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Disaster, Infrastructure and Participatory Knowledge: The Planetary Response Network

using integrated community discussion software. The online infrastructure facilitates worldwide participation even for geographically focused disasters; this widespread public participation means that high-value information can be delivered rapidly and uniformly even for large-scale crises. We discuss lessons learned from deployments, place the PRN's distributed online approach in the context of more localized efforts, and identify future needs for the PRN and similar online crisis mapping projects. The successes of the PRN demonstrate that effective online crisis mapping is possible on a generalized citizen science platform such as the Zooniverse.


INTRODUCTION
During and after a natural disaster or other humanitarian crisis, there is a need for real-or nearreal-time information about an affected area. The informational needs of responders 1 , decision makers, and other stakeholders on the ground often fall under the broad term "situational awareness": these needs pertain to key information about features in the environment that inform decision making at all levels. It is often the case that a relatively small group of responders has an urgent need for accurate, reliably sampled information concerning a large area of interest (AOI). In the digital age, ample relevant data frequently exists but responders often lack the additional resources required to extract this information themselves (Tapia and Moore, 2014). This tension between a flood of data and a trickle of resources is familiar to many citizen science practitioners (Bonney et al., 2009;Wiggins and Crowston, 2011).
There are many types of crowdsourced responses to humanitarian crises, from locally-driven efforts (e.g., Dailey and Starbird, 2014;Brown et al., 2016) to those which filter SMS messages (Munro, 2013) and social media (Hughes and Palen, 2009;Popoola et al., 2013) and those which involve both local and remote participants (Rehman Shahid and Elbanna, 2015;Dittus, Quattrone and Capra, 2017). This work focuses on the application of distributed online citizen science principles and methods to the creation of humanitarian maps, sometimes called crisis maps (e.g., Ziemke, 2012;de Albuquerque, Herfort and Eckle, 2016), based on analysis of satellite imagery. Specifically, this paper offers a case study of the Planetary Response Network (PRN), 2 which since 2014 has provided rapid, accurate, high-value situational awareness to responders and decisionmakers in a disaster context. The PRN is run as a partnership between the Zooniverse, computer science researchers, and humanitarian response and resilience organisations. In the generalized citizen science project typology of Parrish et al. (2018), the 1 In this manuscript, the term "responder" is used to refer to those who use the results of this project to coordinate and execute humanitarian responses on the ground in affected areas. It is distinct from terms such as "volunteer", PRN is a "data generated: active participation: virtual: multiple independent classifications" project type. Within the Disaster Research Center (DRC) typology (for a recent review of this typology, see Strandh and Eklund, 2018), the PRN blends aspects of both the Extending and Emergent types of disaster response organizations. It is an online, distributed project that, within a recently-described geographic citizen science framework (Skarlatidou and Haklay, 2021), uses participatory design principles to capture volunteered geographic information (VGI) in a generalized (i.e., not purely geographic) web application interface.
The field of online distributed humanitarian mapping is relatively new and still evolving (Meier, 2011(Meier, , 2012Ziemke, 2012;Sharma and Joshi, 2019;Turk, 2020). Assessments following the 2010 earthquake in Haiti (e.g., Zook et al., 2010;Harvard Humanitarian Initiative, 2011) and subsequent disasters have shown both the promise of crowdsourced crisis mapping and its challenges. For example, Westrope, Banick and Levine (2014) analyzed the OpenStreetMaps response to Typhoon Haiyan and found that the rapid assessment was valuable to responders but was limited by inaccurate labels, lack of participant training, and uneven coverage of the AOI. More generally, some of the challenges of online crisis mapping are relatively specific to that application, such as the lack of shared technical language between project teams and responders, competing priorities for security, privacy and publicity, and the psychological toll that participation in a project may take on its volunteers (Ziemke, 2012;Liu, 2014). Other challenges, such as data verification and reliability (Haklay, 2013;Kosmala et al., 2016;Parrish et al., 2018) and boundary issues between different involved groups (Shirk et al., 2012;Oswald, 2020), have found multiple solutions in the broader realm of citizen science. In many cases these solutions were known to citizen science practitioners prior to the mainstream emergence of online distributed humanitarian mapping (e.g. participant training, label aggregation and validation, and uniform data "classifier", and "participant", which refer to those who participate in the online citizen science project to provide individual feature labels. coverage; Lintott et al., 2008). Blending best practices from both fields is thus of high potential value to each.
The PRN is slightly different from other crisis mapping efforts (e.g., Ushahidi, Humanitarian OpenStreetMap), in part because it runs on the Zooniverse citizen science platform instead of a platform built specifically for mapping. As such, it must approach the mapping aspect of deployments slightly differently, but benefits from over a decade of lessons learned regarding citizen science project design, data quality and community engagement. It additionally benefits from exposure to the Zooniverse community of over 2 million registered participants. The choice of platform enables the PRN to complement, rather than compete with, existing crisis mapping efforts. This case study aims to describe the project design, present its deployment statistics, and evaluate project outcomes in the context of the field of distributed online crisis mapping, including a summary of lessons learned.

PROJECT DESIGN
The PRN has deployed multiple times for specific disaster responses. To date, the PRN has exclusively made use of satellite imagery for data assessment. Satellites provide verified data over large areas, which facilitates the rapid and broad situational analysis the PRN prioritizes. Within that context, we make project design choices (described in subsections below) that align both with the technical design of the Zooniverse platform and with domain-specific needs.
Deployments of the PRN also adhere to best practices in citizen science (e.g., Lintott and Zooniverse, 2010;Gold, 2019); projects must have a genuine and beneficial outcome whose goal can be expressed in advance. In a disaster relief context, while outcomes may include e.g. providing training data for machine learning algorithms (Isupova et al., 2018;Weber and Kané, 2020), the primary goal of providing useful information to improve situational awareness necessitates that the PRN include partners on the ground. Co-creating crisis maps with local stakeholders is a critical step to prevent the 3 rescueglobal.org abstraction of digital humanitarian projects from those they affect (Mulder et al., 2016).
While the specifics of the PRN pipeline have evolved over time, they generally involve three phases: (1) a planning phase, where all parties consult, gather available data, and decide on urgent situational awareness needs; (2) a crowd labeling phase, where volunteer classifiers assess available imagery (typically answering questions about and marking features on images); and (3) an analysis phase, where crowd labels are aggregated to produce a consensus result and feature assessments are produced. These phases may be repeated several times during a single deployment as more data becomes available and/or the response needs evolve. Following each analysis phase, the PRN produces a "heat map" for each feature of interest and delivers these to responders. In addition to the post-analysis heat maps, the involvement of the crowd facilitates rapid identification of unexpected features that impact situational awareness.
Once a deployment is complete, the PRN organizational team meets to discuss successes, failures, and near-failures, so that we learn from these and improve our future pipeline. Many of the lessons learned discussed below crystallized from these post-deployment self-assessments.

Planning Phase
When a disaster is imminent or has just occurred, PRN partners consult with each other to decide whether a deployment is appropriate. For the PRN deployments to date, our primary domain-expert partner was Rescue Global, a UK response and resilience charity that operates worldwide. 3 Rescue Global's work includes search and rescue activities as well as liaising with local governments and stakeholders.
The advance relationships Rescue Global builds on the ground with local governmental and communitybased organisations are crucial to the project. Given that many disasters strike communities in the global south, the need for local partnerships is especially critical for a response effort (such as the PRN) whose members are primarily from the global north. Rescue Global's ongoing partnerships have included organisations such as the Caribbean Disaster and Emergency Management Agency (CDEMA) and the Mexican Jewish non-profit Cadena, which operates local branches throughout Central and South America. These larger non-profit partnerships have also facilitated relationships with individual communities in these and other regions, which helps Rescue Global communicate local priorities to the PRN team at all stages of a deployment. By partnering directly with a single organization whose expertise includes cultivating multiple local relationships, the PRN team can maximize the chances that a deployment will appropriately address the needs of affected individuals and communities, while minimizing the costs and risks of developing separate relationships from a remote position. The necessity of involving local stakeholders is echoed by many studies of geographical citizen science projects (e.g., Hecker et al., 2019;Skarlatidou and Haklay, 2021).
For each PRN deployment, once Rescue Global confirms they will deploy to the region and would benefit from improved situational awareness, the other partners begin assessing imagery data availability. Satellite data availability can be a complex landscape. Some data is fully open, such as that from NASA's Landsat or ESA's Sentinel constellations. Higher-resolution imagery often comes from commercial providers, which may have their own humanitarian data programs and may also participate in the International Disasters Charter. 4 The date and resolution of available data varies, impacting deployment planning. Satellite imagery is typically not available for at least 24 hours after a disaster, and this can increase due to tasking delays, orbital patterns, and weather. Long delays can force deployment priorities to shift. The resolution of available data affects the labels that can be reliably collected (Battersby, Hodgson and Wang, 2012;See et al., 2013;de Albuquerque, Herfort and Eckle, 2016) and the speed of collection. Considering the needs of the responders and local decision-makers in the context of evolving data availability is critical to ensuring the relevance and utility of crisis maps (Ziemke, 2012;Turk, 2020).
The assessment time for satellite imagery also depends on the complexity of the features being assessed. Citizen science projects across all disciplines must consider tradeoffs between labelling speed and the level of detail captured. For example, collecting binary responses about an image is fast but sacrifices considerable detail compared to drawing individual polygons around each feature. For the PRN, the needs of responders and local stakeholders, not academic researchers, take priority in project design. Responders are accustomed to operating with "good enough" information (Tapia and Moore, 2014), and generally do not require highly granular maps, especially in the early days of a response. PRN deployments have thus generally asked volunteer classifiers to label features with point marks, as this prioritizes the speed of classification while sacrificing precision at a level acceptable to responders. This choice deliberately places responders' immediate needs above the future needs of our computer science partners who use PRN damage labels to train machine learning algorithms between live deployments. 5 The processing of satellite imagery is also part of the planning phase. PRN leadership procures available data, decides which datasets to use for a given deployment, and assembles geo-referenced preevent and post-event image mosaics. Ad-hoc decisions are often required to optimize tradeoffs between cloud cover, image quality, and imaging date, given sparse time-sampling of the AOI in both pre-and post-event imagery. Following assembly and resolution matching of pre-and post-event mosaics, the images are tiled into matched sections of a manageable size for data labeling. Typically, the mosaic is sliced into square sub-images of 500-600 pixels on a side, which will be assessed by volunteer classifiers via the web and mobile devices (described further below). For high-resolution imagery the data labels are collected on image subsections as small as 150 m × 150 m, whereas for medium-resolution imagery this can be as large as 6 km × 6 km. All image subsections are large enough to provide useful context for damage assessments.

Crowd Labeling Phase
The Zooniverse is the world's largest online crowdsourcing platform for citizen research. For a detailed glossary of Zooniverse terms and infrastructure description, we refer the reader to Simpson, Page and De Roure (2014). In the PRN, Zooniverse volunteers typically classify paired preand post-event image subsections as a single unit of data; we refer to these image pairs as "subjects" below. A completed collection of tasks that each volunteer classifier is asked to submit within a workflow for each subject is called a "classification".
Like most Zooniverse projects, the PRN collects multiple classifications per subject. Aggregating multiple independent classifications addresses many data quality challenges identified within citizen science generally (Haklay, 2013;Parrish et al., 2018) and in Earth Observation and crisis mapping specifically (Harvard Humanitarian Initiative, 2011;Liu, 2014;Westrope, Banick and Levine, 2014;Fritz, Fonte and See, 2017). The minimum number of classifications the PRN collects per subject and workflow is generally at least 10. Subjects are served randomly to volunteer classifiers from within a set of subjects. Our design choices contrast with those of other crisis mapping projects, which allow users to choose their own map location and to submit highly detailed labels. Our choices are designed to facilitate rapid, uniform coverage of the entire AOI and to deliver initial results to responders as quickly as possible at their required level of precision. Figure 1 shows a PRN project screenshot showing the classification interface, with examples of both mobile and web interfaces. For deployments where available data may be of variable quality across an AOI, we have found best results with a combination of workflows which filter the subjects in a cascading fashion. Volunteers first assess whether images are "classifiable" (defined as having land visible). Only subjects with a majority of "Yes" responses are added to the feature marking workflow. One advantage of splitting the workflow is that the Yes/No workflow can also be deployed in the Zooniverse mobile app, whose interface facilitates rapid classification. Separating the project into multiple workflows thus optimizes for overall speed of classification without sacrificing completeness of coverage. Classifiers also tend to find this structure more satisfying than in early PRN deployments where feature marking workflows included high fractions of unclassifiable images. Classifiers may access additional resources for help (Katrak-Adefowora, Blickley and Zellmer, 2020) on either a specific task or on the overall project. The Field Guide feature, available for all workflows, allows classifiers to see multiple examples of the different types of labels.
When further data becomes available, the crowd labeling phase may continue with additional rounds of imagery. The project organizers announce each image set to participants in the project's Talk community discussion area; if additional attention is required, the Zooniverse team may also send a newsletter to either the existing project community or a wider Zooniverse audience. We have sustained high levels of engagement over several weeks due to newsletter campaigns and regular data releases. Each deployed Zooniverse project remains active until the PRN partners decide the crowd labeling phase is complete. As soon as participants classify the first image set, the analysis phase of the PRN begins. In the analysis phase, we derive consensus from individual labels by volunteer classifiers. This aggregation step accounts for individual variations in assessment styles and minimizes the impact of the small fraction of classifications that contain errors (Lintott et al., 2008;Simmons et al., 2017) by resolving disagreements among the crowd and arriving at a high-confidence final label set.
In past deployments, individual labels have been aggregated using the Independent Bayesian Classifier Combination (IBCC) machine learning algorithm (Simpson et al., 2013;Ramchurn et al., 2016). This algorithm calculates the reliability of each classifier and combines their labels into a single map by weighting each classifier contribution according to their reliability. The IBCC algorithm is unsupervised, i.e., no ground truthing (physical and/or expert verification of feature labels) is required to produce the crisis maps. However, if expert labeling is available then the algorithm can fold these into the maps as ground truth.
The aggregation incorporates individual pointmarked labels for each feature type, as well as "blank" marks where a classifier indicated there was no feature of interest in the image. The aggregated labels are then turned into "heat maps" for each feature type. A heat map is a color-coded overlay on the satellite image. Figure 2 shows an example of heat maps provided for Dominica following Hurricane Maria in 2017, based on medium-resolution imagery from Planet. The resolution of the heat map grid is chosen to reflect both the resolution of the satellite imagery and the level of map detail required by the responders. These digitized maps are bundled together and forwarded to our partner responders.

PROJECT DEPLOYMENT STATISTICS
The PRN has so far deployed live, time-sensitive projects 4 times: (1) following the two earthquakes with magnitudes 7.8 and 7.5 in Nepal in spring 2015, (2) following the 7.8-magnitude earthquake in 6 The projects we focus on here are at zooniverse.org/projects/vrooje/planetary-responsenetwork-and-rescue-global-caribbean-storms-2017 and Ecuador in April 2016, (3) following Hurricanes Irma and Maria in the Caribbean in autumn 2017, and (4) following Hurricane Dorian in autumn 2019. We have additionally prepared other projects that did not deploy (i.e., they never entered the crowd labeling phase described above). Projects may fail to deploy for a number of reasons, including changes to ground access granted by local governments and revised estimates of event severity (e.g. a hurricane that changes course or dissipates). We choose to focus here on the two most recent deployments of the PRN, as these projects exemplify the general properties of PRN deployments while being similar enough to each other to facilitate comparison. 6 Quantitative and technical details for the PRN Caribbean deployments are given in Appendix 1. The two projects jointly collected over 1 million individual classifications from thousands of online participants. Figure 3 shows classifications collected over time from logged-in and not-logged-in participants for both deployments.
In both projects, Rescue Global joined the PRN team as on-the-ground partners. In the 2019 deployment responding to Hurricane Dorian, we also partnered with 24 Commando Royal Engineers, a unit of the British Army's Royal Engineers who provide military engineering support to 3 Commando Brigade Royal Marines and who had additional assessment needs (see Appendix 1).
Rescue Global has good relationships with multiple governmental and non-governmental organizations (NGOs) active in the Caribbean region. As a result, the heat maps provided by the PRN had wide reach during both deployments. The maps were delivered to over 60 NGOs, the UN and the Caribbean Disaster Emergency Management Authority (CDEMA). Below we analyze deployment outcomes and critically evaluate the PRN to extract several generalized lessons that may be learned from this case study of online citizen science for humanitarian aid. zooniverse.org/projects/mrniaboc/planetary-responsenetwork-hurricane-dorian

EVALUATING PROJECT OUTCOMES
There are many relevant lenses through which to assess the outcomes of the Planetary Response Network. Some are purely related to the citizen science aspect, while others additionally consider our humanitarian objectives. Below we evaluate the PRN in the context of its success in engaging the crowd, the nature of that crowd, the speed of delivery of heat maps, the quality of the data delivered, and evidence of actual use of the maps in the field. We also comment on the process of assessing and learning from failures.

Engagement By, and With, PRN Participants
The overall number of classifiers who participate in a given Zooniverse project can vary from hundreds to hundreds of thousands. Given the duration of each PRN deployment, the fact that thousands of people have participated represents a strong level of participation compared to other short-duration Zooniverse projects. In general, the success of a Zooniverse project is related to both project design and volunteer engagement, rather than project duration (Cox et al., 2015).
The project statistics (see Appendix 1) are also typical of healthy Zooniverse projects. The classification activity over time (Figure 3), while more varied than a typical Zooniverse project, is within expectations for a project with time-sensitive data and staggered data releases (Spiers et al., 2019). The fraction of classifications submitted by logged-in participants is approximately 85% throughout both projects, which is also within normal ranges for successful Zooniverse projects (Cox et al., 2015).
The "Talk" discussion area, where participants can engage more deeply via open-ended discussions and tagging interesting subjects, is a valuable part of the Zooniverse ecosystem. Within the Talk area for each PRN deployment, about 10% of logged-in participants posted at least 1 comment. Figure 4 shows the average word count per post for each participant who posted on Talk. Even among those who choose to join the Talk discussion, participation is not evenly distributed: in both deployments, approximately half of participants who posted on Talk posted a single comment, with a majority (68% and 77% in the 2017 and 2019 deployments, respectively) posting 3 or fewer comments.
The nature of Talk comments varied, from singleword notes tagging an image snapshot with a hashtag (including unexpected features of interest not captured by the main classification interface) to lengthy posts with discussion, comments, and suggestions. The PRN leadership also posted regularly, using Talk to update participants with descriptions of new image datasets and sharing preliminary heatmaps and feedback from responders. As in many Zooniverse projects, we observed "trickledown training" occurring on Talk, in which advice and tips initially shared by the project organizers were subsequently shared by other participants in response to common inquiries from less experienced classifiers.
The Talk environment also allowed us to directly address the risk of participant burnout and secondary trauma inherent to online crisis mapping projects (Ziemke, 2012). To alleviate these risks, the PRN lead created a section of Talk explicitly for taking breaks and regularly reminded people that stepping away from the project was a healthy action that would not endanger those on the ground. Overall, this represented a small fraction of Talk interactions: it was more common that participants expressed sentiments of accomplishment and satisfaction. Still, both participant engagement generally and burnout prevention specifically are important ongoing responsibilities of teams organizing crisis mapping efforts. This is a domain-specific reflection of the need to provide a supportive environment for all 7 Using Bayesian binomial confidence intervals and comparing PRN traffic to overall Zooniverse traffic during the same dates and during a 1-week period outside hurricane season (March 2019), we estimate the probability participants in a citizen science project (Resnik, Elliott and Miller, 2015;Chari, Blumenthal and Matthews, 2019).
The PRN is a virtual and distributed crisis mapping project. Analytics for the landing pages on both Caribbean projects indicate a wide global reach of visitors (over 130 countries represented overall). For both deployments, over 85% of web browser sessions originated in North America and Europe, which is generally consistent with overall Zooniverse traffic during deployment periods. More local participation from Caribbean countries represented less than 1% of browser sessions in either project; however, this fraction is higher than Caribbean traffic Zooniversewide (<0.1% to non-PRN projects). This difference is statistically significant 7 and reflects an increased local interest in the PRN even while overall participation is much more widely distributed.
Therefore, while the PRN does generate some local activity, it primarily provides an opportunity for a global community to meaningfully contribute to a humanitarian aid effort, even (and possibly especially) when its members are too far from the that the fraction of browser sessions from the Caribbean for PRN deployments is consistent with the fraction outside PRN projects is p < 5 × 10 -6 . affected area to offer help in person. This complements humanitarian crowdsourcing projects that are more "ground up" in their origins: whereas those projects often provide highly localized and detailed individual information, the PRN can provide rapid and uniform coverage of a large affected area at a broad level of detail suitable for responders seeking to inform their initial and ongoing allocation of resources. This complementarity reflects the similarities and differences of these two approaches. Specifically, both "ground up" and "top down" crowdsourced crisis mapping efforts often strive to improve knowledge of a specific disaster by blending VGI with traditional sources of geospatial information (Zook et al., 2010), without placing the burden on responders to become experts in either. Locallydriven efforts often harness high levels of relevant local factual and cultural knowledge (Goodchild and Glennon, 2010) that complements the humanitarian skills of response organizations (Strandh and Eklund, 2018). In contrast, the more distributed online projects allow anyone to participate regardless of whether they have the resources or skills to join a locally organized effort. A distributed project such as the PRN, which is hosted on an established citizen science platform, also has access to a high fraction of participants with significant prior experience participating in citizen science, which facilitates accurate label collection and aggregation. Furthermore, the PRN team includes members with substantial experience running citizen science projects, which allows us to translate between our citizen science community and our responder partners. This significantly alleviates boundary issues when planning and deploying a response. We would stress, however, that it is extremely important for a distributed project such as ours to continually center local needs and priorities, including sharing results with local communities (e.g., Mulder et al., 2016) as soon as it is safe to do so.

Data Quality and Delivery Speed
The need for high-quality image labels was a key motivator for hosting the PRN on the Zooniverse. The platform is designed to enable high-quality data collection via proven methods such as collecting multiple independent classifications per subject (Kosmala et al., 2016;Parrish et al., 2018). Ensuring data quality is also a factor in ethical considerations in citizen science (Resnik, Elliott and Miller, 2015). Zooniverse projects have produced data labels whose quality matches and even exceeds that of a single expert (e.g., Lintott et al., 2008;Swanson et al., 2015). Additionally, the aggregation method we use is able to reduce the effect of noisy inputs from individual classifiers and account for individual skill levels in reaching consensus. This is especially important as ground truthing is generally not available in advance, which makes precise calibration challenging. We thus rely on feedback from the field to regularly assess the quality of our heat maps.
There are several potential bottlenecks to delivering heat maps rapidly enough to be of use to responders. These include: • Domain Expertise: Humanitarian crisis mapping is inherently multi-disciplinary (Ziemke, 2012), and several types of expertise are required to successfully deploy all stages of the PRN. These include knowledge of Geographical Information System (GIS) data sources and formats, disaster response and resilience, data science and statistical methods, and citizen science project design. • Data Availability: Satellite and/or aerial imaging data may be unavailable for several reasons. Some of these are outside the project team's control: tasking delays, weather issues, and corrupted data may all mean that needed data is either unavailable or severely delayed. • Data Access: Image data may exist, but not be accessible to the project team. Obstacles to data access can take the form of paywalls, bureaucratic delays, or technological problems (e.g., bandwidth issues).
The PRN has been able to deploy projects with very rapid turnaround, including initial heat map delivery just hours after beginning the crowd labeling phase. This deployment speed is possible in large part due to the work that takes place prior to, and in between, active project phases. Advance preparation is thus a key solution to all of the obstacles described above.
The PRN's advance preparation alleviates the issues described above in several ways. We have carefully assembled the PRN partnership specifically to address the domain expertise needs of a distributed online crisis mapping project. These needs were identified as the PRN initially formed, but have been refined following assessments of successes and failures of deployments. Some of the PRN partnership assembly has included negotiating and building relationships with partners; other aspects have involved training existing team members in new skills, developing new features on the Zooniverse platform that are useful to the PRN, and documenting end-to-end procedures and guidelines for all phases of PRN deployment. These procedures have enabled us to redirect crowd attention to alternative workflows when post-event data is scarce. Pre-and post-event data has sometimes been available to the PRN days before the same data is made openly available due to previously established partnerships with commercial satellite companies.
We therefore strongly agree with the findings of other studies (e.g., Harvard Humanitarian Initiative, 2011; Liu, 2014) that preparation is critical to a successful crisis mapping deployment. We would also note a need to inform preparation with the need to be flexible for each deployment. This is consistent with the idea that advance preparation must prioritize "articulation" work (Hughes and Tapia, 2015), which develops means of inter-and intraorganization information exchange so that this flexibility is possible during time-critical periods without sacrificing efficacy.
Open data is a major benefit to crisis mapping efforts. However, improvements are possible in this area. The best implementations of open data have allowed us to save hours or days in the planning phase of the PRN. For example, humanitarian users of Planet 8 data have access to the full commercial search and download area of the Planet website and API, which significantly streamlines data acquisition. Additionally, Amazon Web Services' Open Data program hosts a copy of processed, mosaiced ESA Sentinel-2 image data with no restrictions on transfer bandwidth. 9 This additionally facilitates GIS image processing in the cloud, which can save further time during live deployments. If more sources of satellite imagery took similar approaches in the future, this would encourage more rapid and more successful crisis mapping projects, including but not limited to the PRN.

Improvements to Crisis Response
Ultimately, the success of a crisis mapping project depends on whether it achieves its stated goals of positively impacting the response effort during and after a particular deployment. This framing encapsulates several of the ECSA's ten principles of citizen science within a humanitarian context. Given that every disaster is different, it is difficult to rigorously quantify the effect of adding a distributed crisis mapping effort to a disaster response. While a project team may be highly motivated (e.g. by academic metrics or funding pressure) to answer questions such as "how much faster will the recovery be now that heat maps are available?", it is not trivial to extract this information even by comparing to previous disasters where the project did not deploy. Attempts to collect uniform quantified feedback in situ during an ongoing response represent a significant local resource demand. For a distributed project such as the PRN, it would be particularly inappropriate for the organizers to either make these demands from their position of safety, or to risk sending personnel into an ongoing response for this purpose. Therefore, feedback to online distributed crowd mapping efforts on the utility of crisis maps is typically qualitative and often arrives after the most active periods of a response effort.
Evidence of improvements during the example deployments considered here vary. They include broad messaging that the heat maps provided were actively used on the ground to inform the ongoing effort, as well as specific examples. The project team collected specific qualitative feedback on the use of the results of the analysis phase. These include the use of road-blockage heat maps to optimize personnel allocation and more quickly restore critical national infrastructure, the incorporation of features flagged directly on Talk into flight plans for aerial assessment and evaluation of airstrips, and the use of building-damage heat maps to target priority areas for rapid ground-truth assessments and subsequent allocation of aid. We also received feedback that responders generally found maps of proportional damage (the fraction of structures in a given area which are damaged) extremely useful, especially as they worked their way to more isolated communities and health centers.
The above examples represent a minimum assessment of the utility of PRN heat maps. Rescue Global also distributed the maps widely to other organizations on the ground, and the remote PRN team did not receive feedback from those organizations as to whether and how the maps were used. Preparation for future deployments will include establishing contact with a wider group of organizations in advance, in part to facilitate end-ofresponse collection of feedback from these groups.

Learning from Failures
Across all PRN deployments, lessons learned from failures underpin the majority of our subsequent successes. As described in the Project Design section above, several of the general best practices described herein arose from specific challenges and failures during deployments, some of which occurred during earlier deployments than those in focus in this paper. For example, prolonged cloud cover immediately following the Nepal earthquakes in 2015 forced the PRN to shift deployment priorities from damage assessment in post-event satellite images to prediction of locations likely to need urgent aid, based on comparing recent pre-event images with (then incomplete) existing building maps. This ad hoc shift subsequently improved our ability to statistically incorporate other sources of geographic information (such as earthquake severity maps) into our analysis pipeline. The value of preparation was further reinforced by another lesson from the PRN Ecuador deployment in 2016, when post-project reflection on deployment delays led us to create project templates including logos, disclaimers and other boilerplate language that could be pre-approved by funders, enabling the team to focus on more pressing issues during a live deployment.
Additionally, communication with other crisis mapping teams and leaders has enabled us to learn from (and thus not repeat) external challenges. For example, early informal discussions with people involved in the 2010 Haiti earthquake and 2013 Typhoon Haiyan responses highlighted the need to ensure our partnerships include local connections and underscored the interdependence between teams who focus on technology-driven solutions (such as the PRN) and more traditional, hierarchical aid organizations (Zook et al., 2010). Discussion with external crisis mapping experts has also been considered alongside the PRN team's expertise in citizen science methods to inform our project design. For example, the design choices described in the Crowd Labeling Phase section above reflect an intent to minimize the biases that can appear in VGI data following a crisis (Goodchild and Glennon, 2010;Zook et al., 2010). All these considerations are especially important in applications where organizations based in the global north deploy to the global south, which is common in disaster and humanitarian aid.
Overall, it is critically important to both communicate with other experts and include an internal reflection phase following each deployment, where the team aggregates both successes and failures into lessons for the next deployment. This should be part of the normal process for any crisis mapping project.

CONCLUSION
The Planetary Response Network is a distributed online crisis mapping project that has deployed multiple times since its creation in 2014. The project approaches crisis mapping through a strong citizen science lens, with particular focus on global community engagement, data quality, and producing outputs with clear utility to responders, decision makers, and other stakeholders. This case study, which focuses on the most recent deployments of the PRN, has produced several lessons learned following evaluation of the project's structure, deployments and outcomes: • Distributed online crisis mapping projects, a particular type of humanitarian citizen science, play a positive role in the digital humanitarian sphere. It is critical that distributed projects such as the PRN continue to center the requirements of local stakeholders at all stages of project deployments. • There is a strong worldwide interest in response efforts following a disaster; distributed online crisis mapping provides an excellent way for people to help even when they are distant and/or cannot afford to financially support aid efforts. • Crisis mapping projects such as the PRN become robust when end-to-end response procedures are established early and the collaboration has prepared as much as possible in advance of deployments. With the addition of citizen science as a key response component, project design and planning become even more important, to ensure that the project makes ethical use of participants' time and contributions. Preestablished procedures must remain flexible to the needs of each specific deployment. • While some labelling requirements vary depending on the type of deployment, other features (e.g. infrastructure damage, access blockages, signs of ad hoc shelters) tend to be high priorities for situational awareness needs across multiple types of disasters. • Responders can generally tolerate more uncertainty in crisis maps than a purely academic study. This affects the design and deployment of a crisis mapping project, another reason it is vital to regularly liaise with responders and local stakeholders. • Point markings, even of extended features, are sufficient to flag features of interest during live deployments. However, these pose challenges when using these labels to train advanced damage detection algorithms. Communication between academic and responder team members is critical to establish priorities in advance and explore ways to alleviate tensions between urgency and precision that satisfy all parties. • Discussion software such as Zooniverse Talk provides an important way for crisis mapping project participants to identify serendipitous features of interest, train each other in advanced feature detection, and remain engaged throughout a deployment. Supervision of the discussion by team leaders is important for guiding self-training as well as identifying and intervening with participants showing signs of secondary distress. The latter is a particularly important ethical consideration. • Feedback between remote project organizers and responders on the ground is often qualitative and anecdotal rather than quantitative or statistical, necessitated by practical reasons of resource allocation priority and risk management. Qualitative feedback can still be extremely useful, provided team members accustomed to quantitative methods adjust their self-assessment techniques.
• It is immensely valuable when satellite image providers make their data open and easily accessible to crisis mapping projects. Providers can enable humanitarian projects to significantly improve deployment and response times by ensuring their open data includes processed data products and that such data is easy to search and acquire.
These lessons may be generalizable to other crisis mapping efforts, particularly those which conform to the virtual and distributed typology of citizen science projects. The successes of the PRN provide evidence that online distributed crisis mapping projects can be effective even when run on a generalized citizen science platform (such as the Zooniverse) not specifically designed for geographical citizen science. By applying best practices of citizen science and involving responders and local stakeholders at all stages of project execution, online distributed crisis mapping can add a valuable layer of information to complement purely community-based response efforts.
Looking forward, work within the PRN partnership continues. In particular, the team leadership is pursuing promising avenues for streamlining the planning phase using more automated image processing techniques. We are also developing a machine learning pipeline, trained on labels provided by project participants, to provide early estimates of heat maps even of new geographical regions (Kuzin et al., 2021). This will allow responders to access highvalue information for early resource allocation. It will also enable the project to direct participant attention to higher-level tasks, easing the tension between urgency and the need for detailed information.
There is also significant potential to develop the PRN further to include more frequent deployments as well as longer-term deployments. Deploying projects in partnership with local stakeholders to address risk reduction and resilience needs, for example, would enable the PRN to provide value at all stages of the disaster life cycle. These deployments would also benefit from the reduced time pressure compared to a response deployment, and they would allow the PRN community to remain active on an ongoing basis.

Appendix 1: Quantitative Details of PRN Caribbean Deployments
Following partner discussions, the PRN activated the planning phase for its 2017 Caribbean deployment on September 8, 2017. After the team processed the available imagery and decided on a project design, the crowd labeling phase was launched on September 12, 2017. Between September 12 and October 5, 5,331 logged-in participants and 6,422 not-logged-in IP addresses provided approximately 650,000 classifications of the islands of Antigua, Barbuda, Dominica, Guadeloupe, Puerto Rico, Turks and Caicos, and the US and British Virgin Islands. This effort, which labeled damage from both Hurricanes Irma and Maria, represents approximately 2.8 years' worth of full-time person-effort. Effort is measured from summing the duration of individual classifications (recorded by the Zooniverse software) and assuming 1 year of FTE is 40 hours' work per week for 52 weeks. A small number of classification durations are extremely high (e.g. 10 minutes or more where the median classification is 10 seconds); this is typical among Zooniverse projects. For the 0.4% of classifications meeting this criterion for this project we assume this represents the classifier pausing their work without closing their browser window, and replace those duration values with the average duration across all non-outlier classifications. For purposes of label aggregation, each not-logged-in IP address is treated as a unique classifier/participant.
The Gini coefficient (Gini, 1936) for classifications per volunteer was 0.82. The Gini coefficient measures inequality in a distribution and ranges from 0 (all classifiers contribute equal amounts) to a limit of 1 (one classifier contributes ~all classifications). In a citizen science context, Gini values between 0.7-0.9 generally indicate that a project can both recruit new participants and retain a loyal community of classifiers (Cox et al., 2015).
The first image set was labeled by volunteer classifiers in approximately 2 hours. The first heat maps were delivered to Rescue Global on the same day, including labels for building damage, road blockage, flooding, and temporary shelters. These features, particularly building damage and road blockage, tend to be of high value to responders following a wide variety of types of disaster. Overall, classifiers submitted over 180,000 individual marks labelling the features of interest. Exact mark counts for each feature type are given in Table A1, with the time series of submitted classifications shown in Figure 3. Additionally, 537 participants from this deployment posted at least 1 comment on the project's "Talk" discussion boards.
The second PRN deployment in the Caribbean began its planning phase on September 5, 2019. This project focused on the northern Bahamas, which were severely affected by Hurricane Dorian. The initial launch of the crowd labeling project was on September 7, 2019; the first data set was labeled by the crowd in under 5 hours. Between September 7 and 16, the project collected approximately 365,000 classifications from 2,404 logged-in participants and 2,105 not-logged-in IP addresses (Gini coefficient: 0.81). Overall, classifiers submitted over 135,000 individual labels of features of interest (see Table A1). The total classification effort represents approximately 1 year of full-time personeffort (replacing 0.2% of high-duration outliers with the average duration, as above). Of the 2,404 logged-in participants who contributed to the project, 219 posted at least 1 comment on Talk.
The features labeled during the 2019 Caribbean deployment were building damage, road blockage, flooding, temporary shelters, underwater hazards and helicopter landing sites (HLSs). The latter two features were added after consultation with 24 Commando Royal Engineers, who were deployed in partnership with Rescue Global to the Bahamas. For the HLS feature assessment, classifiers were asked to examine each subject and assess via a Yes/No question interface whether it contained a 50 m × 50 m plot of clear land (using scale bars added to subjects; see Figure 1, right panel). This example of a specialized label added in consultation with active responders resulted in the identification of 381 high-confidence potential sites, details of which were forwarded for further assessment and potential use in reaching hard-to-access areas of the islands. High-confidence potential sites are defined as those where at least 80% of classifiers (of at least 10) have identified an image subsection as a potential HLS. Table A1: Counts for marks on images applied by classifiers in the PRN, for the two deployments in focus in this case study. The Underwater Hazard feature was not assessed during the 2017 deployment. One additional feature, Helicopter Landing Sites, was assessed in the 2019 deployment using a single-question task (images were not marked).