The invention relates to a method and an arrangement for visualizing an alarm of an automation or control system in a geographic information system (GIS). The arrangement comprises the automation or control system, a human machine interface (HMI) for operating the automation or control system and the GIS. The alarm is displayed on a screen of the

HMI, as is a map-based view indicating a location of at least one technical component which is related to the operation of the automation or control system. The location of the at least one technical component is indicated by visualizing at least one GIS object associated to the at least one technical component.

An automation system may for example be a supervisory control and data acquisition (SCADA) system, and a control system may in particular be a distributed control system (DCS). These automation or control systems are preferably used for monitoring and controlling the operation and performance of geographically distributed industrial

installations, such as large scale manufacturing or production facilities with multiple sites, or utility grids for transmitting and distributing water, gas or electric power.

During the monitoring and controlling of such industrial installations, actions are initiated by an operator to be performed on technical components which is related to the automation or control system. Such technical components may be components belonging to the automation or control system directly, such as controller hardware or components of a communication network, or they may be components of the industrial installation, such as actuators (valves, pumps, motors or switches), sensors, power lines or pipelines.

For automation or control systems, the visualization of alarms is a core feature, wherein an alarm is meant to be any indication of a malfunction or potential problem of the

geographically distributed industrial installation. For visualization purposes, a geographic information system (GIS) may be used in parallel with or integrated into the automation or control system, wherein the GIS displays a map- based view showing the geographic location of the above described technical components . In the map-based view, so called GIS objects are visualized, wherein a GIS object represents a technical component itself or a feature which is related to the technical component, such as one of its hardware elements or software elements or information signals, e.g. input, output or internal signals.

Using the GIS, an operator may benefit from seeing the exact location of an alarm on a map. In US 2012/0032985 A1 , it is for example described that an operator may click on an object of interest in a map-based overview picture which illustrates a power network and several of its substations. By clicking on the object of interest, detailed information corresponding to the object of interest is shown in an integrated manner on the overview picture. The detailed information may for example contain an alarm list of a selected substation.

In ABB's product brochure "Symphony Plus, S+ Operations", document number

3BUS095408, a solution is described for providing a human machine interface (HMI) together with information and alarm management to the operator of a Symphony™ Plus distributed control system. Among others, a GIS is integrated in the HMI, allowing for cross navigation between alarms and GIS objects. This means that within a window of the control system, a map-based view can be opened, as is shown in Fig. 1 1 . Above the map-based view, the latest unacknowledged alarm is shown in the same form as the alarm text entries of an alarm list (27). The operator may then select this alarm and open a context menu (28) corresponding to the alarm, where the context menu (28) contains a function "GIS: Zoom to" (29). By clicking on this function, the map-based view will zoom to the GIS object which corresponds to the selected alarm.

In ABB's brochure "Symphony Plus, S+ Operations 1 .1 .0, User Guide", document number 2VAA001 150C, it is described that the alarm management of S+ Operations comprises the possibility for assigning tags of a tag database to an alarm group, in order to allow for the arranging of tags related to the same process area to be grouped together for easy management. The tag database is the primary element for almost all functions supported by S+ Operations. The term "tag" stands for a signal of the Symphony™ Plus distributed control system, including input and output signals as well as internal signals. Accordingly, each field sensor is associated with a tag. Each tag is associated with a set of attributes, including alarm limits. As a result, if an alarm limit is exceeded by a tag, an alarm is generated. When an alarm for a tag occurs, the corresponding alarm group is alerted and an alarm group button (25) starts blinking. A predefined number of such alarm group buttons are always shown in a row at the top of the window. Each alarm group button represents a navigation link to the corresponding alarm group's primary display. Below the row of alarm group buttons (25), a row of alarm list buttons (26) is situated. Each alarm list button represents a navigation link to a respective alarm and event list. An alarm list button shows the number of active unacknowledged alarms, and is color coordinated with its background color to the alarm of highest priority. The background color blinks if an alarm within the group is unacknowledged.

For an operator of an automation or control system, it is important to maintain an overview of the currently active alarms. This is relatively simple when using a stand-alone alarm list which is shown independently in a separate view or window or even on a separate screen, since the most recent alarms always appear on top.

When the alarm list is integrated in a map-based view in a way as disclosed in US

2012/0032985 A1 , only those alarms are shown which belong to a particular object selected from the map-based view. Any active alarms for a different object are not visualized in the map-based view.

In the above described S+ Operations system with integrated GIS, only the latest unacknowledged alarm is shown directly, while the other alarms are represented by their corresponding alarm group and alarm list buttons. In order to understand where the current alarm is geographically originated from, the operator has to enter a context menu, which may be time consuming if multiple alarms occur almost simultaneously in case of an emergency.

Accordingly, it is an object of the present invention to provide a method and an arrangement for alarm visualization in a geographic information system with which an operator may keep a good overview of currently active alarms.

This object is achieved by a method and an arrangement according to the independent claims.

The method according to the invention comprises the steps of

· transmitting information on a newly generated alarm to the geographic information system (GIS), • identifying the GIS object or group of GIS objects to which the newly generated alarm is associated,

• determining whether the GIS object or group of GIS objects is currently visible in a viewport of the map-based view,

· if so, including or amending a graphic alarm item in the map-based view for visual representation of the newly generated alarm, wherein the graphic alarm item is shown in connection with the GIS object or group of GIS objects,

• if not, including or amending a graphic direction item in the map-based view for visual representation of the direction in which the viewport needs to be panned for making the GIS object or group of GIS objects visible.

In the arrangement according to the invention, the automation or control system is configured to transmit information on a newly generated alarm to the geographic information system, and the geographic information system is configured to

· identify the GIS object or group of GIS objects to which the newly generated alarm is associated,

• determine whether the GIS object or group of GIS objects is currently visible in a viewport in the map-based view,

• if so, include or amend a graphic alarm item in the map-based view for visual representation of the newly generated alarm, wherein the graphic alarm item is shown in connection with the GIS object or group of GIS objects,

• if not, include or amend a graphic direction item in the map-based view for visual representation of the direction in which the viewport needs to be panned for making the GIS object or group of GIS objects visible.

A possible approach to visualize the currently active alarms in a map of a GIS would be to always show a corresponding graphic alarm item in close proximity to the location of the component to which the active alarm belongs to. At the moment, it is only known to visualize alarms using symbols having different colors.

However, the inventors have realized that this solution does not tackle the challenges discussed above. In particular, it may make it hard for the operator to maintain a good overview of the currently active alarms, as the number of alarms of the industrial installation may easily reach several hundred, and as the operator usually pans and zooms the viewport in order to investigate a certain area of the map. Accordingly, there would always be a risk of losing track of the alarms. In particular, newly raised alarms may belong to a part of the industrial installation which is currently not visible in the GIS view. According to the present solution, the newly generated alarm is in any case represented in the map-based view by a graphic item. However, in order to avoid losing track of alarms, it is proposed to detect whether the GIS object or objects to which the new alarm belongs would be situated in the visible area, also called viewport, of the map-based view. If so, a graphic alarm item is depicted at or in close proximity to the GIS object or group of GIS objects. In case that the GIS object or group of GIS objects lies outside the current viewport, a graphic direction item is shown at or close to the edge of the viewport, pointing in the direction of the alarm location or locations.

The proposed solution increases the safety of operating a distributed industrial installation, since the probability of an operator overlooking an alarm in a GIS is reduced considerably. The solution also improves the accessibility and usability, i.e., user friendliness, of alarm visualization in a GIS.

The information on the newly generated alarm may be transmitted to the GIS by either reacting to a request from the GIS for sending such newly generated alarms, or by pro- actively sending the newly generated alarm or alarms without waiting for a request. In other words, either a polling or a push mechanism may be applied.

In order to get a faster system response and to reduce refresh latencies, it is proposed here to prefer the push technology over the polling, e.g., to use web sockets, for transmitting a new alarm from the SCADA/DCS into the GIS component. As a result, the system

responsiveness is increased and alarm display latency is reduced.

The GIS component may be a system which either works independently from the automation or control system, or is integrated into the automation or control system; and which operates either as a standalone component or as a web-based component.

Preferably, the newly generated alarm is pushed to the GIS immediately after its generation or, alternatively, after a predetermined waiting time. By introducing a waiting time, it becomes possible to send newly generated alarms in a group-wise manner or to cumulate new alarms and to sort and push them according to their level of severity or according to a predefined priority. The arrival of the newly generated alarm may be illustrated not only by amending or newly introducing the graphic alarm item and/or graphic direction item, but by applying additional graphic, acoustic and/or haptic means so that the operator experiences in a sensorial way that something has happened. For example, new alarms may flash or blink shortly, and the map-based view may temporarily zoom and pan to the affected area, unless it is locked. Apart from this graphic emphasis, there may be a sound and/or the mouse may vibrate shortly.

The graphic alarm item and/or the graphic direction item may be an alarm group symbol for representing multiple alarms, indicating the total number and/or the severity level of the alarms. This is advantageous in order to reduce the cluttering of the map-based view with individual alarm symbols. In particular, the alarm group symbol may be applied if two or more alarms originate from the same GIS object. For example, if a water pumping station is represented by one GIS object, but contains several automation system tags, the alarms belonging to these tags may be displayed using an alarm group symbol that indicates their count and priority distribution, but at the same time allows an operator to interact with the alarms individually. The grouping of the alarms may be preferably performed automatically, based on a spatial proximity of the corresponding geographic locations and/or using custom rules. Accordingly, unlike the above described S+ Operations alarm grouping, the grouping suggested here is not pre-defined by user, but is determined automatically depending on the geographic location of the technical components to which the alarms belong, but also depending on the current zoom-level and panning position of the viewport.

Advantageously, the graphic direction item may be an arrow which is positioned close to one of the edges of the map-based view so that it points outwardly, away from the center of the map-based view, in the direction of the retrieved geographic location. In other words, if the newly generated alarm belongs to a GIS object or group of GIS objects which is located outside the current viewport, i.e. outside the map extent which is currently shown on the screen, an arrow close to one of the four edges of the map's viewport, i.e. close to the top, right, bottom or left of the window showing the map, may indicate two things: that a new alarm has arrived, by flashing, changing its color and/or size, and where the component is located to which the new alarm belongs, by pointing in that direction. An alternative solution of the graphic direction item may be a color band which is positioned around the edges of the map-based view. The color of that band may be changed in one part of the band, wherein the position of that part corresponds to the direction in which the newly generated alarm can be found if panned to.

If more than one alarm has newly arrived without being directly acknowledged by the operator, multiple corresponding arrows may be shown at the corresponding edges or the color of the color band may be changed in correspondingly multiple parts. The visible area of the map-based view may be panned towards the GIS object or GIS objects after having received a user input action performed with respect to the arrow or the color band. For example, if the user or operator clicks on the arrow or moves a pointer over the arrow, the panning may be performed. One arrow may represent multiple alarms corresponding to the same retrieved geographic location or to a group of neighbouring geographic locations and wherein the graphic representation of the arrow is varied according to the total number and/or severity level of alarms to which it is pointing. Accordingly, it is proposed to use arrows, also called navigation arrows, at the four edges of the GIS view for pointing towards the location of alarms that are outside of the viewport. In this way, no alarms shown in a GIS can be missed by an operator. The navigation arrows do not only represent the priority of alarms in a given area by, e.g., using different colors or arrow style, they also indicate their number, e.g., using the symbol size. In addition, they navigate to the affected area when an operator clicks on them with a mouse.

The at least one GIS object may be represented by a two-dimensional or three-dimensional shape, preferably described by data according to the Simple Feature Access data model. The Simple Feature Access data model is an OGC/ISO standard, defining among others the following features: Geometry, Point, Multipoint, LineString, MultiLineString, Polygon,

MultiPolygon, Triangle, CircularString, Curve, MultiCurve, CompoundCurve, CurvePolygon, Surface, MultiSurface, PolyhedralSurface, TIN and GeometryCollection.

Until now, SCADA/DCS alarm lists and GIS alarm representations have been displayed in a disparate fashion, i.e., no direct mutual interaction was possible. It is proposed to

synchronize the alarm view in the GIS with an alarm view in the automation or control system, i.e. in the SCADA/DCS, by highlighting an alarm in an alarm list directly when the corresponding graphic alarm item is selected by a user in the map-based view.

The selection may be performed, for example, by moving a mouse over it, clicking on it, marking it via a rectangle etc. Accordingly, no further user interaction, such as a selection of a synchronization action from a context menu, is required; instead the synchronization is performed immediately and automatically.

The alarm list may be displayed in a separate window from the map-based view or may be integrated in the map-based view. In an alternative embodiment, the synchronization between GIS and SCADA/DCS view may be realized without any selection, just depending on what is shown in the GIS map. For example, a GIS map may be predefined by a user as a master view, and then an alarm list or a trend could be defined as slave view which would always display information relating to the same objects visible in the GIS master view, and vice versa.

Further, the graphic alarm item or graphic direction item may be highlighted in the map- based view when the corresponding alarm is selected by a user from an alarm list in the SCADA/DCS. This means that moving a mouse over an alarm list entry or selecting multiple entries in a SCADA/DCS may highlight an alarm symbol or multiple alarm symbols in a GIS and vice-a-versa.

Even further, a miniaturized alarm list may be shown next to the graphic alarm item or graphic direction item in the map-based view when the graphic alarm item or graphic direction item is selected by a user, respectively, wherein the alarm list contains the alarm or alarms corresponding to the graphic item.

Analogously, a miniaturized map-based view containing the graphic alarm item and/or graphic direction item may be shown next to the corresponding alarm in an alarm list of the automation or control system, when the alarm is selected by a user from the alarm list.

This means that in the conventional SCADA/DCS alarm list, dynamically generated mini-GIS screenshots may be displayed next to an alarm message; and in the GIS, a popup or dialogue containing a mini-alarm list may be shown when an alarm is selected in the GIS or the mouse is moved over it. Accordingly, it is proposed to link the SCADA/DCS alarm lists and GIS alarm views so that alarms get automatically highlighted in each of the components simultaneously. When an alarm is selected in a SCADA/DCS alarm list, e.g., by moving a mouse over it or clicking on it with a mouse, the SCADA/DCS will automatically transmit the necessary commands to the GIS where the alarm will be highlighted. Also, alarms currently visible in a GIS view can be highlighted in the SCADA/DCS alarm list, e.g., by showing close the alarm a symbol or by using a different background color for this alarm.

The invention and its embodiments will become even more apparent from the example and its embodiments described below in connection with the appended drawings which illustrate:

Fig. 1 an alarm polling mechanism to load alarms into a GIS component

Fig. 2 an alarm push mechanism, directly from a DCS server to GIS components, Fig. 3 an alarm push mechanism, indirectly via a DCS client,

Fig. 4 an alarm group symbol for visualizing aggregated alarms,

Fig. 5 further possible alarm group symbols,

Fig. 6 an expansion of an alarm group symbol on mouse-over,

Fig. 7 navigation arrows showing the number and priority of alarms outside of the

current GIS view port,

Fig. 8 synchronization between a DCS alarm list and a GIS alarm view,

Fig. 9 a GIS mini alarm list opening in the GIS after mouse-over or mouse click,

Fig. 10 a mapping table between SCADA/DCS tags and GIS objects,

Fig. 1 1 a DCS window with integrated GIS, as known from the art.

Fig. 1 shows a SCADA/DCS system comprising a SCADA/DCS server 1 and an embedded (2, 3) or a loosely coupled (4, 5) GIS component. A polling mechanism is used to load alarms from the SCADA/DCS server 1 into the various GIS components 2, 3, 4, 5. As shown in Figs. 2 and 3, it is proposed to preferably use an event-driven push technology, for example Remote Procedure Call (RPC), SOAP or web sockets, for inserting alarms from a SCADA/DCS server 6 into a GIS component 7, 8, 9 and/or 10 for visualization purposes.

After having received a newly generated alarm, the GIS component 7, 8, 9 or 10 first associates the SCADA/DCS alarm with a corresponding GIS object, or group of GIS objects, as it is further explained in connection with Fig. 10 below. The geometry, e.g. point, line, polygon, etc., of the corresponding symbol for visualizing the GIS object or group of GIS objects may be loaded from a database management system using a standard Web Feature Service (WFS) protocol or some other protocol type in a GML, KML, JSON or other vector format that can hold the object's coordinates. For example, a group of closely related GIS objects may be represented by a polygon surrounding the geographic area where the corresponding technical components are located. Once the GIS object or group of GIS objects is known, the visual representation of an alarm, i.e. a graphic alarm item, is rendered on the screen with the map-based view of the

SCADA/DCS and of the distributed industrial installation controlled by it. If the spatial extent of the operator's viewport covers the affected location, the alarm symbol is shown directly within the map (see Fig. 4). If this is not the case, then the GIS component may temporarily zoom-in/out and pan to the affected area for a short period of time, and a graphic direction item, for example a navigation arrow, is introduced or changed in its appearance at the edge of the map-based view, i.e. at the edge of the map's viewport (see Fig. 7). The new alarm may be highlighted, e.g., by changing the appearance of the alarm symbol or graphic direction item, and/or by attracting the operator's attention in any other suitable fashion, e.g., using a sound, or vibrating a mouse.

Once the operator has acknowledged a newly arrived alarm, the appearance of the corresponding navigation arrow, alarm group symbol and/or individual alarm symbol may be changed again, for example from flashing or blinking towards a steady representation. The further means for emphasizing the arrival of the new alarm, like sound and vibration, may cease. The GIS system may automatically determine the right zoom level and extent of the map- based view in case that multiple alarms arrive simultaneously or within a short period of time, to avoid unnecessary changes of the viewport, which would result in a too fast zooming in and out or panning. It may also be provided for the GIS operator to lock the current GIS view, in order to prevent this automatic focusing from happening if it is undesirable.

In some cases, multiple alarms may be associated with one and the same GIS object or GIS component, e.g., pressure and flow alarms may be associated with one water pump or one pumping station. Normally, these alarms might overlap when represented using a symbol for each alarm individually. They become stacked on top of each other and only the latest alarm would therefore be visible. In such situations, it is vital to present alarms in a GIS in such a way that the operator is aware of the seriousness of the situation, understands the number and importance of active as well as past alarms, and has access to all alarms.

Therefore, it is proposed here to use an alarm group symbol that represents a group of spatially co-located alarms belonging to one and the same GIS object, or, alternatively, represents alarms belonging to several neighboring GIS objects that are located too close to each other, so that a drawing of individual alarm symbols would cause the symbols to overlap and hide one another. Alarm group symbols are shown in Figs. 4, 5 and 6.

The important property of the alarm group symbol is that it provides details of the number of alarms that are within the group and their severity. This can be achieved by using a different size for the alarm group symbol for representing the total number of alarms contained within, by using a suitable label, indicating for example the alarm count, and/or by using colors (e.g., red, yellow, green) or symbol types or shapes (e.g., square, triangle) for representing the severity levels or priorities of the included alarms.

Fig. 4 shows for example a circular first alarm group symbol 1 1 representing six alarms, and a second circular alarm group symbol 12 representing four alarms. The symbol may be divided into sectors in order to indicate a level of severity of the alarm. For example, in the second alarm group symbol 12, a quarter of the symbol may be colored in red to indicate that one alarm is of this highest severity, another quarter may be colored in yellow representing one alarm of medium severity, and the rest half of the symbol may be green for the remaining two alarms of low severity.

Alternative alarm group symbols are shown in Fig. 5, where in the third circular alarm group symbol 13 the number of alarms belonging to each level of severity is not only indicated by the size of the sector, but it is given directly by a corresponding number shown within the sector. The fourth and fifth alarm group symbols 14 and 15 are of quadratic shape and show at their right hand edge the severity level of the represented alarms via a traffic light arrangement, where only those colors show up which are actually present with their corresponding severity level in the corresponding alarm group. For example, the fifth alarm group symbol 15 indicates that only yellow = medium severe alarms are present, while the fourth alarm group symbol 14 represents all three severity levels.

Once a new alarm has arrived, this may be indicated to the operator by a flashing of the whole alarm group symbol, or just by a flashing of the sector of the alarm group symbol which corresponds to the same level of severity as the new alarm.

When an operator interacts with an alarm group symbol, e.g., by moving the mouse over it or by selecting it using a mouse click, the alarm group symbol may change its visual representation and provide easy access to individual alarms contained within, as is indicated in Fig. 6, where the circular alarm group symbol expands into a zoomed-in view of the corresponding geographical area so that the corresponding alarms can be visualized individually at their respective geographic location.

Regardless of the alarm grouping concept in place, there will always be situations when the operator zooms-in or pans the map in the GIS viewport in such a way that some of the alarms will not be visible. To avoid such situation, it is proposed here to use one or multiple graphic direction items, in particular navigation arrows. The navigation arrows are placed in the proximity of the top, right, bottom or left edges of the viewport and point in the direction of alarms that are currently not visible in the viewport, as is shown in Fig. 7. The area surrounded by the two rectangles 16 and 17 in Fig. 7 is the area where the navigation arrows 18, 19, 20 are located. The outer rectangle 17 indicates at the same time the outer limitations of the current viewport, i.e. of the currently visible area of the map-based view. The size of the arrows indicates the number of alarms in that direction, i.e. the bigger the arrow the more alarms are there in the given direction, and the color of the arrows indicates the alarm priority or severity level, e.g., the color is governed by the highest priority alarm. These two important properties can be represented using other suitable means, e.g., arrow style, label, etc..

The direction of the arrow may also be dynamic and may change fluently depending on the viewport's position as the operator pans and/or zooms or interacts with the viewport in any other way. When the operator clicks on a navigation arrow, the GIS viewport may pan and zoom into the area containing the alarm or alarms. After that, if the number of hidden alarms is not too high in one direction, there may be only one alarm group symbol visible on the screen. However, if the number of alarms is too high or when it is beneficial to group several alarms due to their spatial proximity, the navigation arrow may zoom and pan to a surrogate point so that all the grouped alarms are visible in the viewport. The grouping of alarms can be done fully automatically based on spatial proximity of alarms or using custom rules that consider the alarm priority, age, historical occurrences, etc.

While interacting with alarms in a GIS, it is recognized to be beneficial to have a live link with a conventional alarm list of the SCADA/GIS, which is typically displayed using a table. A bidirectional interaction is proposed here, as can be seen from Fig. 8, where the map-based GIS view is shown on the right hand side and the SCADA/GIS alarm list on the left hand side. The GIS view of Fig. 8 can be switched into a mode where it displays those alarms that are selected in the traditional alarm list, by automatically adapting the location and zoom- level. The selected alarm is here identified by a specific symbol. Alternatively, the selected alarm in the alarm list may be highlighted or flashing. Vice-a-versa, the traditional alarm list may highlight and/or filter alarms that are currently visible in the GIS viewport. Such interaction can be disabled or enabled and the selection of a master-slave control may be fully configurable per operator. The traditional alarm list and map-based GIS view can be displayed as separate windows, as in Fig. 8, and/or on different screens or alternatively on different machines via a remote connection and linked using a suitable communication mechanism.

It may be envisaged that the GIS would be the primary Graphical User Interface from where the entire SCADA/DCS is controlled. In that case, it is regarded to be of potential benefit to the operators to visualize mini alarm lists in the GIS view as is shown in Fig. 9. When the operator selects a GIS object, indicated in Fig. 9 by a circle, a mini-alarm list containing alarms related to tags or signals associated with the object, may appear directly in the map- based view. The mini alarm list can have a form similar to the one in Fig. 9 and may appear when the mouse is moved over the GIS object or after a selection of the GIS object via mouse-click.

Figure 10 illustrates a mapping table which may be used for associating the SCADA/DCS alarm with a corresponding GIS object, or group of GIS objects. In a first column 21 of the mapping table, a unique identifier is stored for each mapping. A second column 22 contains a unique name for each SCADA/DCS tag, where a SCADA/DCS tag represents an information signal from the SCADA/DCS. An alarm represents a state of a corresponding tag. Attributes for a tag may for example be alarm limits. Accordingly, when the SCADA/DCS tag or signal exceeds one of the alarm limits, a corresponding alarm is generated. From the third column 23, a unique identifier of a GIS object can be derived, where the GIS object belongs to a predefined GIS layer. The name of the GIS layer can be seen in a fourth column 24.

In the solution described above, an improved visualization of alarms in a GIS is achieved among others by:

• pushing new alarms into a GIS and applying a specific way of their visualization; · interactive navigation arrows at the edge of the screen pointing in the direction of active alarms to avoid missing an alarm;

• intelligent dynamic spatial aggregation, i.e., clustering, of alarms to limit the number of arrows on the screen at any point in time;

• synchronization of a GIS alarm view with a traditional list or table-based alarm list, where selections or filters in one view are automatically applied in the other and where the GIS displays alarms according to filter criteria in the traditional alarm list; displaying of a mini alarm list in a GIS after an interaction with an alarm

representation in a GIS, e.g., in form of a tooltip;

a way to visualize multiple alarms originating from the same geographical location in order to avoid spatial overlaps.