatively, the data may be provided as user-readable encoded text, and the apparatus may have a key pad or the like to enable a user to enter the encoded text, the text then being decoded by the MPU  28 . 
     In the case where more than one map or atlas page covers a particular position, the apparatus may be arranged to display the details (including the position in the grid block on the graphical display  26 ) of only a selected one of the maps or pages, but with an indication that others are available. A push button may then be provided so that the user can cycle through the available details. Also, in the case where the map positions relate to different sets of the maps, and the selected map ceases to cover the current position, the MIPU  28  may be programed initially to select for display another map from the same series. 
     As described above, the graphical display  26  shows only the current relative position in the grid block. Alternatively, it may be arranged also to show previous positions so that a trail is built up as the user moves their position, with the current position displayed differently to the previous positions, for example blinding. In this case, a push button may be provided whose operation causes the previous positions to be cleared if the display becomes too cluttered. 
     The graphical display may be modified so that it is transparent and can be registered with the appropriate grid block of the map so as to show the current position in that grid block. In this case, the data which is associated with the map would include data from which the actual size of the grid blocks on the paper of the map can be determined. 
     In a simpler form of the apparatus, the graphical display may be omitted. 
     The apparatus may be modified so as to display the current position also in terms of latitude and longitude. 
     The data which is associated with each map may be encrypted, and the MPU  28  may he programmed to perform a suitable decryption algorithm so as to hinder the use of unauthorised data. 
     In the embodiment described above, the MPU  28  tests each of the maps in a predetermined order to determine which of the maps covers the current position. Once a map has been found which covers the current position, if that map then ceases to cover it, the current position should then be covered by a map covering an adjacent part of the earth&#39;s surface. Accordingly, the data provided for each map may also include information on the maps covering adjacent areas, and the MPU  28  may be programmed to use that information in order to increase the speed with which the next map may be found. 
     For maps drawn according to some cartographic projections, Formula 2 above based on a flat-earth model may not be appropriate. In order to deal with this, the MPU  28  may be programmed to be able to perform different transformation functions, for example for flat-earth, Mercator and transverse Mercator, and the data associated with each map or series of maps or atlas may include parameters defining the transformation function to be used for that map, or that series of maps or atlas, or for particular sets of maps in the series or atlas. Alternatively, the data may include the transformation function(s) itself, which is then loaded into the apparatus when the data is read. 
     The information which is provided so that the relationship between the wide-area co-ordinate system and the map&#39;s coordinate system need not be of the form described with reference to Formula 2 above. Instead, it may consist of the wide-area co-ordinates of a predetermined position on the map, the north direction on the map, and the scale of the map. 
     The apparatus need not include a GPS receiver, but instead may have an input to receive an output from a separate GPS receiver. Also, the apparatus may be built in to some other apparatus having other functions. Furthermore, the display may be separately housed, for example on the dashboard of a vehicle, whilst other parts of the apparatus are mounted elsewhere. 
     The invention may be put into effect by providing an accessory for a standard palm-top computer, such as a Psion Organiser. The accessory may comprise a GPS receiver connected to an interface card which may be inserted into one of the card slots of the palm-top computer. Accordingly, the computer, interface card and GPS receiver can perform the functions described above, with the display of the palm-top computer being used to provide both the text display  24  and the graphical display  26  described above. 
     It will be appreciated that many other modifications and developments may also be made and that the description above is not to be taken as limiting the scope of the invention.