A surveillance system

Abstract

A method and a system for optimizing the data retention time period in a video surveillance system. Based on geofencing data from users (1) of a surveillance zone (17) a data processing proposes changes to the length of the data retention time period, in dependence of detected duration non-occupation of the surveillance zone (17).

Description

[0001] A SURVEILLANCE SYSTEM.

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a surveillance system, in particular but not exclusively to the storage and retention of image data recorded in the surveillance system.

[0004] BACKGROUND OF THE INVENTION

[0005] Video surveillance in conjunction with e.g. intrusion detection in specific zones such e.g. restricted areas and private premises, such as homes or businesses. Recorded images from the video surveillance may then be saved for an appropriate time helping in the subsequent identification of the intruders, e.g. if an intrusion involves criminal acts. For various reasons images from the video surveillance cannot and should not be retained infinitely, one reason being limited storage capacity of the necessary media for storing the images. Still, even with a limited retention period the storage capacity needed for image data may be substantial if for some reason there are frequent long recordings, in high resolution and high frame rate. In the end there is only the user to pay for this storage capacity irrespective whether the user provides the storage locally or, as is the outset of the present invention, paying a service provider for remote storage in a retention period, e.g. per subscription. The need for retention, i.e. the length of the data retention time period, may differ depending on the nature of the zone and for a given zone even change over time. Taking as an example home zone such as a flat or a house with a garden. In the home zone it would normally not be necessary or desirable to record images when the normal occupants, in the following referred to as users, are present. Detecting user presence and tailoring actions in the zone accordingly, i.e. geofencing, is well established. For smart phones, which usually have one or more Global Navigation Satellite Systems (GNSS) implemented on them may use a geofencing feature implemented on the smart phone. That is to say, defining a geofencing perimeter distance around the smart phone, allowing detection when the home zone comes within the perimeter as user with the phone approaches the known location of the home. Vice versa, a geofencing perimeter distance can be defined around the known location of the home to define the home zone allowing detection of the phone being within the perimeter of the home. Detection of presence in a given zone is inter alia disclosed in US2020 / 0304971. Examples of actions could be turning on the lights when first user enters an empty zone or tuning off lights once last user leaves and the zone again, or vice versa turning on alarm and video surveillance once the last user leaves and off again upon arrival of the first user. Depending on the household of the home zone it may be that the home zone is only left unoccupied for a few hours on normal working days, whereas over a weekend it could be left unoccupied for a couple of days or continuously have at least one user present. In holiday seasons prolonged periods where there are no users present in the zone could occur. On this background it is an object of the invention to facilitate the adaptation of the need for data storage to the actual circumstances.

[0006] US 2019 / 0141362 Al discloses a surveillance camera and surveillance data management system which includes a video management system with a data retention module for determining when surveillance data is deleted. Blocks of the surveillance data are deleted based on the ages of the blocks, which are calculated based on the difference between a recorded start date for each block and the current time and adjusted to subtract durations of periods of system downtime that occurred since the start date. The ages are also calculated using a counter, which is initialized to a predetermined value for each block and incremented or decremented only during uptime.

[0007] SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to overcome this problem, and to provide a surveillance system allowing the storage capacity for recorded images to be adapted to current needs.

[0009] According to a first aspect of the invention, this and other objects are achieved by a surveillance system, the surveillance system comprising at least one surveillance camera having a field of view covering an area within a zone, a detector adapted to detect the presence of a user in the zone, a communications connection adapted to communicate images from the at least one surveillance camera to a data storage device, said data storage device comprising digital storage media adapted to store the images for the length of a predetermined data retention time period in accordance with a data retention plan, the data retention plan defining the predetermined data retention time period, a communications connection adapted to communicate data regarding the detected presence of one or more users in the zone to a control center, a data processing device associated with the control center and adapted to, based on the data regarding the detected presence of one or more users in the zone to determine a non-detection time period where no user was present in the zone, and in dependence of said non-detection time propose, via a user interface, to at least one of said one or more users a change in the data retention plan.

[0010] According to a second aspect of the invention, this and other objects are achieved by a surveillance method, the surveillance method comprising: providing at least one surveillance camera having a field of view covering an area within a zone, providing a detector adapted to detect the presence of a user in the zone, providing a communications connection, communicating, via the communications connection, images from the at least one surveillance camera to a data storage device comprising digital storage media, and storing in said data storage device the images for the length of a predetermined data retention time period in accordance with a data retention plan, the data retention plan defining the predetermined data retention time period, providing a communications connection adapted to communicate data regarding the detected presence of one or more users in the zone to a data processing device associated with a control center and, determining, based on the data regarding the detected presence of one or more users in the zone, a non-detection time period where no user was present in the zone, providing a user interface, and proposing, in dependence of said non-detection time, a change in the data retention plan to at least one of said one or more users via the user interface.

[0011] According to a third aspect of the invention, this and other objects are achieved by a computer program product for a computing device, the computer program product comprising computer program code adapted to perform, when the computer program product is run on a processing unit of the computing device, the steps of: detecting the presence of one or more users in a zone, controlling the storage of images from at least one surveillance camera on a data storage device, said data storage device comprising digital storage media adapted to store the images for the length of a predetermined data retention time period in accordance with a data retention plan, the data retention plan defining the predetermined data retention time period, based on the detected presence of one or more users in the zone, determining a non-detection time period where no user was present in the zone, and in dependence of said non-detection time proposing, via a user interface, to at least one of said one or more users a change in the data retention plan.

[0012] These aspects of the invention allow the storage capacity for recorded images to be adapted to current needs of the user.

[0013] According to an embodiment of the first aspect of the invention, the surveillance system comprises a user specific tracking device for each of said one or more users, and the detector is adapted to detect the presence of the user in the zone based on the tracking device. Using a tracking device, such as a smart phone, is efficient in detecting the presence in the zone.

[0014] According to an embodiment of the first aspect of the invention, the tracking device is further adapted to communicate location information to the control center and the data processing device is adapted to propose to the at least one or more users the change in the data retention plan based on the location information. Using the location information of the tracking device allows the system to take into account the possibility of a user being so far away from the zone that that in itself would justify the suggestion of e.g. an extension of the data retention plan.

[0015] According to an embodiment of the first aspect of the invention the data processing device is adapted to propose, as the change in the data retention plan, an increase of the length of the predetermined data retention time period. Proposing an extension to a user makes sense when the user has been away for so long that image data is likely to be deleted before the user returns. This would be undesirable as if the user returns to the zone, e.g. a home zone, to find that there has been an intrusion such as a burglary, images of the perpetrator may already have been deleted according to current data retention plan. Not only the duration of the absence but also the distance from the zone, as mentioned above, may be relevant for this.

[0016] According to an embodiment of the first aspect of the invention, the data processing device is adapted to propose to the at least one of said one or more users the change in the data retention plan, if the duration of said non-detection time period is exceeding a predetermined fraction of the length of the predetermined data retention time period. This allows the proposal of the change of the data retention plan to be made in due time.

[0017] According to an embodiment of the first aspect of the invention, the data processing device is adapted to, based on the location information from each of the one or more users, to modify the predetermined fraction. This will allow the system to take into account also the distance of the user from the zone, e.g. to shorten the predetermined fraction and make the proposal of change earlier, if the user the receive the proposal is far away from the zone.

[0018] According to an embodiment of the first aspect of the invention, the length of the predetermined data retention time period is zero. This allows the system to make the proposal of implementing a data retention plan to a user who does not yet have one, once the system detects a prolonged non-occupation of the zone and / or if the system detects that users are so far away, that they cannot return soon.

[0019] According to an embodiment of the second aspect of the invention, the surveillance method further comprises the provision of a user specific tracking device for each of said one or more users and, using the detector, detecting the presence of the user in the zone based on the tracking device. Using a tracking device, such as a smart phone, is efficient in detecting the presence in the zone and thus facilitates the implantation of the method.

[0020] According to an embodiment of the second aspect of the invention, the surveillance method further comprises: communicating, by the tracking device, location information to the control center and proposing, by the data processing device, the change in the data retention plan to the at least one or more users based on the location information. Using the location information of the tracking device allows the system to take into account the possibility of a user being so far away from the zone that that in itself would justify the suggestion of e.g. an extension of the data retention time of the data retention plan.

[0021] According to an embodiment of the second aspect of the invention, the surveillance method further comprises: proposing, by the data processing device, as the change in the data retention plan, an increase of the length of the predetermined data retention time period. Proposing an extension to a user makes sense when the user has been away for so long that image data is likely to be deleted before the user returns. This would be undesirable as if the user returns to the zone, e.g. a home zone, to find that there has been an intrusion such as a burglary, images of the perpetrator may already have been deleted according to current data retention plan. Not only the duration of the absence but also the distance from the zone, as mentioned above, may be relevant for this.

[0022] According to an embodiment of the second aspect of the invention, the surveillance method further comprises: proposing, by the data processing device, to the at least one of said one or more users the change in the data retention plan, if the duration of said non-detection time period exceeds a predetermined fraction of the length of the predetermined data retention time period. This will allow the system to take into account also the distance of the user from the zone, e.g. to shorten the predetermined fraction and make the proposal of change earlier, if the user is far away from the zone.

[0023] According to an embodiment of the second aspect of the invention, the surveillance method further comprises: modifying, by the data processing device, the predetermined fraction based on the location information from each of the one or more users. This will allow the system to take into account also the distance of the user from the zone, e.g. to shorten the predetermined fraction and make the proposal of change earlier, if the user to receive the proposal is far away from the zone.

[0024] According to an embodiment of the second aspect of the invention, the surveillance method further comprises: proposing, by the data processing device, as the change in the data retention plan, a reduction of the length of the predetermined data retention time period. This will allow to change the data retention time of the data retention plan to a more data storage efficient plan if it is likely that the user does not need the current data retention plan, e.g. if the once the one or more users change their daily habits or if a previous change is no longer relevant, e.g. after a prolonged extraordinary absence.

[0025] It is noted that the invention relates to all possible combinations of features recited in the claims.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention. In the drawings:

[0028] Fig. 1 schematically shows a user in a home zone with a camera surveillance system and communication to a remote data centre,

[0029] Fig. 2 schematically shows the user geographically remote from the home zone and the data centre, and

[0030] Fig. 3 shows a flow chart of an embodiment of the method according to the invention, schematically shows

[0031] As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention. Like reference numerals refer to like elements throughout.

[0032] DETAILED DESCRIPTION

[0033] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

[0034] Turning first to Fig. 1 a camera surveillance system covering a surveillance zone, is shown. The zone can be any relevant area, such as restricted areas and private premises, e.g. homes or businesses. It may be solely indoors, such as in a flat or a house, or outdoors, such as a garden, or combinations thereof. In the illustrated example it is assumed that the zone is a home zone 17 of a at least one user 1. More specifically Fig. 1 shows the user 1 on a garden path 2 leading from the public pavement 3 along a street 4 in front of a house 5 to which the garden path 2 leads. The house 4 is surrounded by a garden 6 which also accommodates a parking area 7 for vehicles (not shown). The garden 6 is delimited by the public pavement 3 on one side and hedges 8 on the remaining sides. The camera surveillance system comprises not just one, but for the example two, outdoors surveillance cameras 9, one having a field of view overlooking the parking area and one having a field of view overlooking the garden path. There may be further surveillance cameras 9 with other fields of view inside the house 5. The surveillance cameras 9 are adapted to communicate images from the surveillance cameras 9 to a data storage device to a control center 10, e.g. through a router and the internet 13. This communication may be via a local device such as a Zigbee bridge 11 used in home automation to control other facilities such as heating, lighting, doorbells, remote controls, surveillance cameras 9, etc., the Zigbee bridge 11 normally being coupled to the router. Some of these such as heating, lighting, and surveillance cameras 9 may be controlled in dependence of user presence. The presence of a user 1 in the home zone 17 may be detected based on a tracking device such as a smart phone 12 using a detector, such as the Zigbee bridge 11, detecting when the user’s smart phone 12 logs on to the WiFi router of the house, to which the ZigBee bridge 11 would normally also be connected. Alternatively, the aforementioned geofencing feature implemented on the user’s smart phone 12 may be used. That is to say, the phone uses GNSS data to see if it is within or without a defined perimeter distance around the home defining the home zone or vice versa. The phone will not send the actual position but only if it is inside or outside of the defined zone. For privacy reasons, the surveillance cameras 9 will typically be disabled when a user 1 is present, i.e. the surveillance cameras 9 will only start the recoding if no user 1 is present at all.

[0035] The control center 10 would contain or have one or more associated data storage devices 14 with digital storage media adapted to store the images for the length of a predetermined data retention time period in accordance with a data retention plan, e.g. in dependence of a subscription agreement between a user 1 and a provider running the control center.

[0036] Often there will be more users 1 having a regular presence in the monitored home zone 17, such as husband, wife, children, boyfriends, girlfriends, parents etc. Regular presence largely ensures that undesired events such as burglary or other intrusion will be detected well before the expiry of the predetermined data retention time period, i.e. before old images are deleted according to plan. This in turn means predetermined data retention time period may be kept restively shorth and accordingly the necessary storage capacity on the digital storage media kept low. This in turn will be reflected in the cost of the user, since the service of the storage by the provider, is typically subscription based, e.g. the user paying a fixed monthly subscription fee for a predetermined retention time period, say 14 or 30 days.

[0037] Under certain circumstances, e.g. during holidays or business trips a single user 1 or a group of users 1, such as a family, could leave the home zone 17 unoccupied for a longer duration. If this duration, be it due to negligence or simply lack of awareness exceeds the predetermined retention period there is a risk of the users returning to a burglarized home zone 17 only to learn that the images of the perpetrators have already been deleted according to retention plan.

[0038] To avoid this, the communications connection adapted to communicate data regarding the detected presence of one or more users in the home zone 17 to the control center. In the control center or associated with the control center 10 a data processing device 15, such as a computer running suitable software, is adapted to determine a non-detection time period where no user was present in the home zone 17, based on the data received regarding the detected presence of one or more users in the zone 17. Based on suitable criteria, the software may then, in dependence of said non-detection time propose to at least one of said one or more users 1 a change in the data retention plan. This may be made via a suitable user interface, such as e.g. the screen of the smart phone 12, be it in the form of a notification from an app, via a RSC message, SMS text message, received email or the like. In this respect it should be noted that if email is used the interface could likely be a lap-top, a stationary computer, a tablet or the like.

[0039] Since in most situations frequent or regular occupation of the home zone 17 is the norm and longer periods of non-occupation rare, the typical first proposal by the data processing device, as the change in the data retention plan, would be an increase of the length of the predetermined data retention time period. The increased length of the predetermined retention time period need not necessarily be a fixed value. It could be a dynamic value such as the sum of the longest absence of the user 1 from the home zone 17 and an additional period of a fixed length. However, upon return to the normal situation with frequent or regular occupation, a proposal to shorten the data retention time period again should evidently be made to reduce again the necessary storage capacity of the digital storage media 14. The opposite may evidently also be the case, say if the longer retention period is the default, which could be the case if the zone 17 is at a summer house or the like.

[0040] Preferably, the software should take into account a rejection by the user 1 of a change and not send any new proposals for a certain time.

[0041] In order to allow the user 1 to consider whether the current length of the data retention period is sufficient, the data processing device 15 is adapted to propose the change in the data retention plan in due time. That is to say, if the duration of said non-detection time period is exceeding a predetermined fraction of the length of the predetermined data retention time period. This could be after 23 days if the length of the data retention time period is 30 days or after 10 days if the length of the data retention time period is 14 days. These fractions are of course just a matter of choice for the skilled person and other fractions than 23 / 30 or 10 / 14 could be chosen.

[0042] It may, however, be envisaged that there are situations where these fractions are too large, e.g. if the user 1 or all the users are so far away that increases the likelihood that they may not return before the end of the predetermined data retention time period. If as illustrated in Fig. 2 the user is at a remote location 18 on the other side of the Earth 19 the trip to the home zone 17 is bound to take some time. Accordingly, if the tracking device has a capability of detecting and sending location data, which is normally the case for smart phones 12, this information could be sent to the control center 10 either as direct communication 16 or via equipment in the home zone 17. Thus, if the data processing device assess that the user 1 is too far from the home zone 17 to return before the expiry of the retention time the user a smaller fraction will be set and the user prompted with the proposal of changing the retention time and the subscription at an earlier stage, to give the user more time to consider.

[0043] A further reason to propose to the user 1 a change in the data retention plan could be that a security event is detected, e.g. by automated analysis of the recorded images from the surveillance cameras 9, and the retention time for the recordings is about to end. In this situation too, an increase in the retention time could be proposed.

[0044] It can also be envisaged that the subscription is part of a general broader subscription for the home automation system covering a lot more services than retention of image data from surveillance cameras 9. Until the user 1 actually installs a surveillance camera the retention will be relevant, and the predetermined retention time period set to zero. The upon installation and set-up the user could be prompted to select a retention time period, as 14 or 30 days. It may also be that the user simply found the subscription price too high if the home zone 17 is occupied on a regular basis. Then, if the user 1 is far away or for a longer time but not subscribed, offer a subscription could be offered via the user interface. This is likely to be more effective and less disturbing than untargeted and thus many subscriptions offers, and might be perceived as care and interest, similar to reminders from the dentist or the like.

[0045] One way of executing the method on the data processing device in the control center 10 is illustrated in the flow chart of Fig. 3 representing a software program to be run on the data processing device 15 in or associated with the control center 10. There will of course be numerous other ways of securing the needs of the user 1, depending on the needs of the user 1 and the changes in the user’s behaviour.

[0046] The program is initialized in box 100, and in box 110 a retention time period, such as 14 or 30 days, is set, in accordance with e.g. subscription terms. In box 120 the program check the home zone 17 for occupants, i.e. users 1. If occupants are detected the program checks, in box 130, the elapsed time since the home zone 17 was last occupied and if that exceeds is less than a certain threshold TH1, e.g. a day, there is regular use and a pattern counter indicating daily use is incremented in box 140. Then in box 150 it is checked whether the pattern counter exceeds a certain threshold TH4 indicating prolonged daily occupation. If that is not the case the program returns to box 120 to check for occupation. If, on the other hand, this is the case it is in box 160, checked whether a notification rejection flag_2 is set, meaning that, at least for a given time period, the user 1 is not going to receive prompts regarding changes to the retention plan. If set the program returns to box 120 checking for occupants. If rejection flag_2 is not set, the proposal of a reduction of the retention period is, in box 170, sent to the user 1 via the user interface.

[0047] If in box 180 the user 1 accepts the reduction the rejection flag_2 is cleared in box 190 and the new reduced retention time is set in box 110. If on the other hand the user 1 rejects the rejection flag_2 is set in box 200, and the program returns to box 120 checking for occupants.

[0048] If in box 120 the program determines that there is no occupation it is determined in box 210 whether the duration of non-occupation exceeds a threshold T2, e.g. the aforementioned fraction of the length of the predetermined data retention time period. If the threshold TH2 is exceeded, it is in box 220 checked whether a notification rejection flag l is set, meaning that, at least for a given time period, the user 1 is not going to receive prompts regarding changes to the retention plan. If rejection flag l set the program returns to box 120 checking for occupants. If notification rejection flag l is not set, a proposed increase is, in box 230, sent to the user 1 via the user interface. If in box 240 the user 1 accepts the increase the notification rejection flag l is cleared in box 250 and the new reduced retention time is set in box 110. Otherwise, the notification rejection flag l is set in box 260, and the program returns to box 120 checking for occupants.

[0049] If instead it is determined that the time elapsed since last occupation is below the threshold TH2 the distance of the user 1 from the home zone 17 is checked in box 270 and factored in to yield a new threshold TH3. If in box 280 the program determines that the threshold TH3 is exceeded and no rejection flag l set in box 220 the proposed increase is made in box 230.

[0050] If in box 180 the user 1 accepts the reduction the rejection flag_2 is cleared in box 190 and the new reduced retention time is set in box 110. If on the other hand the user 1 rejects the rejection flag_2 is set in box 260, and the program returns to box 120 checking for occupants.

[0051] It is to be understood that the term pattern counter is not to be understood as a simple counter counting e.g. only the number of successive days during which an occupation of the home zone 17 occurred. Rather, it is to be understood as the outcome of a function retention_time(pl,p2,. . .), where parameters pl, p2 etc. could be part of a large geofencing data set showing occupation, duration thereof, duration of non-occupation, user distance to the home zone, regular other whereabouts of users such as workplace, as well as other data such as time of year, user specific inputs, etc., allowing the data processor to derive patterns, in turn allowing the data retention plan and the associated subscription to be tailored to the specific home zone 17 and updated when patterns change, e.g. when children move out to live on their own. Likewise, the use of remote location information could be used to tailor the data retention plan and subscription to the home zone 17. It should be noted that although there may be several users 1 associated with the home zone 17, there will typically only be one associated with and responsible for the subscription. The proposals may be made to this single user. The proposal may, however, also be made or more or even all of the users 1 of the home zone 17.

[0052] The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

Claims

CLAIMS1. A surveillance system, the surveillance system comprising at least one surveillance camera having a field of view covering an area within a zone, a detector adapted to detect the presence of a user in the zone, a communications connection adapted to communicate images from the at least one surveillance camera to a data storage device, said data storage device comprising digital storage media adapted to store the images for the length of a predetermined data retention time period in accordance with a data retention plan, the data retention plan defining the predetermined data retention time period, a communications connection adapted to communicate data regarding the detected presence of one or more users in the zone to a control center, a data processing device associated with the control center and adapted to, based on the data regarding the detected presence of one or more users in the zone to determine a non-detection time period where no user was present in the zone, and in dependence of said non-detection time period propose, via a user interface, to at least one of said one or more users a change in the data retention plan.

2. A surveillance system according to claim 1, comprising a user specific tracking device for each of said one or more users, and wherein the detector is adapted to detect the presence of the user in the zone based on the tracking device.

3. A surveillance system according to claim 2, wherein the tracking device is further adapted to communicate location information to the control center and the data processing device is adapted to propose to the at least one or more users the change in the data retention plan based on the location information.

4. A surveillance system according to any one of the preceding claims, wherein the data processing device is adapted to propose, as the change in the data retention plan, an increase of the length of the predetermined data retention time period.

5. A surveillance system according to any one of the preceding claims, wherein the data processing device is adapted to propose to the at least one of said one or more users the change in the data retention plan, if the duration of said non-detection time period is exceeding a predetermined fraction of the length of the predetermined data retention time period.

6. A surveillance system according to claim 5, wherein the data processing device is adapted to, based on the location information from each of the one or more users, to modify the predetermined fraction.

7. A surveillance system according to claim 4, wherein the length of the predetermined data retention time period is zero.

8. A surveillance method, the surveillance method comprising: providing at least one surveillance camera having a field of view covering an area within a zone, providing a detector adapted to detect the presence of a user in the zone, providing a communications connection, communicating, via the communications connection, images from the at least one surveillance camera to a data storage device comprising digital storage media, and storing in said data storage device the images for the length of a predetermined data retention time period in accordance with a data retention plan, the data retention plan defining the predetermined data retention time period, providing a communications connection adapted to communicate data regarding the detected presence of one or more users in the zone to a data processing device associated with a control center and, determining, by the data processing device, based on the data regarding the detected presence of one or more users in the zone, a non-detection time period where no user was present in the zone, providing a user interface, and proposing, in dependence of said non-detection time period, via the user interface, a change in the data retention plan to at least one of said one or more users via the user interface.

9. A surveillance method according to claim 8, wherein the surveillance method further comprises the provision of a user specific tracking device for each of said one or more users and, using the detector, detecting the presence of the user in the zone based on the tracking device.

10. A surveillance method according to claim 9, wherein the surveillance method further comprises: communicating, by the tracking device, location information to the control center and proposing, by the data processing device, the change in the data retention plan to the at least one or more users based on the location information.

11. A surveillance method according to any one of claims 8 to 10, wherein the surveillance method further comprises: proposing, by the data processing device, as the change in the data retention plan, an increase of the length of the predetermined data retention time period.

12. A surveillance method according to any one of claims 8 to 11, wherein the surveillance method further comprises: proposing, by the data processing device, to the at least one of said one or more users the change in the data retention plan, if the duration of said non-detection time period exceeds a predetermined fraction of the length of the predetermined data retention time period.

13. A surveillance method according to claim 12, wherein the surveillance method further comprises: modifying, by the data processing device, the predetermined fraction based on the location information from each of the one or more users.

14. A surveillance method according to any one of claims 8 to 10, wherein the surveillance method further comprises: proposing, by the data processing device, as the change in the data retention plan, a reduction of the length of the predetermined data retention time period.

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