# Efficient, Decentralized Detection of Qualitative Spatial Events in a Dynamic Scalar Field

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## Abstract

**:**

## 1. Introduction

## 2. Background

## 3. Model

#### 3.1. Algorithm Preliminaries

Algorithm 1 Update gradient vectors. | |

1: Restrictions: Geosensor network, $G=(V,E)$; sensor function $\mathit{sense}:V\times T\to \mathbb{R}$; communication neighborhood $\mathit{nbr}:V\to {2}^{V}$; identifier function $\mathit{id}:V\to \mathbb{N}$; reliable communication. | |

2: State transition system: $\langle \{idle,peak,pitx\},\left\{\right(idle,peak\left)\right),$ $(idle,pitx)\}\rangle $ | |

3: Local variables: the last sensing value, ${s}_{l}$, initialized empty; list of upstream neighbors, $\mathit{Nu}$, initialized empty; list of downstream neighbors, $\mathit{Nd}$, initialized empty; an ascent vector, $\mathit{av}$, initialized empty; the last ascent vector, ${\mathit{av}}_{l}$, initialized empty; a strong peak id, $\mathit{pkid}$, initialized empty; a last peak id, ${\mathit{pkid}}_{l}$, initialized empty; a strong pit id, $\mathit{ptid}$, initialized empty; neighbors’ peak identifiers, $\mathit{Pkcell}$, initialized empty; neighbors’ pit identifiers, $\mathit{Ptcell}$, initialized empty; a weak peak’s ascent bridge, ${\mathit{wpk}}_{av}$, initialized empty; | |

IDLE, PEAK, PITX | |

4: $\mathit{When}$ $\stackrel{\u02da}{\mathit{sense}}$ changes | |

5: set ${s}_{l}:=\mathit{sense}$ | - - Save the last sensed value |

6: broadcast (upd8, $\stackrel{\u02da}{\mathit{id}}$, $\stackrel{\u02da}{\mathit{sense}}$, $\mathit{pkid}$, $\mathit{ptid}$) | |

IDLE | |

7: Receiving (upd8, i, s, ${c}_{pk}$, ${c}_{pt}$) | |

8: update $\mathit{Nu}$ and $\mathit{Nd}$ | - - Based on s value |

9: update $\mathit{Pkcell}$ and $\mathit{Ptcell}$ | |

10: if $\left|\mathit{Nu}\right|>0$ and $\left|\mathit{Nd}\right|>0$ then | |

11: set $\mathit{av}:=max\left(\mathit{Nu}\right)$ | - - Likewise for descent vector |

12: if $\mathit{pkid}\ne \mathit{av}$’s peak identifier then | |

13: set $\mathit{pkid}:=\mathit{av}$’s peak identifier | |

14: broadcast (udsf, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{pkid}$, $\mathit{ptid}$, $"\mathrm{channel}"$) | |

15: if $\left|\mathit{Nu}\right|=0$ then | - -Likewise for Nd |

16: set ${\mathit{av}}_{l}:=\mathit{av}$ and set $\mathit{av}:=\varnothing $ | |

17: if $|{\mathit{wpk}}_{av}|>0$ then | |

18: set ${\mathit{av}}_{l}:={\mathit{wpk}}_{av}$ and set ${\mathit{wpk}}_{av}:=\varnothing $ | |

19: Receiving (udsf, i, ${n}_{pk}$, ${n}_{pt}$, $\mathit{sf}$) | |

20: if $\mathit{sf}="\mathrm{channel}"$ then | |

21: update $\mathit{Pkcell}$ | - - Update a neighbor’s peak |

identifier | |

22: if $i=\mathit{av}$ and ${n}_{pk}\ne \mathit{pkid}$ then | |

23: set $\mathit{pkid}:={n}_{pk}$ | |

24: broadcast (udsf, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{pkid}$, $\mathit{ptid}$, $"\mathrm{channel}"$) |

#### 3.2. Discrete Surface Networks

**Figure 1.**Identification of a strong and a weak peak, ascent vectors and an ascent bridge (Contour lines describe a scalar field showing the difference in elevation between consecutive contour lines. The sensed values can be estimated using a contour map).

**Figure 2.**Representative pass (Contour lines describe a scalar field showing the difference in elevation between consecutive contour lines. The sensed values can be estimated using a contour map.)

## 4. Algorithm

#### 4.1. Monitoring Events Occurring on Peaks and Pits

**Figure 3.**Stable/Unstable Morse complexes. This figure is adapted from [30].

#### 4.1.1. Algorithm 1: Update Gradient Vectors

#### 4.1.2. Algorithm 2: Monitor Peak Movement

- A node in a peak state that detects a new ascent vector (a higher neighbor) sends a wipk message to its ascent neighbor and transitions to an idle state (Algorithm 2, Lines 34–40).
- A wipk message is forwarded along the ascent vector until it reaches a peak (Algorithm 2, Lines 9 and 12).
- The peak receiving an wipk message evaluates whether it is a strong or a weak peak (Algorithm 2, Line 7). If it is a strong peak, this node transitions to a PEAK state (Algorithm 2, Line 27). If not, the node unicasts the wipk message via its ascent bridge (Algorithm 2, Line 29) and the algorithm continues from Step 2 above.

Algorithm 2 Monitoring a peak movement. | |

1: Fragment extend: Algorithm 1 | |

2: Local variables: the flag for receiving wipk messages, $\mathit{bpk}$, initialized $\mathit{false}$; neighbors adjacent to a potential peak, $\mathit{Tcells}$, initialized empty; | |

IDLE | |

3: Receiving (wipk, ${p}_{s}$, i, ${p}_{pk}$) | |

4: if $\mathit{pkid}={p}_{pk}$ then | |

5: if $\left|\mathit{Nu}\right|=0$ then | |

6: set $\mathit{bpk}:=$ true | |

7: broadcast (etcl, $\stackrel{\u02da}{\mathit{id}}$) | - - To identify a strong peak |

8: else | |

9: send (wipk, ${p}_{s},\stackrel{\u02da}{\mathit{id}},\mathit{pkid}$) to $\mathit{av}$ or ${\mathit{wpk}}_{av}$ | |

10: else | |

11: if $\left|\mathit{Nu}\right|>0$ then | |

12: send (wipk, ${p}_{s},\stackrel{\u02da}{\mathit{id}},\mathit{pkid}$) to $\mathit{av}$ or ${\mathit{wpk}}_{av}$ | |

13: Receiving (etcl, i) | |

14: if $\stackrel{\u02da}{\mathit{av}}=i$ then | |

15: send (cetc, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{true}$, $\mathit{pkid}$, $\stackrel{\u02da}{\mathit{sense}}$) to a node i | |

16: else | |

17: send (cetc, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{false}$, $\mathit{pkid}$, $\stackrel{\u02da}{\mathit{sense}}$) to a node i | |

18: Receiving (cetc, i, bflag, ${n}_{pk}$, ${n}_{s}$) | |

19: if $\mathit{bflag}=\mathit{false}$ then | |

20: set ${\mathit{wpk}}_{av}:=\{i,{n}_{pk},{n}_{s}\}$ | - - Set ascent bridge |

21: if $i\notin \mathit{Tcells}$ Tcells then | |

22: $:=\mathit{Tcells}\cup \left\{i\right\}$ | |

23: if $\left|\mathit{Tcells}\right|=\left|\mathit{Nd}\right|$ then | |

24: if $\left|\mathit{Nu}\right|=0$ then | |

25: if $\left(\right|\mathit{av}|>0$ and $|{\mathit{wpk}}_{av}|=0)$ or $\left(\right|\mathit{av}|=0$ and $|{\mathit{wpk}}_{av}|=0)$ then | - - A strong peak |

26: set $\mathit{av}:=\varnothing $; set $\mathit{bpk}:=\mathit{false}$; set $\mathit{Tcells}:=\varnothing $ | |

27: become PEAK | |

28: if $|{\mathit{wpk}}_{av}|>0$ then | - - A weak peak |

29: send (wipk, ${p}_{s},\stackrel{\u02da}{\mathit{id}},\mathit{pkid}$) to ${\mathit{wpk}}_{av}$ | |

30: else | |

31: send (wipk, ${p}_{s},\stackrel{\u02da}{\mathit{id}},\mathit{pkid}$) to $\mathit{av}$ or ${\mathit{wpk}}_{av}$ | |

PEAK | |

32: Receiving (upd8, i, s, ${c}_{pk}$, ${c}_{pt}$) | |

33: update $\mathit{Nu}$, $\mathit{Nd}$, $\mathit{Pkcell}$, and $\mathit{Ptcell}$ | |

34: if $\left|\mathit{Nu}\right|>0$ then | |

35: set $\mathit{av}:=max\left(\mathit{Nu}\right)$ | |

36: if $\mathit{pkid}\ne \mathit{av}$’s peak identifier then | |

37: set $\mathit{pkid}:=\mathit{av}$’s peak identifier | |

38: broadcast (udsf, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{pkid}$, $\mathit{ptid}$, $"\mathrm{channel}"$) | |

39: send (wipk, $\stackrel{\u02da}{\mathit{id}},\stackrel{\u02da}{\mathit{id}},\mathit{pkid}$) to $\mathit{av}$ | |

40: become IDLE |

#### 4.1.3. Algorithm 3: Monitor Peak Disappearance

- When a wipk message reaches a PEAK node, the node checks if the peak identifier contained in that message matches its own peak identifier. If not, this event triggers the PEAK node to return a rwpk message back down the ascent vector to the origin of the wipk message (Algorithm 3, Line 5 or Line 24).
- When the origin of a wipk message subsequently receives a rwpk message, it confirms a peak disappearance. The node then broadcasts swpk messages that trigger the update of peak identifiers (Algorithm 3, Line 12) in nodes below it in the surface.

Algorithm 3 Monitoring a peak disappearance. | |

1: Fragment extend: Algorithm 1, 2 | |

IDLE | |

2: Receiving (wipk, ${p}_{s}$, i, ${p}_{pk}$) | |

3: if $\mathit{pkid}\ne {p}_{pk}$ then | |

4: if $\left|\mathit{Nu}\right|=0$ then | |

5: send (rwpk, ${p}_{s},\stackrel{\u02da}{\mathit{id}},\mathit{pkid},{p}_{pk}$) to i | |

6: Receiving (rwpk, ${p}_{s}$, i, ${p}_{pk}$, ${p}_{pkl}$) | |

7: if $\stackrel{\u02da}{\mathit{id}}={p}_{s}$ and $\mathit{pkid}={p}_{pk}$ then | |

8: A peak identifier has been already changed because of udsf messages. | |

9: if $\stackrel{\u02da}{\mathit{id}}={p}_{s}$ then | |

10: if $\mathit{pkid}\ne {p}_{pk}$ then | |

11: set $\mathit{pkid}:={p}_{pk}$ | - - Peak disappearance |

12: broadcast (swpk, $"\mathrm{disappearance}"$, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{pkid}$, ${p}_{pkl}$) | |

13: update $\mathit{Pkcell}$ | - - Update neighbors’ peak |

identifier | |

14: else | |

15: send (rwpk, ${p}_{s},\stackrel{\u02da}{\mathit{id}},{p}_{pk},{p}_{pkl}$) to a node sent wipk message | |

16: Receiving (swpk, $\mathrm{e}$, i, ${p}_{pk}$, ${p}_{pkl}$) | |

17: if $\mathrm{e}="\mathrm{disappearance}"$ then | |

18: if $\mathit{pkid}={p}_{pkl}$ then | |

19: set $\mathit{pkid}:={p}_{pk}$ | - - Set new peak id |

20: broadcast (swpk, “disappearance", $\stackrel{\u02da}{\mathit{id}}$, $\mathit{pkid}$, ${p}_{pkl}$) | |

21: update $\mathit{Pkcell}$ | |

PEAK | |

22: Receiving (wipk, ${p}_{s}$, i, ${p}_{pk}$) | |

23: if $\mathit{pkid}\ne {p}_{pk}$ then | |

24: send (rwpk, ${p}_{s},\stackrel{\u02da}{\mathit{id}},\mathit{pkid},{p}_{pk}$) to i |

#### 4.1.4. Algorithm 4: Monitor Peak Appearance

- If a node detects pit movement, it broadcasts wnpk messages in order to infer peak appearance. (Algorithm 4, Line 5).
- When a node receives a wnpk message from a descent neighbor (i.e., via a descent vector, dv), it delivers a wnpk message to its ascent neighbor (i.e., via an ascent vector, av). By following each node’s ascent vector, this message reaches a peak (Algorithm 4, Line 10).
- If a node receives a wnpk message from a downhill node with the same pit identifier (but not a descent vector), this node broadcasts wnpk messages to its neighbors to trigger a wnpk message to peaks. These nodes usually exist on boundaries where two catchment areas meet (Algorithm 4, Line 12).
- If a node with the highest value among neighbors takes a wnpk message in an IDLE state, it will first check whether it is a strong peak or a weak peak (Algorithm 4, Line 20). If it is a strong peak, this node will broadcast swpk messages to create a new catchment area (i.e., a new peak appearance; Algorithm 4, Line 35).
- If a node receives a swpk message from an ascent neighbor, it will update its peak identifier that is indicated in the swpk message. It then broadcast an swpk message in order to trigger the construction of the new catchment area (Algorithm 4, Line 43).

Algorithm 4 Monitoring a peak appearance. | |

1: Fragment extend: Algorithm 1, 2, 3 | |

2: Local variables: the flag for receiving wnpk messages, $\mathit{bnpk}$, initialized $\mathit{false}$; | |

IDLE | |

3: Receiving (cebc, i, bflag, ${n}_{pt}$, ${n}_{s}$) | |

4: if A pit movement is confirmed then | |

5: broadcast (wnpk, $\mathit{ptid},\stackrel{\u02da}{\mathit{id}},\mathit{false}$) | |

6: Receiving (wnpk, ${n}_{pt}$, i, $\mathit{flag}$) | |

7: if $\left|\mathit{Nu}\right|>0$ and $\mathit{bnpk}=\mathit{false}$ then | |

8: if $\mathit{flag}=\mathit{false}$ then | |

9: if $\left(\right|\mathit{dv}|>0$ and $i=\mathit{dv})$ or $\left(\right|{\mathit{wpt}}_{dv}|>0$ and $i={\mathit{wpt}}_{dv})$ then | |

10: send (wnpk, $\mathit{ptid}$, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{false}$) to $\mathit{av}$ or ${\mathit{wpk}}_{av}$ | |

11: else | |

12: broadcast (wnpk, $\mathit{ptid},\stackrel{\u02da}{\mathit{id}},\mathit{true}$) | |

13: set $\mathit{bnpk}:=\mathit{true}$ | |

14: else | |

15: if $i\in \mathit{Nd}$ and $\mathit{ptid}={n}_{pt}$ then | |

16: send (wnpk, $\mathit{ptid}$, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{false}$) to $\mathit{av}$ or ${\mathit{wpk}}_{av}$ | |

17: set $\mathit{bnpk}:=\mathit{true}$ | |

18: if $\left|Nu\right|=0$ and $\mathit{bnpk}=\mathit{false}$ and $\mathit{bpk}=\mathit{false}$ then | |

19: set $\mathit{bnpk}:=\mathit{true}$; set $\mathit{Tcells}:=\varnothing $ | |

20: broadcast (ispk, $\stackrel{\u02da}{\mathit{id}}$) | |

21: Receiving (ispk, i) | |

22: if $\stackrel{\u02da}{\mathit{av}}=i$ then | |

23: send (rspk, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{true}$, $\mathit{pkid}$, $\stackrel{\u02da}{\mathit{sense}}$) to a node i | |

24: else | |

25: send (rspk, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{false}$, $\mathit{pkid}$, $\stackrel{\u02da}{\mathit{sense}}$) to a node i | |

26: Receiving (rspk, i, bflag, ${n}_{pk}$, ${n}_{s}$) | |

27: if $\mathit{bflag}=\mathit{false}$ then | |

28: set ${\mathit{wpk}}_{av}:=\{i,{n}_{pk},{n}_{s}\}$ | |

29: if $i\notin \mathit{Tcells}$ then | |

30: Tcells $:=\mathit{Tcells}\cup \left\{i\right\}$ | |

31: if $\left|\mathit{Tcells}\right|=\left|\mathit{Nd}\right|$ then | |

32: if $\left|\mathit{Nu}\right|=0$ and $\mathit{bpk}=\mathit{false}$ then | |

33: if $\left(\right|\mathit{av}|>0$ and $|{\mathit{wpk}}_{av}|=0)$ or $\left(\right|\mathit{av}|=0$ and $|{\mathit{wpk}}_{av}|=0)$ then | - - A strong peak |

34: set $\mathit{av}:=\varnothing $; set $\mathit{bpk}:=\mathit{false}$; set $\mathit{Tcells}:=\varnothing $ ; set $\mathit{pkid}:=\stackrel{\u02da}{\mathit{id}}$ | |

35: broadcast (swpk, $"\mathrm{appearance}"$, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{pkid}$, ${p}_{pkl}$) | |

36: become peak | - - Peak appearance |

37: if $|{\mathit{wpk}}_{av}|>0$ then | - - A weak peak |

38: A weak peak appearance | |

39: Receiving (swpk, e, i, ${p}_{pk}$, ${p}_{pkl}$) | |

40: if $e=$ “appearance” then | |

41: if $\mathit{pkid}={p}_{pkl}$ and $\mathit{av}=i$ then | |

42: set $\mathit{pkid}:={p}_{pk}$ | |

43: broadcast (swpk, $"\mathrm{appearance}"$, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{pkid}$, ${p}_{pkl}$) |

#### 4.1.5. Summary

#### 4.2. Monitoring Events Occurring on Passes

Algorithm 5 Monitoring events occurring on passes. | |

1: Fragment extend: Algorithm 1, 2, 3, 4 | |

2: Local variables: list of passes, $\mathit{Passes}$, initialized empty; list of pass cluster, Passcluster, initialized empty; list of a representative pass, Repass, initialized empty; list of a last representative pass, ${\mathit{Repasse}}_{l}$, initialized empty; | |

3: Receiving (uppc, i, $\mathit{Npasscluster}$) | |

4: if $\mathit{Passes}$ have the same associated peaks (pits) then | |

5: update$\mathit{Passcluster}$ and $\mathit{Repass}$ | |

6: if $\mathit{Repass}=\varnothing $ and ${\mathit{Repasse}}_{l}\ne \varnothing $then | |

7: send (wirp, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{Passcluster}$) to a node i | |

8: if $\mathit{Passcluster}\ne \mathit{Npasscluster}$ then | |

9: broadcast (uppc, $\stackrel{\u02da}{\mathit{id}}$, $\mathit{Passcluster}$) | |

10: Receiving (wirp, i, $\mathit{Nrepass}$) | - - Infer pass events |

11: if this node is a representative pass then | |

12: infer events occurring on a pass | |

13: else | |

14: deliver a wirp message to a representative pass. |

#### 4.3. Scalability

## 5. Experiments

#### 5.1. Experimental Setup

#### 5.2. Overall Scalability

**Figure 6.**Overall scalability for monitoring events (averaged over 10 randomized networks and 10 consecutive time steps in each evolution).

**Figure 7.**Number of messages generated by spatial events at each evolution step: dots are deemed to be outliers.

#### 5.3. Latency

#### 5.4. Load Balance

**Figure 10.**Load balance for communication messages (averaged over 10 networks of 4000 nodes and 10 consecutive evolution).

#### 5.5. Accuracy

Network sizes | 1000 | 2000 | 4000 | 8000 | 16,000 |
---|---|---|---|---|---|

Peak | 0.77 | 0.86 | 0.89 | 0.93 | 0.94 |

Pass | 0.84 | 0.91 | 0.90 | 0.88 | 0.91 |

**Figure 11.**Positive predictive value (PPV)-recall curves for network sizes of 1000–16,000 nodes on a Level 4 surface.

## 6. Discussion

## 7. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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Jeong, M.-H.; Duckham, M.
Efficient, Decentralized Detection of Qualitative Spatial Events in a Dynamic Scalar Field. *Sensors* **2015**, *15*, 21350-21376.
https://doi.org/10.3390/s150921350

**AMA Style**

Jeong M-H, Duckham M.
Efficient, Decentralized Detection of Qualitative Spatial Events in a Dynamic Scalar Field. *Sensors*. 2015; 15(9):21350-21376.
https://doi.org/10.3390/s150921350

**Chicago/Turabian Style**

Jeong, Myeong-Hun, and Matt Duckham.
2015. "Efficient, Decentralized Detection of Qualitative Spatial Events in a Dynamic Scalar Field" *Sensors* 15, no. 9: 21350-21376.
https://doi.org/10.3390/s150921350