Abstract:
A display having a screen providing historical water depth information and a screen providing fish school detection information is presented. The water depth information and the fish school detection information may be derived from an echo signal, and can be displayed simultaneously or alternated separately on the display by switching. When the water depth reaches or is less than a predetermined value, an alarm may be provided on the display and/or an alarm sound may be generated. As the alarm is displayed the brilliance of the display may be raised to a maximum brilliance, making the display brighter so the contents are easier to see.

Description:
This application is a divisional of application Ser. No. 09/662,627, filed on Sep. 15, 2000 now U.S. Pat. No. 6,595,315, the entire contents of which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 11-264478; 11-267893; and 11-344595 filed in Japan on Sep. 17, 1999; Sep. 22, 1999; and Dec. 3, 1999, respectively under 35 U.S.C. §119. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to an underwater detecting apparatus for performing measurement of water depths, detection of a school of fish, or the like by using ultrasonic waves, and a depth measuring apparatus having the same function. Besides, the present invention relates to a display apparatus which is installed in a ship or the like and displays an alarm on a screen in an emergency, and which is provided with a function of displaying guidance as to a key, and a depth measuring apparatus having the same function. 
   A depth measuring apparatus in which an ultrasonic signal is emitted into the water from an echo sounder transducer provided on a ship&#39;s hull and the water depths are measured on the basis of received echo signals, has been conventionally used. In such a conventional depth measuring apparatus, measured data are recorded on a recording sheet and are outputted. 
   However, in the conventional depth measuring apparatus, paper as a recording medium is used, and this is a consumption article so that this is costly, and this is also inconvenient in handling such as exchange of recording sheets or preservation thereof. Besides, in the depth measuring apparatus of such a recording sheet system, a mechanism for making a record on the recording sheet, and this mechanism includes movable portions such as a gear and belt, so that there is also a problem that the apparatus is inferior in reliability. 
   Then, instead of the above recording sheet system, it is conceivable to make a system in which measurement data are displayed on a display instrument such as a liquid crystal display. According to this, the recording sheet becomes unnecessary, and data can be electrically stored, and further, any movable mechanism is not necessary, so that the reliability is improved, and the defects of the conventional apparatus can be overcome. 
   However, since a display instrument mounted on a ship is generally small and the space of a display screen is also limited, when a depth measurement result is displayed on the screen, it is demanded that the largest possible amount of information is displayed efficiently and to be easy to see. 
   Besides, a ship is equipped with various measuring apparatuses such as a radar apparatus, a fish finder, a tidal current meter, a water temperature meter, and a depth measuring apparatus, and information measured by these apparatuses is displayed as an image on a screen of a display instrument. Here, for example, when a water depth to the seabed measured by the depth measuring apparatus becomes a definite value or less, since there is a fear that the ship strands, that must be informed by an alarm. Thus, it is designed such that the alarm to indicate water depth abnormality is displayed on the screen of the display instrument of the depth measuring apparatus, and this alarm is also given by sound. The depth measuring apparatus provided with such an alarm display function is disclosed in, for example, Japanese Patent Unexamined Publication No.223583/1993. 
   At the time of a night cruise or the like of a ship, since the surroundings of the display instrument in a room are dark, when the brilliance of the display instrument is raised, a reflection is formed in a glass window or dazzle is given to a navigator, so that it is not preferable in view of safety of the cruise. Then, at a place where the surroundings are dark, the display instrument is set at low brilliance when in use. 
   However, in the case where some alarm is raised in a state where the display instrument is set at low brilliance, the contents of the alarm displayed on the screen are difficult to understand, since the display screen is dark. In order to accurately grasp this, the brilliance of the screen should be again set at high brilliance to confirm the alarm. Thus, it takes a time to take measures, and there has been a danger that a serious accident such as stranding is caused by a time delay during this. Besides, in an instrument which gives an alarm by sound as well, in the case where there are noises in the surroundings, it is difficult to recognize the alarm sound, so that the danger is further increased. 
   Besides, a ship is equipped with various measuring apparatuses such as a radar apparatus, a fish finder, a tidal current meter, a water temperature meter, and a depth measuring apparatus, and information measured by these apparatuses is displayed as an image on a screen of a display apparatus. In the display apparatus, an operation portion in which a plurality of keys are arranged is provided, and these keys are operated so that various settings, instructions and the like are executed. 
   For example, in a depth measuring apparatus, since the position of the bottom of a ship on which a transducer of an ultrasonic beam is provided is not coincident with the position of the sea surface, a draft value of a difference therebetween is inputted through a key in advance, and a correction of a water depth value is performed by using this draft value when the water depth is calculated. Besides, in order to prevent stranding of a ship, an alarm water depth value is set by a key, and when the water depth to the seabed becomes this or less, an alarm is activated. 
   By the way, since various keys are provided on the operation portion, it is difficult for a person unskilled in the operation to completely understand and memorize the functions of the respective keys. Thus, in general, this kind of apparatus is provided with a help function for displaying guidance as key functions. That is, when a mode is set to a help mode and a predetermined operation is performed, an explanation of the function, operation procedure, and the like for each key are displayed on the screen, and an operator performs the key operation while referring to this. By using such help function, the load of operation is reduced, and the operator can perform a desired key input without puzzlement. 
   However, in a conventional apparatus, after the mode is set to the help mode, in order to display the guidance of the key, it has been necessary to search and select an item (function, key name, etc.) related to the key from the menu or contents displayed on the help screen. In this case, when the menu or contents are hierarchically constructed, the screen is changed every time the item is selected, and it is necessary to repeat such an operation that a related item is searched and selected on the switched screen, so that a lot of trouble is required. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the foregoing circumstances, and an object of the present invention is to meet such a demand that when a depth measurement result is displayed on a screen, the largest possible amount of information is displayed efficiently and to be easy to see. 
   Another object of the present invention is to enable quick measures to be taken in an emergency by making it possible to instantaneously grasp the contents of an alarm displayed on a display instrument in a case where the alarm is given, and by certainly giving the alarm even in a case where there are noises in surroundings. 
   Still another object of the present invention is to display guidance as to a key in a help mode by a very simple operation. 
   In order to achieve the above objects, according to the present invention, a first screen in which water depth information obtained from an echo signal is historically displayed and a second screen on which fish school detection information obtained from the echo signal is displayed are displayed on a display instrument. 
   By doing so, since the screen of the water depth information and the screen of the fish school detection information are displayed on one display instrument, an apparatus having both functions of a depth measuring apparatus and a fish school detecting apparatus is realized, and the display instrument can be used for both. The water depth information and the fish school detection information are not displayed together on the same screen (such a display mode is adopted also in a conventional fish detector), but the water depth information and the fish school detection information are displayed as independent screens, so that more accurate and detailed information can be displayed in a mode easy to see. 
   In the present invention, the first screen and the second screen may be displayed side by side on the display instrument, or either one of them may be displayed by switching the screen. 
   Besides, in the present invention, when the apparatus having both functions of depth measurement and fish school detection is constructed, in order to simplify circuits, it is preferable to make such construction that the water depth is measured by using ultrasonic detection signals of two frequencies used for fish school detection. 
   Besides, in the present invention, in the case where the water depth information over a past definite period is historically displayed on the screen for displaying the water depth information, it is preferable that display contents are renewed by scroll in an area unit of 1/n of all display area, in making a display easy to see. In this case, it is also possible that the display screen of the water depth information is divided in two, the water depth information over the past definite period is displayed on one part of the display screen, the latest water depth information including the present measured depth is displayed on the other part of the screen, and the display contents of the one screen are renewed in an area unit of 1/n by scrolling the one display screen. 
   Besides, in the present invention, in order to measure depths in the wide range from shallow waters to deep waters and to make measurement results clearly displayed in both cases, it is preferable to automatically switch the frequency of the ultrasonic wave to a high frequency or low frequency according to a water depth. 
   Besides, in the present invention, in order to facilitate the prediction of change of the seabed easily on the screen, it is preferable that on the basis of the past water depth data, the change of the water depth up to now is calculated, and the result is displayed on the screen with a mark. 
   Besides, in the present invention, in order to secure a large display region to make it easy to see when draft is corrected, it is preferable that when a draft value is set on the screen, an oscillation line is not moved but a water depth scale is shifted upward. 
   Moreover, in order to achieve the above objects, in the present invention, a low brilliance screen on which a predetermined image is displayed is changed into high brilliance at an alarm time. 
   By doing so, at the alarm time, since the screen of the display instrument is automatically switched from low brilliance to high brilliance, the screen becomes bright and the contents of the displayed alarm can be grasped quite clearly. Besides, in an apparatus in which an alarm is given by sound as well, even in the case where there are noises in surroundings and an alarm sound is difficult to hear, the screen is changed into high brilliance so that the alarm can be visually recognized. 
   Here, in addition to changing the brilliance of the display screen to the high brilliance, if the display screen is made to flash on and off, attention can be aroused still more. 
   Besides, on the display screen, if a seabed line is historically displayed on the basis of a water depth measurement value, an alarm water depth mark as a reference to give an alarm is displayed, and the alarm is displayed together with the seabed line and the alarm water depth mark, the grasp of the contents of the alarm becomes easy still more. 
   Furthermore, in order to achieve the above objects, in the present invention, when a key is pushed in a help mode, guidance as to the key is displayed. 
   By doing so, even if related items are not searched and selected one by one from the menu, contents or the like, the guidance as to the key is instantaneously displayed by merely pushing the objective key, so that the operation becomes very simple and working efficiency is greatly improved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an underwater detecting apparatus according to an embodiment of the present invention. 
       FIG. 2  is a front view of a display unit of the underwater detecting apparatus. 
       FIG. 3  is a view showing an example of a display on the screen. 
       FIG. 4  is a block diagram of an underwater detecting apparatus according to another embodiment of the present invention. 
       FIG. 5  is a view showing an example of a fish school display screen. 
       FIG. 6  is a view showing an example of a water depth display screen. 
       FIGS. 7A and 7B  are views showing an example of a display screen renewed by scroll. 
       FIG. 8  is a view showing another example of a display screen renewed by scroll. 
       FIG. 9  is a view showing another example of a display screen renewed by scroll. 
       FIGS. 10A and 10B  are views showing an example of a water depth display screen in the case where a frequency is switched. 
       FIG. 11  is a flowchart in the case where shallow waters and deep waters are measured through frequency switching. 
       FIG. 12  is a flowchart showing another embodiment. 
       FIGS. 13A and 13B  are views showing examples of a screen displaying a mark indicating the trend of an inclination of the seabed. 
       FIGS. 14A to 14G  are views showing display patterns of the mark. 
       FIG. 15  is a view showing the principle for obtaining a change in water depth. 
       FIGS. 16A and 16B  are an example of a display screen for explaining a disadvantage by a draft correction. 
       FIGS. 17A and 17B  are an example of a display screen of a draft correction of the present invention. 
       FIG. 18  is a view showing the principle of the draft correction. 
       FIG. 19  is a block diagram of an underwater detecting apparatus according to another embodiment of the present invention. 
       FIG. 20  is a block diagram of an underwater detecting apparatus according to another embodiment of the present invention. 
       FIG. 21  is an electrical block diagram of a depth measuring apparatus of another embodiment of the present invention. 
       FIG. 22  is a view showing an example of a display instrument. 
       FIG. 23  is a view showing a case where an alarm is displayed on the display instrument. 
       FIG. 24  is a flowchart showing an operation. 
       FIG. 25  is an electrical block diagram of a depth measuring apparatus of another embodiment of the present invention. 
       FIG. 26  is a view showing an example of a display apparatus. 
       FIG. 27  is a view showing a case where a help screen is displayed on the display apparatus. 
       FIG. 28  is a view showing a case where guidance is displayed on the display apparatus. 
       FIG. 29  is a flowchart showing an operation. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. 
     FIG. 1  is a block diagram showing an example of an underwater detecting apparatus of the present invention. In the drawing, reference numeral  1  designates a transducer provided on a ship&#39;s hull and for receiving and transmitting an ultrasonic wave, which is constituted by a transducer  1   a  for a high frequency and a transducer  1   b  for a low frequency. Reference numeral  2  designates a switching portion for switching the operation of the transducer  1  between a wave transmitting unit and a wave receiving unit and between a high frequency signal and a low frequency signal; reference numeral  3 , a transmitting circuit for sending a transmission signal to the transducer  1  through the switching portion  2 ; reference numeral  4 , a receiving circuit for receiving an echo signal from the seabed received by the transducer  1  through the switching portion  2 ; and reference numeral  5 , an A/D converter for converting an analog signal received by the receiving circuit  4  into a digital signal. A transmitter and receiver block  15  is constituted by the switching portion  2 , the transmitting circuit  3 , the receiving circuit  4 , and the A/D converter  5 . 
   Reference numeral  6  designates a CPU for calculating fish school information on the basis of the output from the A/D converter  5 ; reference numeral  7 , a memory constituted by a ROM storing a program for the CPU  6  and a RAM storing various data; and reference numeral  8 , a display control portion for making the fish school information displayed on a display instrument  9 , and a fish detection block  16  is comprised with the CPU  6 , the memory  7 , and the display control portion  8 . 
   Reference numeral  10  designates a CPU for calculating water depth information on the basis of the output from the A/D converter  5 ;  11 , a memory constituted by a ROM storing a program for the CPU  10  and a RAM storing various data; and  12 , a display control portion for making the water depth information displayed on the display instrument  9 , and a depth measurement control block  17  is constituted by the CPU  10 , the memory  11 , and the display control portion  12 . 
   The display instrument  9  is made of a liquid crystal display, and displays the fish school information obtained by the CPU  6  and the water depth information obtained by the CPU  10 . The details will be described later. Reference numeral  13  designates an operation portion provided with keys for performing various settings or the like; and  14 , an external storage device for extracting data of the water depth information and storing. 
   Next, the operation of the underwater detecting apparatus of the above structure will be described in brief. When a transmission instruction is given to the transmission circuit  3  from the CPU  6 , the transmission circuit  3  outputs a transmission signal, and an ultrasonic signal is emitted from the transducer  1  into the water through the switching portion  2  which is switched to the wave transmission side. Here, the ultrasonic wave of a high frequency of, for example, 200 KHz is transmitted from the transducer  1   a  as described later, and the ultrasonic wave of a low frequency of, for example, 50 KHz is transmitted from the transducer  1   b . Switching of these frequencies is performed by the switching portion  2 . 
   The ultrasonic wave emitted from the transducer  1  is reflected by a school of fish or the seabed, and its echo signal is received by the transducer  1 . This echo signal is received by the receiving circuit  4  through the switching portion  2  which is switched to the wave reception side, and is converted into the digital signal by the A/D converter  5  and is given to the CPU  6 . The CPU  6  discriminates the school of fish on the basis of the echo signal, calculates the fish school detection information, and stores the result in the memory  7 . The display control portion  8  outputs the data read out from the memory  7  to the display instrument  9 , and the fish school information is displayed on the display instrument  9 . 
   On the other hand, the output of the A/D converter  5  is also given to the CPU  10 , and the CPU  10  calculates the water depth to the seabed on the basis of the echo signal and stores the result in the memory  11 . The display control portion  12  outputs the water depth data read out from the memory  11  to the display instrument  9 , and the water depth information is displayed on the display instrument  9 . 
   The external storage device  14  takes in the water depth data obtained by the CPU  10  and stores this. This external storage device  14  is constituted by, for example, a card reader writer, in which a not-shown memory card is inserted, and the water depth data for the past 24 hours are stored in this memory card. Alternatively, the external storage device  14  may be constituted by a personal computer, and in this case as well, the downloaded water depth data for the past 24 hours are recorded in the memory device of the personal computer. 
   As described above, the underwater detecting apparatus of  FIG. 1  has both functions of the fish school detecting apparatus and the depth measuring apparatus. Since the ultrasonic detection signals of two frequencies of 200 KHz and 50 KHz used for fish school detection are also used for measurement of the water depth, the transducer  1  and the transmitter and receiver block  15  can be used jointly, and the circuits are simplified. 
   Incidentally, in the case where the fish school is displayed on the display instrument  9 , since it is demanded to display the fish school in detail by increasing the amount of information, the number of detection pulse signals emitted from the transducer  1  becomes large. For example, the number of pulses in the case where the water depth scale is displayed in a 20 m range is 1000 pulses/minute or more, and in the case of a 200 m range, it becomes 100 pulses/minute or more. 
   On the other hand, in the case where the water depth is displayed, since the undulations of the seabed does not change very much in a short time, the number of emitted detection pulses will be small. For example, in the case where it is displayed in the 20 m range, the number of pulses is 36 pulses/minute or more, and in the case of the 200 m range, it becomes 12 pulses/minutes or more. Actually, based on partial signals of the detection pulse signals emitted for fish school detection and their echo signals, the water depth information is displayed. Thus, by peak holding the echo signals for the fish school display or performing an averaging process, the water depth information can be displayed. Incidentally, in the case where the fish school information is displayed, there is a case where the seabed is not displayed by switching of the range. But, in the case where the water depth information is displayed, the seabed must be displayed without fail. 
     FIG. 2  shows an example of a display unit  20  of the underwater detecting apparatus. In the display unit  20 , the operation portion  13  is provided at a right side of the display instrument  9 . The display instrument  9  is made of a TFT (Thin Film Transistor) color liquid crystal display, and is a small display with a display area of, for example, 133×97 mm. 
     FIG. 3  shows an example of a display screen  41  of the display instrument  9 . In  FIG. 3 , two independent screens  41   a  and  41   b  are displayed side by side on the display screen  41 . The screen  41   a  is a screen on which the water depth information obtained by the depth measurement control block  17  of  FIG. 1  is displayed, and the screen  41   b  is a screen on which the fish school detection information obtained in the fish detection block  16  of  FIG. 1  is displayed. 
   On the screen  41   a , reference numeral  42  designates a water depth scale provided at intervals of 50 meters; reference numeral  43 , a seabed line displayed on the basis of measured water depth values, and the seabed line  43  is continuously displayed as a history on the basis of the data for a definite period. That is, the seabed line  43  at the position of a right end  46  of the screen  41   a  is the latest seabed line measured presently, and toward the left from this position, the seabed line for at least the past 15 minutes is displayed. Reference numeral  44  designates a measured present water depth indication, and the water depth value of 81.3 m of the seabed line  43  at the end  46  is displayed. Reference numeral  45  designates an alarm water depth line as the reference to give the alarm. The alarm water depth line  45  can be arbitrarily set, and here, it is set to 15 m, and when the water depth is at this one or less, the alarm will be produced. Reference numeral  47  designates a display portion for displaying the set alarm water depth value. 
   Besides, on the screen  41   b , reference numeral  48  designates a water depth scale provided at intervals of 25 meters;  49 , a seabed line; and  50 , an image of a school of fish displayed on the basis of echo signals, and the display contents are similar to a display screen of a conventional fish detector. Incidentally, reference numeral  51  designates an oscillation line indicating the position of the transducer  1  fixed to the ship&#39;s hull. 
   In this way, the screen  41   a  displaying the water depth information and the screen  41   b  displaying the fish school detection information are displayed side by side, so that a large amount of information can be displayed on the limited display screen  41 , and an efficient display can be made. In  FIG. 3 , although the screen  41   a  and the screen  41   b  are respectively displayed on half of the display screen  41 , for example, the screen  41   a  of the water depth information may be display on ⅕ of the display screen  41 , and the screen  41   b  of the fish school detection information may be displayed on the remaining ⅘, and the ratio can be arbitrarily selected. 
   Incidentally, since the latest image is always displayed at the position of the right end  46  on the display screen  41   a  of the water depth information, it is necessary to make renewal by sending the image at regular intervals. The time interval of the image feed is set to, for example, 15 minutes in a normal mode. Besides, by making a mode a fast-forwarding mode, it can be sent at high speed corresponding to the range. For example, it can be sent at 10 second intervals for the 10 m range, 15 second intervals for the 20 m range, 1 minute intervals for the 60 m range, 1.5 minute intervals for the 100 m range, and 2 minute intervals for the 200 m range. However, even in this case, a measurement value is displayed at 1 second intervals on the water depth data value indication  44 . The display screens of the water depth information in the following embodiments are also the same. 
   On the other hand, in the operation portion  13  of  FIG. 2 , reference numeral  21  designates a draft key for setting a draft value;  22 , an alarm key for setting the foregoing alarm water depth line  45 ;  23 , an illumination adjusting key for adjusting the panel illumination of the operation portion  13 ;  24 , a brilliance adjusting key for adjusting the screen brilliance of the display instrument  9 ;  25 , an automatic key for changing the automatic/manual operation of range, sensitivity or the like;  26 , a color key for adjusting the color tone of the display instrument  9 ;  27 , a minus key for scrolling the display of the water depth history displayed on the display instrument  9  in a reverse direction (left direction) or for decreasing set values; and  28 , a plus key for scrolling the display of the water depth history displayed on the display instrument  9  in a positive direction (right direction) or for increasing set values. 
   Reference numeral  29  designates a range switch for changing the display range of the display instrument  9 ;  30 , a gain switch for adjusting sensitivity;  31 , a mode switch for switching a mode; and  32 , a power switch of the display unit  20 . The mode of the mode switch includes a NAV mode displaying a basic screen, a DBS (Depth Below Surface) mode displaying a screen by draft correction, a HISTORY mode displaying past water depth history in a graph form, a LOGBOOK mode displaying the past water depth history in a table form in which a time is made to correspond to a water depth value, an OSDATA mode displaying the position and speed of a ship, a water depth, or the like, a HELP mode displaying an explanation of operation and function, and a MENU mode for selecting a menu of various functions. 
   In  FIG. 3 , although the two display screens  41   a  and  41   b  are displayed side by side at the same time, it is also possible to display either one by switching the display screens  41   a  and  41   b .  FIG. 4  shows a circuit for that purpose, and the same portions as those of  FIG. 1  are designated by the same symbols. In  FIG. 4 , a selector switch  18  is provided between a display control portion  8  and a display instrument  9  and between a display control portion  12  and the display instrument  9 . This selector switch  18  may be provided at a suitable place of the display unit. Since the other constructions are the same as  FIG. 1 , the explanation is omitted. 
   In  FIG. 4 , in the case where the selector switch  18  is switched to a side “a”, the display instrument  9  is connected to the side of the fish detection control block  16 , and a screen  41   b  of fish school detection information as shown in  FIG. 5  is displayed on the display instrument  9 . Since this screen  41   b  is the same as the screen  41   b  of  FIG. 3 , the same portions are designated by the same symbols and the explanation is omitted. However, in  FIG. 5 , since the fish school detection information can be displayed on the whole area of the screen, the display area is naturally wider than that of  FIG. 3 , and a larger amount of fish school information can be displayed. 
   Besides, in  FIG. 4 , in the case where the selector switch  18  is switched to a side “b”, the display instrument  9  is connected to the side of the depth measurement control block  17 , and a screen  41   a  of water depth information as shown in  FIG. 6  is displayed on the display instrument  9 . Since this screen  41   a  is also the same as the screen  41   a  of  FIG. 3 , the same portions are designated by the same symbols and the explanation is omitted. Besides, also in this case, since the water depth information can be displayed on the whole area of the screen, the display area becomes wide and the water depth history for a longer time can be displayed. 
   In this way, by switching the screen  41   a  of the water depth information and the screen  41   b  of the fish school detection information to display one of them on the display instrument  9 , it is possible to select necessary information and to display it in detail, and it is possible to display a large amount of information efficiently on the limited screen. 
   By the way, in  FIG. 2 , when the mode switch  31  is set to the HISTORY mode, a screen as shown in  FIG. 7A  is displayed. In the HISTORY mode, the display screen  41   a  is divided in two, and a screen  61  on which water depth information over the past 24 hours is historically displayed and a screen  62  on which water depth information for the latest 5 minutes including the present measured depth (in this example, 47.5 m) is historically displayed are displayed. Reference numerals  43   a  and  43   b  designate seabed lines, and with respect to the seabed line  43   a  of the screen  61 , only the contour line is displayed. On the other hand, with respect to the seabed line  43   b  of the screen  62 , a geological portion is displayed in, for example, a form painted by red. Incidentally, reference numeral  42  designates a water depth scale; and  44 , a present water depth value. 
   The screen  61  can be scrolled in the right and left directions, and by this, the change of the seabed line  43   a  for the past 24 hours can be traced. That is, when the plus key  28  of  FIG. 2  is pressed, the screen  61  is scrolled forward (direction of arrow A), and when the minus key  27  is pressed, the screen  61  is scrolled backward. 
   In this case, when the screen is renewed by the scroll operation, if data is renewed one by one, it takes a very long time to renew. On the other hand, when one page of the screen  61  is renewed, although a time for renewal can be shortened, there is a problem that the connection of data is hard to understand and the history of change of the seabed line  43   a  can not be accurately grasped, since data before the renewal disappears from the screen  61 . 
   Then, in this embodiment, display contents are renewed by scroll in an area unit of ½ of all display area. That is, in  FIG. 7A , in the case where the width of all display area is made w, by pressing the plus key  28  once, the screen  61  is scrolled in the direction of the arrow A, and a display area  63  of a width of w/2 on the right half of the screen  61  is moved to the left half of the screen  61  as shown in  FIG. 7B . Then, a new seabed line  64  is displayed on the right half of the screen  61 . The case where scroll is made in the direction reverse to the arrow A by the minus key  27  is also the same, and when the key  27  is pressed once, the left half of the screen  61  is moved to the right half and renewal is made. Incidentally, reference numeral  66  designates a time indication, and the time positioned in the direction of the arrow A is new. 
   Like this, by renewing the screen  61  every ½ screen, since the half of the screen before the renewal is displayed on the screen after the renewal, the connection of data becomes easy to understand, and the history of the change of the seabed line  43   a  can be easily grasped. Besides, as compared with the case where data is renewed one by one, the renewal speed can be raised. Incidentally, in this example, although renewal is made in a unit of ½ of all display area, restriction is not made to this, but in general, renewal can be made in a unit of 1/n, and renewal can be made in an arbitrary unit of ⅓, ¼ or the like. 
   The foregoing scroll system can also be adopted in the water depth display screen in the case where the screen of the fish school and the screen of the water depth as in  FIG. 3  are displayed side by side.  FIG. 8  shows an example of this case, and the same portions as those of  FIG. 3  are designated by the same symbols. The screen  41   a  on which the water depth information is displayed is renewed in an area unit of 1/n of all display area by pressing the plus key  28  or the minus key  27  once. The scroll system can be applied to not only the case where the screen of the fish school and the screen of the water depth as in  FIG. 3  are displayed side by side, but also the case where the water depth display screen  41   a  as in  FIG. 6  is displayed alone. 
     FIG. 9  shows still another embodiment of the scroll system. Here, in the two screens  41   a  and  41   b  in  FIG. 8 , the display screen  41   a  of the water depth information is divided in two similarly to  FIGS. 7A and 7B , so that three screens  411 ,  412  and  41   b  are displayed on the screen  41 . The water depth information for the past 24 hours is displayed on the screen  411 , and the water depth information for the latest 5 minutes including the present measurement water depth (in this example, 81.3 m) is historically displayed on the screen  412 . The screen  411  is designed such that the display contents are renewed by scroll in an area unit of 1/n of all display area similarly to the above. 
   As described above, even in the case where the display screen  41  is divided in two or three so that the display region becomes narrow, since the display contents are renewed in a unit of 1/n of a display area, the connection of data can be maintained, so that the screen becomes easy to see. 
   Next, switching of a frequency of an ultrasonic signal will be described. In a conventional depth measuring apparatus, one of a transducer for a high frequency and a transducer for a low frequency is used to measure the depth. However, since the high frequency has a short wavelength and a large amount of attenuation, it is hard to reach a deep place in the water and the depth of deep waters can not be measured. On the other hand, since the low frequency has a long wavelength and a small amount of attenuation, it reaches a deep place in the water, but shallow waters can not be measured with accuracy. Besides, when the low frequency is used, ultrasonic transmission is obstructed by bubbles near the water surface generated after a ship has passed, that is, a so-called bubble cut phenomenon is apt to occur, so that there is a case where accurate depth measurement becomes impossible. 
   In order to solve such a disadvantage, in this embodiment, as described above, the transducer  1  of  FIG. 1  is constituted by the transducer  1   a  for the high frequency and the transducer  1   b  for the low frequency. Then, the frequency of an ultrasonic wave is automatically changed in accordance with the depth, so that in shallow waters where the water depth is less than, for example, 30 m, an ultrasonic signal of the high frequency (200 KHz) is emitted from the transducer  1   a , and in deep waters where the water depth is 30 m or more, an ultrasonic signal of the low frequency (50 KHz) is emitted from the transducer  1   b . This switching is performed by the switching portion  2 . Incidentally, the range where depth measurement can be made is, for example, 2 m to 200 m. 
   In this way, when the two frequencies are used so that the shallow waters are measured with the high frequency and the deep waters are measured with the low frequency. For the shallow waters, the depth measurement can be accurately made without receiving the influence of the bubble cut, and for the deep waters, the depth measurement can be made to a deeper place with the low frequency. Thus, in either event, measurement results are clearly displayed on the display screen  41 . 
     FIGS. 10A and 10B  show the water depth information display screen  41   a  of the display instrument  9  in the case where the frequency is changed in the manner as described above.  FIG. 10A  shows a screen in the case where depth measurement is made with the high frequency (200 KHz), and  FIG. 10B  shows a screen in the case where depth measurement is made with the low frequency (50 KHz). In the drawings, the same portions as those of  FIG. 6  are designated by the same symbols and the explanation is omitted. Incidentally, reference numeral  65  designates a frequency indication.  FIG. 10A  shows the screen of shallow waters, and the range of the water depth scale  42  is in a unit of 10 m.  FIG. 10B  shows the screen of deep waters, and the range of the water depth scale  42  is in a unit of 50 m. In the case where the frequency is switched from the high frequency to the low frequency, with the screen  41   a  being also switched from  FIG. 10A  to  FIG. 10B , when the automatic key  25  ( FIG. 2 ) is operated to set the AUTO mode, the range of the water depth scale  42  is automatically switched so that the seabed line  43  is displayed on the screen  41   a  without fail. On the other hand, in the case where the manual mode is set, the range of the water depth scale  42  is switched by the range switch  29 . 
     FIG. 11  is a flowchart in the case where shallow waters and deep waters are measured through the foregoing frequency switching. First, the transducer  1  sends the ultrasonic wave to measure the water depth (step S 1 ). The high frequency is used as the frequency of the ultrasonic wave at this time. Incidentally, the high frequency or the low frequency can be selected as the ultrasonic wave sent from the transducer  1  with the screen (not shown) of the MENU mode. 
   Next, it is judged whether the measured depth is 30 m or more (step S 2 ). This value of 30 m can be changed to an arbitrary value by setting. If it is less than 30 m (NO at step S 2 ), a measurement place is shallow waters, so that the transducer  1   a  sends the ultrasonic wave of the high frequency (200 KHz) to measure the depth (step S 3 ), and displays the results on the display instrument  9  as in  FIG. 10A  described before (step S 5 ). If the depth is 30 m or more (YES at step S 2 ), the measurement place is deep waters, so that the transducer  1   b  sends the ultrasonic wave of the low frequency (50 KHz) to measure the depth (step S 4 ), and the results are displayed on the display instrument  9  as in  FIG. 10B  (step S 5 ). Then it is judged whether the measurement is ended (step S 6 ), and if the measurement is not ended (NO at step S 6 ), the procedure returns to step S 1  and the foregoing operation is repeated, and if the measurement is ended (YES at step S 6 ), the operation is ended. 
     FIG. 12  is a flowchart showing another embodiment in the case where shallow waters and deep waters are measured through frequency switching. In this embodiment, priority is given to the high frequency when depth measurement is made, and at the point when the measurement with the high frequency becomes impossible, switching is automatically made to the low frequency. First, the transducer  1   a  sends the high frequency (200 KHz) to measure the water depth (step S 11 ). Then it is judged whether an echo signal is received by the receiving circuit  4  (step S 12 ). If the echo signal is received (YES at step S 12 ), measurement data based on that are displayed on the display instrument  9  as in  FIG. 10A  (step S 14 ).Then it is judged whether the measurement is ended (step S 15 ). If the measurement is not ended (NO at step S 15 ), the procedure returns to step S 12  and the depth measurement with the high frequency is continued. 
   If the echo signal with the high frequency carrier comes not to be received (NO at step S 12 ), the transducer  1   b  sends the low frequency (50 KHz) to measure the water depth (step S 13 ), and measurement data based on that are displayed on the display instrument  9  as in  FIG. 10B  (step S 14 ). Then it is judged whether the measurement is ended or not (step S 15 ). If the measurement is not ended (NO at step S 15 ), the procedure returns to step  12  and the depth measurement with the low frequency is continued, and if the measurement is ended (YES at step S 15 ), the operation is ended. 
     FIGS. 13A and 13B  show an embodiment in which a mark  70  indicating the trend of an inclination of the seabed is displayed on the display screen  41   a  in addition to the water depth information. Whether the mark  70  is displayed or not can be selected through the screen of the MENU mode. In  FIGS. 13A and 13B , the same portions as those of  FIG. 6  are designated by the same symbols and the explanation is omitted. The mark  70  is constituted by a semicircle  71  and an arrow  72  indicated in its inside, and the degree of the inclination of the arrow  72  from the horizontal indicates the trend of the average inclination of the seabed for a past time period. In the case of  FIG. 13A , since the inclination of the seabed is gentle on average, the inclination of the arrow  72  is small. However, in the case of  FIG. 13B , since the inclination of the seabed is steep on average, the inclination of the arrow  72  is large. 
     FIGS. 14A through 14G  are views showing display patterns of the mark  70 .  FIG. 14A  shows an indication of a case where an average inclination angle θ is −90°≦θ&lt;−60°;  FIG. 14B  shows an indication of a case of −60°≦θ&lt;−30°;  FIG. 14C  shows an indication of a case of −30°≦θ&lt;−1°;  FIG. 14D  shows an indication of a case of −1°≦θ≦1°;  FIG. 14E  shows an indication of a case of 1°&lt;θ≦30°;  FIG. 14F  shows an indication of a case of 30°&lt;θ≦60°; and  FIG. 14G  shows an indication of a case of 60°&lt;θ≦90°. 
   The average inclination angle θ of the seabed is obtained by calculating the changes in the water depth up to now on the basis of the past water depth data.  FIG. 15  shows this principle. In  FIG. 15 , t 1 , t 2 , . . . tn are timings of ultrasonic detection pulses emitted from the transducer  1  for the past T minutes (for example, one minute). Water depths at respective points of time measured by the ultrasonic detection pulses are made D 1 , D 2 , . . . Dn, respectively. Here, when the speed of a ship is v per minute, a distance L in which the ship moves for T minutes becomes L=v×T. On the other hand, an average value D of the water depth measured for T minutes becomes D=(D 1 +D 2  . . . +Dn)/n, and an average value D′ of the water depth measured similarly at the point T minutes before becomes D′=(D 1 ′+D 2 ′ . . . +Dn′)/n, and a difference between the present average water depth D and the average water depth D′ T minutes before becomes ΔD=D−D′. Thus, the average inclination angle θ of the seabed for T minutes can be obtained from the following expression.
 
θ=tan −1 ΔD/L=tan −1 ΔD/(v×T)
 
   By the value of the average inclination angle θ, the patterns of the mark  70  of  FIGS. 14A through 14G  are determined. Incidentally, the value of T can be determined by parameters such as a speed or steering time (time from steering to a point when a ship is actually rotated). 
   Incidentally, in the above example, although the difference between the present average water depth D and the average water depth D′ T minutes before is obtained, the water depth value Dn at the point T minutes before may be used in place of the average water depth D′ T minutes before. Besides, in the above example, although the existence of display of the mark  70  is selected on the screen of the MENU mode, instead of this, the mark  70  may be automatically displayed when the measured water depth becomes a definite value or less. Besides, as the mark  70 , various shapes made of symbols or figures can be used, and restriction is not made to the above example. 
   Thus, by displaying the mark  70  indicating the trend of the inclination of the seabed on the display screen  41   a  in addition to the water depth information, the change of the seabed can be easily predicted on the screen. That is, in the case of the seabed with many undulations, it is difficult to judge by only using the display of the seabed line  43  whether the seabed becomes shallow on average or becomes deep, and is also difficult even at a place where the inclination of the seabed is gentle. However, in this embodiment, since the average change of the water depth is graphically displayed by the mark  70  on the basis of the past depth measurement data, the change of the seabed can be easily predicted. Besides, a slight inclination of the seabed can also be grasped by the mark  70 . Thus, an accident such as stranding can be prevented in advance. 
   Next, screen display in the case where a draft correction is made will be described. The draft correction is a correction which is necessary since a fixed position of the transducer  1  is lower than the actual water surface.  FIG. 18  is a view showing the principle of the draft correction. The transducer  1  is fixed on the lower surface of a ship&#39;s hull  80 , and the ship&#39; hull  80  sinks in the water by Z 2  from the water surface  82 . Thus, the water depth measured with an ultrasonic wave  83  emitted from the transducer  1  is a water depth value Z 1  from the bottom of the ship to the seabed, and is not an actual water depth value Z. Thus, in the case where the actual water depth value Z should be displayed in the foregoing DBS mode, it is necessary to add the draft value Z 2  to the measured water depth value Z 1 . This correction is the draft correction. 
   By the way, when such draft correction is carried out by a conventional apparatus, there occurs a disadvantage that an oscillation line is shifted downward on the display screen and the display region of data will be narrower. This will be explained with reference to  FIGS. 16A and 16B .  FIG. 16A  shows a screen in the case where a draft value is 0 and an oscillation line  51  is positioned at 0 of the water depth scale  42 . In this case, the water depth information is displayed on the whole region of the screen  41   a . On the other hand,  FIG. 16B  is a screen in the case where the draft value is set to 5 m, and the oscillation line  51  is shifted by X to the position of 5 m of the water depth scale  42 . Thus, a display region Y of the water depth information will be narrower. As the draft value becomes large, the oscillation line  51  shifts downward and the display region Y becomes further narrow. 
   While, in IMO (International Maritime Organization) standards as international standards, in the case of a 20 m range, it is obliged to display a screen with 5 mm or more per a water depth of 1 m, in other words, it is required to secure 5 mm×20=100 mm for a vertical size of the display region Y. However, since the screen  41   a  is a small screen as described above and the size is limited, when the oscillation line  51  is shifted downward and the display region Y becomes narrow as set forth above, it becomes impossible to satisfy the IMO standards. Especially in the case of a shallow range, since the amount of shift of the oscillation line  51  is large, the display region Y is greatly decreased, and a part of the seabed line  43  disappears from the screen and becomes hard to see. On the other hand, since anything is not displayed on a portion above the oscillation line  51 , this portion results in a useless region. 
   Then, in this embodiment, when the draft value is set, the oscillation line is not moved but the water depth scale is shifted upward, so that the above disadvantage will be overcome.  FIGS. 17A and 17B  show a screen for explaining this case.  FIG. 17A  shows the screen for the case where the draft value is 0, and the oscillation line  51  is positioned at 0 of the water depth scale  42 . This screen is the same as the screen of  FIG. 16A , and the water depth information is displayed on the whole region of the screen  41   a . On the other hand,  FIG. 17B  shows the screen in the case where the draft value is set to 5 m, and the position of the oscillation line  51  is not changed as compared with  FIG. 17A . The water depth scale  42  is shifted upward by 5 m. Thus, the display region Y does not become narrow, and the same display region as the case of  FIG. 17A  is secured. 
   Incidentally, in order to perform the draft correction, the mode switch  31  in  FIG. 2  is made the DBS mode, and the draft key  21  is operated. When the draft key  21  is pressed, a not-shown draft value set screen is displayed. The draft value is initialized on this screen, and the plus key  28  or the minus key  27  is pressed to renew the set value, so that setting of the draft value is performed on the screen. The draft value setting can be made also on the fish school display screen  41   b.    
   In this way, by shifting the water depth scale upward, the size of the display region Y will be constant and broad, and it will become possible to satisfy the IMO standards. Besides, since a wasteful region is not produced on the screen and the display region Y does not change even in the case of a shallow range, it does not become hard to see. Thus, information can be efficiently displayed on the small display screen  41   a.    
     FIG. 19  is a block diagram of an underwater detecting apparatus according to another embodiment of the present invention, which is a modified example of the embodiment of  FIG. 1 . In  FIG. 19 , the same portions as those of  FIG. 1  are designated by the same symbols and the explanation is omitted. In  FIG. 19 , the fish detection control block  16  and the depth measurement control block  17  in  FIG. 1  are integrated to make a fish detection and depth measurement control block  84 . That is, the CPU  6  is made to have the function of the CPU  10  in the depth measurement control block  17  as well, the memory  7  is made to have the function of the memory  11  as well, and the display control portion  8  is made to have the function of the display control portion  12  as well. As a result, the circuits can be simplified. 
   The operation of  FIG. 19  is basically the same as that of  FIG. 1 . A transmission signal is outputted from the transmitting circuit  3  on the basis of a transmission instruction from the CPU  6 , and an ultrasonic wave is sent from the transducer  1 . An echo signal reflected by a school of fish or the seabed is received by the transducer  1 , and is supplied to the CPU  6  through the receiving circuit  4  and the A/D converter  5 . The CPU  6  calculates fish school detection information based on the echo signal, and at the same time, the CPU holds the peak of the echo signal or performs an averaging processing to calculate a water depth to the seabed, and stores the respective calculation results in the memory  7 . The display control portion  8  outputs data read out from the memory  7  to the display instrument  9 , and the fish school detection information and the water depth information are displayed side by side on the display instrument  9  as shown in  FIG. 3 . 
     FIG. 20  is a block diagram showing another embodiment of an underwater detecting apparatus of the present invention, which is a modified example of the embodiment of  FIG. 4 . In  FIG. 20 , the same portions as those of  FIG. 4  are designated by the same symbols and the explanation is omitted. Also in  FIG. 20 , similar to  FIG. 19 , the fish detection control block  16  and the depth measurement control block  17  in  FIG. 4  are integrated to make a fish detection and depth measurement control block  84 . The CPU  6  is made to have the function of the CPU  10  in the depth measurement control block  17  as well, the memory  7  is made to have the function of the memory  11  as well, and the display control portion  8  is made to have the function of the display control portion  12  as well. With this construction, the circuits can be simplified. 
   The operation of  FIG. 20  is basically the same as with  FIG. 4 . A transmission signal is outputted from the transmitting circuit  3  based on a transmission instruction from the CPU  6 , and an ultrasonic wave is sent from the transducer  1 . An echo signal reflected by a school of fish or the seabed is received by the transducer  1 , and is provided to the CPU  6  through the receiving circuit  4  and the A/D converter  5 . The CPU  6  calculates fish school detection information on the basis of the echo signal, and at the same time, the CPU holds the peak of the echo signal or performs an averaging processing to calculate a water depth to the seabed, and stores the respective calculation results in the memory  7 . 
   In the case where the selector switch  18  is switched to the side “a”, the display control portion  8  reads out the fish school detection information from the memory  7  and output it to the display instrument  9 , and the fish school detection information is displayed on one screen  41  of the display instrument  9  as shown in  FIG. 5 . In the case where the selector switch  18  is switched to the side “b”, the display control portion  8  reads out the water depth information from the memory  7  and outputs it to the display instrument  9 , and the water depth information is displayed on one screen  41  of the display instrument  9  as shown in  FIG. 6 . 
   Incidentally, since the water depth data for the past 24 hours are stored in the external storage device  14  of  FIGS. 1 ,  4 ,  19  and  20  as described hereinbefore, it is possible to read out the stored data and to display it on the screen  41   a  when necessary. At this readout, it is possible to specify an arbitrary time zone and to extract only the data during that. This operation is performed by making the mode switch  31  of  FIG. 2  the LOGBOOK mode and by specifying the time zone in the table displayed at this time in which times are made to correspond to water depth values. 
   In the above embodiments, although the underwater detecting apparatus having both functions of the fish school detecting apparatus and the depth measuring apparatus, the embodiments shown in  FIGS. 7 ,  10  through  15  and  17  can also be applied to a single depth measuring apparatus. As the circuit structure therefor, the same as that of  FIG. 19  can be used, and it can be realized by replacing the CPU  6 , the memory  7  and the display control portion  8  of the fish detection and depth measurement control block  84  by those which are necessary for only the depth measurement. Besides, the display unit  20  can also be constructed as the same one shown in  FIG. 2 . 
   Besides, although the above embodiments show the example in which the display instrument  9  is comprised with the liquid crystal display, it can also be comprised with a CRT, an EL display or the like. 
   According to the present invention, since a large amount of underwater detection information can be displayed on a space of a limited display screen efficiently and effectively and to be easy to see, even if a display instrument is small, it becomes possible to accurately and quickly grasp necessary information. 
     FIG. 21  is an electrical block diagram of a depth measuring apparatus according to an embodiment of the present invention. In the drawing, reference numeral  101  designates a transducer which is provided on the bottom of a ship and transmits/receives an ultrasonic wave;  102 , a transmission/reception switching circuit for switching the operation of the transducer  101  to a transmission side and reception side;  103 , a transmitting portion for producing a transmission signal to the transducer  101  through the transmission/reception switching circuit  102 ;  104 , a receiving portion for receiving an echo signal from the seabed received by the transducer  101  through the transmission/reception switching circuit  102 ; and  105 , an A/D converter for converting an analog signal received by the receiving portion  104  into a digital signal. A measurement means for measuring water depth is constituted by the transducer  101 , the transmission/reception switching circuit  102 , the transmitting portion  103 , the receiving portion  104 , the A/D converter  105 , and a control portion  106  which will be explained hereinafter. 
   Reference numeral  106  designates a control portion including a CPU and for calculating the water depth to the seabed on the basis of output from the A/D converter  105 ;  107 , a memory including a ROM storing a program of the CPU and a RAM storing various data;  108 , a display control circuit for making the various data of the memory  107  displayed on a display instrument  109 ;  109 , the display instrument comprised with a liquid crystal display driven by the display control circuit  108 ;  110 , a brilliance adjustment circuit for adjusting the brilliance of a screen of the display instrument  109  on the basis of an instruction from the control portion  106 ;  111 , a backlight whose brightness is adjusted by the brilliance adjustment circuit  110 ; and  112 , an alarm circuit for driving a buzzer  113  as an alarm unit in response to an alarm signal from the control portion  106 . A display apparatus of the present invention is constituted by the control portion  106 , the memory  107 , the display control circuit  108 , the display instrument  109 , the brilliance adjustment circuit  110 , the backlight  111 , the alarm circuit  112 , and the buzzer  113 . Incidentally, although the alarm circuit  112  and the buzzer  113  are provided to give an alarm by sound as well, they may be omitted in the case where an alarm is merely displayed on the screen of the display instrument  109 . 
   In the depth measuring apparatus constructed as explained hereinbefore, when a transmission instruction is supplied to the transmitting portion  103  from the control portion  106 , a transmission signal is outputted from the transmitting portion  103 , and an ultrasonic search pulse signal is emitted in a beam form from the transducer  101  to the seabed through the transmission/reception switching circuit  102  switched to the transmission side. This ultrasonic beam is reflected by the seabed, and its echo signal is received by the transducer  101 . This echo signal is received by the receiving portion  104  through the transmission/reception switching circuit  102  switched to the reception side, is converted into a digital signal by the A/D converter  105 , and is supplied to the control portion  106 . 
   The control portion  106  calculates a time from the emission of the ultrasonic search pulse signal to the reception of the echo signal, obtains the water depth to the seabed by calculation on the basis of the time measured, and stores the result in the memory  107 . The display control circuit  108  outputs data read out from the memory  107  to the display instrument  109 , and various data including the water depth value to the seabed are displayed on the display instrument  109 . 
     FIG. 22  shows an example of the display instrument  109 . In the display instrument  109 , an operation portion  131  is provided adjacent to a display screen  121 . On the display screen  121 , reference numeral  122  designates an oscillation line indicating a position of the transducer  101  on the bottom of a ship;  123 , a water depth scale provided at 5 meter intervals; and  124 , an alarm water depth mark as a reference to give an alarm. In this embodiment, although the alarm water depth mark  124  is indicated by a line, it may be displayed by an arrow or the like other than the line. The alarm water depth mark  124  can be set arbitrarily, and here, it is set at 15 m, and an alarm is given when the measured water depth is this one or less. Reference numeral  125  designates a seabed line displayed on the basis of water depth measurement values, and is continuously displayed as a history on the basis of data for a definite period. That is, the seabed line  125  at a position of a right end  127  of the display screen  121  is the presently measured latest seabed line, and a past seabed line measured for a past time period is displayed toward the left from this position. Reference numeral  126  designates a present water depth measurement value, and the water depth value of 47.5 m of the seabed line  125  at the end  127  is displayed. 
   On the other hand, in the operation portion  131 , reference numeral  132  designates a draft key for setting the draft;  133 , an alarm key for setting the foregoing alarm water depth mark  124 ;  134 , an illumination adjustment key for adjusting the panel illumination of the operation portion  131 ;  135 , a brilliance adjustment key for adjusting the brilliance of the display screen  121 ;  136 , an automatic key for switching the automatic/manual operation of a range, sensitivity or the like;  137 , a color key for adjusting the color tone of the display screen  121 ;  138 , a minus key for renewing the display of water depth history displayed on the display screen  121  in a reverse direction (left direction) and for decreasing a set value; and  139 , a plus key for renewing the display of water depth history displayed on the display screen  121  in a positive direction (right direction) and for increasing the set value. 
   The alarm key  133  has not only the function of setting the alarm water depth mark  124  but also the function of stopping an alarm sound and restoring the brilliance of the display screen  121  as will be described hereinafter. The brilliance adjustment key  135  adjusts the brilliance of the display screen  121  by adjusting the brightness of the backlight  111  through the brilliance adjustment circuit  110  of  FIG. 21 . As described hereinbefore, at the time of a night cruise, an adjustment is made such that the backlight  111  is made dark and the display screen  121  is made to have low brilliance. On the other hand, in the case where the surroundings are bright as in daylight, the display screen  121  is adjusted to normal brilliance. 
   Reference numeral  140  designates a range switch for switching a display range;  141 , a gain switch for adjusting sensitivity;  142 , a mode switch for switching a mode; and  143 , a power switch of the display instrument  109 . The mode includes a BASIC mode for displaying a basic screen, a TEXT mode for displaying the water depth history in a table form, a CONTOUR mode for displaying the water depth history in a graph form, a HELP mode for displaying the explanation of operations and functions, and the like. 
   With the display in  FIG. 22 , the present water depth value is 47.5 m, and does not reach 15 m as the set value of the alarm water depth mark  124 , so that there is no fear of stranding, and therefore, an alarm is not produced. However, if the present water depth value reaches 15 m, an alarm to stranding is outputted as water depth abnormality. That is, if the calculated water depth value is 15 m or less, the control portion  106  reads out alarm data from the RAM  107  and outputs this to the display control circuit  108 . Then, an alarm  128  “SHALLOW DEPTH ALARM”, as shown in  FIG. 23 , indicating that the ship enters a shallow depth region is displayed on the display screen  121  of the display instrument  109 . 
   At the same time as this, the control portion  106  provides the alarm circuit  112  with an alarm signal, so that the alarm circuit  112  operates to ring the buzzer  113 . This ringing sound may be a continuous sound or may be an intermittent sound. Besides, instead of the buzzer  113 , a bell or the like can also be used as an alarm unit. Alternatively, an electrical synthetic sound may also be used. Further, a terminal for outputting the alarm signal to the outside is provided and an equipment such as an alarm unit may be externally fitted to this terminal. If there is adopted a system in which the alarm signal is transmitted to another place in the ship by wireless, it is possible to recognize the occurrence of the alarm even at a remote place. 
   Besides, the control portion  106  outputs the alarm signal as set forth above, and at the same time, sends a brilliance control signal to the brilliance adjustment circuit  110 . In response to this control signal, the brilliance adjustment circuit  110  makes the backlight  111  bright to change the display screen  121  into high brilliance even in the case where the display screen  121  is set at low brilliance by the brilliance adjustment key  135 . With this arrangement, since the alarm  128  is displayed brightly and clearly, the contents can be instantaneously and easily grasped, and measures to prevent stranding can be quickly taken. Besides, even in the case where the alarm sound is hard to hear because of surrounding noises, the display screen is changed from low brilliance to high brilliance, so that the alarm can be given by light. Incidentally, even in the case where the display screen  121  is set at normal brilliance as in daylight, the brilliance is made higher than that to brighten the screen. 
   Here, the high brilliance is not necessarily required to be the maximum brilliance, but may be, for example, brilliance of about 80% of the maximum brilliance. In brief, the brilliance has only to be such that the contents of the alarm  128  can be clearly and visually recognized. However, for the purpose of arousing attention by making the change of brilliance of the screen large, it is preferable to raise the brilliance to the maximum brilliance. Besides, in addition to making high brilliance, if the display screen  121  is made to flash on and off, the attention is further aroused, and if the display screen  121  is made to flash on and off in the state of the maximum brilliance, it is further effective. 
   Besides, in  FIG. 23 , since the alarm  128  is displayed together with the alarm water depth mark  124  and the seabed line  125 , it is possible to intuitively recognize that the seabed line  125  exceeds the line of the alarm water depth mark  124  (water depth value is the reference value or less), and the contents of the alarm  128  can be grasped more easily. 
   In order to stop the alarm sound of the buzzer  113  after the alarm contents are confirmed, the alarm key  133  is pressed once. When the alarm key  133  is pressed, the control portion  106  sends an alarm stop instruction to the alarm circuit  112 , and the alarm circuit  112  stops driving of the buzzer  113  in response to the alarm stop instruction. Also, the control portion  106  sends an alarm display erase instruction to the display control circuit  108 , and the alarm  128  of the display screen  121  is also erased. At the same time as this, the control portion  106  sends a brilliance return signal to the brilliance adjustment circuit  110 , and the brilliance adjustment circuit  110  receives this, makes the backlight  111  dark, and restores the brilliance of the display screen  121  to the former low brilliance. 
   In this way, with this embodiment, since both the alarm and brilliance can be reset by pressing the alarm key  133  once, there is a merit that the operation becomes simple. However, the present invention is not necessarily limited to this method, but may be modified such that the alarm sound is stopped by pressing the alarm key  133  once and the brilliance is restored by pressing the brilliance adjustment key  135  once. Besides, instead of resetting the alarm sound and the brilliance by the key operation, it is also possible to automatically stop the alarm sound and to restore the brilliance after a predetermined time set by a built-in timer (not shown) of the control portion  16  has elapsed. 
     FIG. 24  is a flowchart showing the foregoing operation, and shows the procedure executed by the CPU of the control portion  106 . The control portion  106  measures the water depth on the basis of the transmission/reception timing of the ultrasonic signals, and displays the resultant water depth on the display screen  121  of the display instrument  109  (step S 21 ), and then, the control portion judges whether or not the water depth is abnormal, that is, whether or not the water depth is not larger than the set value of the alarm water depth mark  124  (step S 22 ). If the water depth is not abnormal (NO at step S 22 ), the procedure is ended. While, if the water depth is abnormal (YES at step S 22 ), the alarm  128  is displayed on the display screen  121  of the display instrument  109  through the display control circuit  108 , and the alarm circuit  112  is driven to produce the alarm by the buzzer  113  (step S 23 ). At the same time as this, the brilliance of the display screen  121  is raised to the maximum value through the brilliance adjustment circuit  110  (step S 24 ). 
   Thereafter, it is judged whether or not the alarm key  133  is pressed (step S 25 ), and if the alarm key  133  is not pressed (NO at step S 25 ), steps S 23  and S 24  are repeated. If the alarm key  133  is pressed (YES at step S 25 ), an instruction is given to the alarm circuit  112  to stop ringing of the buzzer  113 , and the alarm  128  displayed on the display screen  121  is erased (step S 26 ). Besides, at the same time as this, the control portion  106  gives an instruction to the brilliance adjustment circuit  110  and restores the brilliance of the display screen  121  to the former low brilliance (step S 27 ). 
   The present invention is not limited to only the foregoing embodiment, but is capable of adopting various modifications in addition. For example, in the above embodiment, although the liquid crystal display is adopted as the display instrument  109 , a CRT can be adopted instead of this. Besides, the alarm  128  of the display screen  121  may be displayed to flash on and off, and the volume of the alarm sound by the buzzer  113  may be increased as the water depth becomes shallow. 
   Further, the display apparatus of the present invention can be generally widely applied to an apparatus for outputting an emergency alarm in response to some change, such as a fish finding apparatus giving an alarm when a school of fish are detected, a radar plotter for giving an alarm when another ship enters a sea area within a predetermined distance from the present ship, or a water temperature meter for giving an alarm at a changing point of water temperature, in addition to the depth measuring apparatus described above. Besides, the present invention can be applied to not only such a single apparatus but also an apparatus of a combination of, for example, the fish finding apparatus and the depth measuring apparatus. 
   According to the present invention, since the screen of the display instrument comes to have high brilliance at the alarm time, the contents of the alarm can be instantaneously grasped and measures can be quickly taken. Besides, in the case where the alarm is given by sound as well, even in the case where the alarm sound is hard to hear because of surrounding noises or the like, the alarm can be recognized by the change in the brilliance of the screen, and hence, it becomes possible to prevent an accident such as stranding in advance. 
     FIG. 25  is an electrical block diagram of a depth measuring apparatus of another embodiment of the present invention. In the drawing, reference numeral  201  designates a transducer which is provided on the bottom of a ship and transmits and receives an ultrasonic wave;  202 , a transmission/reception switching circuit for switching the operation of the transducer  201  to a transmission side and reception side;  203 , a transmitting portion for giving a transmission signal to the transducer  201  through the transmission/reception switching circuit  202 ;  204 , a receiving portion for receiving an echo signal from the seabed received by the transducer  201  through the transmission/reception switching circuit  202 ; and  205 , an A/D converter for converting an analog signal received by the receiving portion  204  into a digital signal. A measurement means for measuring water depth is constituted by the transducer  201 , the transmission/reception switching circuit  202 , the transmitting portion  203 , the receiving portion  204 , the A/D converter  205 , and a control portion  206  which will be described hereinafter. 
   Reference numeral  206  designates a control portion including a CPU and for calculating the water depth to the seabed on the basis of output from the A/D converter  205 ;  207 , a memory including a ROM storing a program of the CPU and a RAM storing various data;  208 , a display control circuit for making the various data of the memory  207  displayed on a display portion  209 ;  209 , the display portion comprised with a liquid crystal display driven by the display control circuit  208 ; and  210 , an operation portion in which a plurality of keys described later are arranged. A display apparatus  200  of the present invention is constituted by the control portion  206 , the memory  207 , the display control circuit  208 , the display portion  209 , and the operation portion  210 . 
   In the depth measuring apparatus constructed as above, when a transmission instruction is given to the transmitting portion  203  from the control portion  206 , a transmission signal is outputted from the transmitting portion  203 , and an ultrasonic pulse signal is emitted in a beam form from the transducer  201  to the seabed through the transmission/reception switching circuit  202  switched to the transmission side. This ultrasonic search signal is reflected by the seabed, and its echo signal is received by the transducer  201 . This echo signal is received by the receiving portion  204  through the transmission/reception switching circuit  202  switched to the reception side, is converted into a digital signal by the A/D converter  205 , and is given to the control portion  206 . 
   The control portion  206  calculates a time from the emission of the ultrasonic pulse search signal to the reception of the echo signal, obtains the water depth to the seabed by calculation on the basis of the measured time, and stores the result in the memory  207 . The display control circuit  208  outputs data read out from the memory  207  to the display portion  209 , and various data including the water depth value to the seabed are displayed on the display portion  209 . 
     FIG. 26  shows an example of the display apparatus  200 . In the display apparatus  200 , the operation portion  210  is provided adjacent to the display portion  209  comprised with the liquid crystal display. On the display portion  209 , reference numeral  222  designates an oscillation line indicating the position of the transducer  201  on the bottom of a ship;  223 , a water depth scale provided at 5 meter intervals; and  224 , an alarm water depth line as a reference to produce an alarm. The alarm water depth line  224  can be arbitrarily set, and here, it is set at 15 m, and an alarm is given when the water depth is at this one or less. Reference numeral  225  designates a seabed line displayed on the basis of water depth measurement values, and is continuously displayed as a history on the basis of data for a predetermined definite time period. That is, the seabed line  225  at the position of a right end line  227  of the display portion  209  is the presently measured latest seabed line, and a past seabed line measured for a past definite time is displayed toward the left from this position. Reference numeral  226  designates a measured present water depth value, and the water depth value of 47.5 m of the seabed line  225  at the end  227  is displayed. 
   On the other hand, in the operation portion  210 , reference numeral  232  designates a draft key for setting a draft value;  233 , an alarm key for setting the foregoing alarm water depth line  224 ;  234 , an illumination adjustment key for adjusting the panel illumination of the operation portion  210 ;  235 , a brilliance adjustment key for adjusting the screen brilliance of the display portion  209 ;  236 , an automatic key for switching the automatic/manual operation of a range, sensitivity or the like;  237 , a color key for adjusting the color tone of the display portion  209 ;  238 , a minus key for renewing the display of the water depth history displayed on the display portion  209  in a reverse direction (left direction) and for decreasing a set value; and  239 , a plus key for renewing the display of the water depth history displayed on the display portion  209  in a positive direction (right direction) and for increasing the set value. 
   Reference numeral  240  designates a range switch for switching a display range of the display portion  209 ;  241 , a gain switch for adjusting sensitivity;  242 , a mode switch for switching a mode; and  243 , a power switch of the display apparatus  200 . The mode includes a BASIC mode for displaying a basic screen, a TEXT mode for displaying the water depth history in a table form, a CONTOUR mode for displaying the water depth history in a graph form, a HELP mode for displaying the explanation of operations and functions, and the like. In  FIG. 26 , the mode is set to the BASIC mode by the mode switch  242 . 
   Next, the procedure for displaying the guidance of a key in the help mode will be described. As shown in  FIG. 27 , when the mode switch  242  is switched to the position of “HELP”, the mode is set to the help mode, and a window for a help screen  228  is displayed on the display portion  209 . On the help screen  228 , an expression of “PUSH ANY KEY” is displayed, which indicates a state waiting any key to be pushed. 
   Next, when any one of the keys  232  through  239  in the operation portion  210  is pushed, as shown in  FIG. 28 , guidance  229  as to the pushed key is displayed in the help screen  228 . Here, there is shown a case where the draft key  232  for setting the draft value is pushed, and in the guidance  229 , as the explanation of the function of the draft key  232 , it is explained that the key is a key used when a draft value is inputted. Besides, the procedure of key operations in the case where the draft value is inputted is also displayed. 
   The operator looks at this guidance  229 , and inputs the draft value by the draft key  232 , the minus key  238 , and the plus key  239 . The inputted draft value is displayed at the portion of “DRAFT” at the right upper portion of the display portion  209 . Since  FIG. 28  shows the state before the draft value is inputted, the draft value is 0 m. When the draft value is inputted, the oscillation line  222  is shifted downward by an amount corresponding to the draft value, and the image of the seabed line  225  is also shifted downward together with that by the same amount. 
   In this way, in the case where the function and operation procedure of keys in the operation portion  210  are unknown, after the mode is set to the help mode by the mode switch  242 , when a key whose guidance is desired to be displayed is pushed, the guidance  229  as to the key is immediately displayed, so that the function and operation procedure of the key can be easily known without performing any complicated operation. 
     FIG. 29  is a flowchart showing the foregoing operation, and shows the procedure executed by the CPU of the control portion  206 . The control portion  206  reads out the mode set by the mode switch  242  (step S 31 ), and judges whether or not the mode is the help mode (step S 32 ). If it is not the help mode (NO at step S 32 ), the procedure is ended, and if it is the help mode (YES at step S 32 ), the help screen  228  of  FIG. 27  is displayed on the display portion  209  through the display control circuit  208  (step S 33 ). The image data of this help screen  228  is stored in the memory  207  in advance. 
   Next, it is judged whether or not any one of the keys  232  through  239  of the operation portion  210  is pushed (step S 34 ), and if a key is not pushed (NO at step S 34 ), the display of the help screen  228  is continued (step S 33 ), and if a key is pushed (YES at step S 34 ), the guidance  229  as to the key is displayed on the help screen  228  (step S 35 ). The image data of this guidance  229  corresponding to each key is also previously stored in the memory  207 . 
   Next, it is judged again whether or not a key is pushed (step S 36 ). Here, if another key is pushed (YES at step S 36 ), the guidance as to the key is displayed on the help screen  228  (step S 35 ), and renewal of the guidance  229  is performed. That is, while the mode is set to the help mode, each time a new key is pushed, the guidance  229  as to the key is renewed and is displayed. 
   If a key is not pushed at step S 36  (NO at step S 36 ), it is judged whether or not the help mode is ended, that is, the mode switch  242  is switched to another mode (step S 37 ), and if it is switched to another mode (YES at step S 37 ), the help screen  228  is erased and the procedure is ended (step S 38 ). If the help mode is not ended (NO at step S 37 ), the procedure returns to step S 36 , and the foregoing steps are repeated. 
   The present invention can adopt various modifications in addition to the foregoing embodiment. For example, in the foregoing embodiment, although the liquid crystal display is adopted for the display portion  209 , a CRT can be adopted in place of this. 
   Besides, in the above embodiment, although the mode display in the case where the help mode is set by the mode switch  242  is “HELP”, the help mode in the present invention indicates the mode for displaying the guidance as to keys, and is not necessarily limited to only the case where the mode name is “HELP”. Thus, as long as a guidance mode, a guide mode, or a mode such as an abstract X mode is a mode for displaying the guidance as to keys, all of them are included in the help mode of the present invention. 
   Besides, in the above embodiment, although the guidance  229  is displayed by pushing an actual key of the operation portion  210  in the help mode, it is also possible that keys of the operation portion  210  are displayed on the help screen  228 , and by pushing this key, the guidance  229  of the key may be displayed. In this case, the display portion  209  has only to be constituted by a touch panel. 
   Besides, in the above embodiment, although the guidance  229  is renewed each time a key is pushed in the help mode, it is also possible that the guidance  229  may be displayed side by side as a multi-window each time a key is pushed. 
   Further, the display apparatus of the present invention can be widely applied to another apparatus provided with keys and a display portion, such as a fish finding apparatus, a tidal current meter, or a water temperature meter, in addition to the depth measuring apparatus described above. Besides, the present invention can be applied to not only such a single apparatus but also an apparatus of a combination of, for example, the fish finding apparatus and the depth measuring apparatus. 
   According to the present invention, by merely pushing an objective key in a help mode, the guidance as to the key is instantaneously displayed, so that it becomes unnecessary to search and select an item from a menu and contents contrary to the prior art, the operation becomes very simple, and the working efficiency is greatly improved.