18 March, 2013
The Consumers’ Association Which? magazine is probably not the first place to turn to when you look for usability studies. Especially not if you’re interested in computer technology – for that, there are sources such as PC Magazine on the consumer side, and professional magazines such as Interactions from Association for Computing Machinery (ACM) Special Interest Group on Computer-Human Interaction (SIGCHI).
Over the past few years, Which? is reviewing, testing and recommending Satnavs (also known Personal Navigation Devices – PNDs). Which? is an interesting case because it reaches over 600,000 households and because of the level of trust that it enjoys. If you look at their methodology for testing satnavs , you’ll find that it does resemble usability testing – click on the image to see the video from Which? about their methodology. The methodology is more about everyday use and the opinion of the assessors seems to play an important role.
Professionals in geographical information science or human-computer interaction might dismiss the study as unrepresentative, or not fitting their ways of evaluating technologies, but we need to remember that Which? is providing an insight into the experience of the people who are outside our usual professional and social context – people who go to a high street shop or download an app and start using it straightaway. Therefore, it’s worth understanding how they review the different systems and what the experience is like when you try to think like a consumer, with limited technical knowledge and understanding of maps.
There are also aspects that puncture the ‘filter bubble‘ of geoweb people – Google Maps are now probably the most used maps on the web, but the satnav application using Google Maps was described as ‘bad, useful for getting around on foot, but traffic information and audio instructions are limited and there’s no speed limit or speed camera data‘. Waze, the crowdsourced application received especially low marks and the magazine noted that it ‘lets users share traffic and road info, but we found its routes and maps are inaccurate and audio is poor‘ (both citations from Which? Nov 2012, p. 38). It is also worth reading their description of OpenStreetMap when discussing map updates, and also the opinions on the willingness to pay for map updates.
There are many ways to receive information about the usability and the nature of interaction with geographical technologies, and some of them, while not traditional, can provide useful insights.
20 October, 2012
The Spatial Data Infrastructure Magazine (SDIMag.com) is a relatively new e-zine dedicated to the development of spatial data infrastructures around the world. Roger Longhorn, the editor of the magazine, conducted an email interview with me, which is now published.
In the interview, we are covering the problematic terminology used to describe a wider range of activities; the need to consider social and technical aspects as well as goals of the participants; and, of course, the role of the information that is produced through crowdsourcing, citizen science, VGI with spatial data infrastructures.
At the State of the Map (EU) 2011 conference that was held in Vienna from 15-17 July, I gave a keynote talk on the relationships between the OpenStreetMap (OSM) community and the GIScience research community. Of course, the relationships are especially important for those researchers who are working on volunteered Geographic Information (VGI), due to the major role of OSM in this area of research.
The talk included an overview of what researchers have discovered about OpenStreetMap over the 5 years since we started to pay attention to OSM. One striking result is that the issue of positional accuracy does not require much more work by researchers. Another important outcome of the research is to understand that quality is impacted by the number of mappers, or that the data can be used with confidence for mainstream geographical applications when some conditions are met. These results are both useful, and of interest to a wide range of groups, but there remain key areas that require further research – for example, specific facets of quality, community characteristics and how the OSM data is used.
Reflecting on the body of research, we can start to form a ‘code of engagement’ for both academics and mappers who are engaged in researching or using OpenStreetMap. One such guideline would be that it is both prudent and productive for any researcher do some mapping herself, and understand the process of creating OSM data, if the research is to be relevant and accurate. Other aspects of the proposed ‘code’ are covered in the presentation.
In March 2008, I started comparing OpenStreetMap in England to the Ordnance Survey Meridian 2, as a way to evaluate the completeness of OpenStreetMap coverage. The rational behind the comparison is that Meridian 2 represents a generalised geographic dataset that is widely use in national scale spatial analysis. At the time that the study started, it was not clear that OpenStreetMap volunteers can create highly detailed maps as can be seen on the ‘Best of OpenStreetMap‘ site. Yet even today, Meridian 2 provides a minimum threshold for OpenStreetMap when the question of completeness is asked.
So far, I have carried out 6 evaluations, comparing the two datasets in March 2008, March 2009, October 2009, March 2010, September 2010 and March 2011. While the work on the statistical analysis and verification of the results continues, Oliver O’Brien helped me in taking the results of the analysis for Britain and turn them into an interactive online map that can help in exploring the progression of the coverage over the various time period.
Notice that the visualisation shows the total length of all road objects in OpenStreetMap, so does not discriminate between roads, footpaths and other types of objects. This is the most basic level of completeness evaluation and it is fairly coarse.
The application will allow you to browse the results and to zoom to a specific location, and as Oliver integrated the Ordnance Survey Street View layer, it will allow you to see what information is missing from OpenStreetMap.
Finally, note that for the periods before September 2010, the coverage is for England only.
Some details on the development of the map are available on Oliver’s blog.
23 March, 2011
This post reviews the two books about OpenStreetMap that appeared late in 2010: OpenStreetMap: Using and Enhancing the Free Map of the World (by F. Ramm, J. Topf & S. Chilton, 386 pages, £25) and OpenStreetMap: Be your own Cartographer (by J. Bennett, 252 pages, £25). The review was written by Thomas Koukoletsos, with some edits by me. The review first covers the Ramm et al. book, and then compares it to Bennett’s. It is fairly details, so if you want to see the recommendation, scroll all the way down.
OpenStreetMap: Using and Enhancing the Free Map of the World is a comprehensive guide to OpenStreetMap (OSM), aimed at a wide range of readers, from those unfamiliar with the project to those who want to use its information and tools and integrate them with other applications. It is written in accessible language, starting from the basics and presenting things in an appropriate order for the reader to be able to follow, slowly building the necessary knowledge.
Part I, the introduction, covers 3 chapters. It presents the OSM project generally, while pointing to other chapters wherever further details are provided later on. This includes how the project started, a short description of its main interface, how to export data, and some of its related services such as OpenStreetBugs and OpenRouteService. It concludes with a reference on mapping parties and the OSM foundation. This gives all the necessary information for someone new to OSM to get a general idea, without becoming too technical.
Part II, addressing OSM contributors, follows with chapter 4 focusing on how GPS technology is used for OSM. The balance between the technical detail and accessibility continues, so all the necessary information for mapping is presented in an easily digested way even for those not familiar with mapping science. The following chapter covers the whole mapping process using a very comprehensive case study, through which the reader understands how to work in the field, edit and finally upload the collected data. Based on this overview, the next chapter is slightly more technical, describing the data model followed by OSM. The information provided is necessary to understand how the OSM database is structured.
Chapter 7 moves on to details, describing what objects need to be mapped and how this can be done by using tags. The examples provided help the user to move from simpler to more complicated representations. The importance of this chapter, however, is in emphasising that, although the proposed tagging framework is not compulsory, it would be wise to do it as this will increase the consistency in the OSM database. The chapter ends with a suggestion of mapping priorities, from ‘very important’ objects and attributes to ‘luxury’ ones. Chapter 8 continues with map features, covering all other proposed mapping priorities. The split between the two chapters guides the user gradually from the most important features to those covered by expert OSM users, as otherwise mapping might have been far too difficult a task for new participants.
Chapter 9 describes Potlatch, an online editor which is the most popular. The description is simple and complete, and by the end the user is ready to contribute to the OSM database. The next chapter refers to JOSM, an offline editor designed for advanced users, which is more powerful than Potlatch but more difficult to use – although the extensive instructions make the use of this tool almost as easy as Potlatch. Chapter 11 concludes the review of editors by providing basic information on 5 other editors, suitable for desktop or mobile use. Chapter 12 presents some of the tools for mappers, designed to handle the OSM data or perform quality assurance tests. Among the capabilities described are viewing data in layers, monitoring changes in an area, viewing roads with no names, etc. The second part ends, in Chapter 13, with a description of the OSM licensing framework, giving the reader a detailed view of what source of data should be avoided when updating OSM to save it from copyright violations.
Part III of Ramm et al. is far more technical, beginning with how to use OSM on web pages. After providing the necessary information on tiling used for the OSM map (Mapnik and Tiles@Home servers), chapter 14 moves on to the use of OSM with Google Maps or with OpenLayers. Code is provided to assist the learning process. Chapter 15 provides information on how to download data, including the ability to download only changes and update an already downloaded version, explained further in a following chapter.
The next three chapters dive into cartographic issues, with chapter 16 starting with Osmarender, which helps visualising OSM data. With the help of many examples, the reader is shown how this tool can be used to render maps, and how to customise visualisation rules to create a personal map style. Chapter 17 continues with Mapnik, a more efficient tool than Osmarender for large datasets. Its efficiency is the result of reading the data from a PostgreSQL database. A number of other tools are required to be installed for Mapnik; however, they are all listed with basic installation instructions. The chapter concludes with performance tips, with an example of layers used according to the zooming level so that rendering is faster. The final renderer, described in chapter 18, is Kosmos. It is a more user-friendly application than the previous two, and the only one with a Graphical User Interface (GUI). The rules used to transform OSM data into a map come from the wiki pages, so anyone in need of a personal map style will have to create a wiki page. There is a description of a tiling process using Kosmos, as well as of exporting and printing options. The chapter concludes by mentioning Maperitive, the successor to Kosmos to be released shortly.
Chapter 19 is devoted to mobile use of OSM. After explaining the basics of navigation and route planning, there is a detailed description of how to create and install OSM data on Garmin GPS receivers. Additional applications for various types of devices are briefly presented (iPhones, iPods, Android), as well as other routing applications. Chapter 20 closes the third part of the book with an extensive discussion on licence issues of OSM data and its derivatives. The chapter covers the CC-BY-SA licence framework, as well as a comprehensive presentation of the future licence, without forgetting to mention the difficulties of such a change.
Part IV is the most technical part, aimed at those who want to integrate OSM into their applications. Chapter 21 reveals how OSM works, beginning with the OSM subversion repository, where the software for OSM is managed. Chapter 22 explains how the OSM Application Programming Interface (API) works. Apart from the basic data handling modes (create, retrieve, update or delete objects and GPS tracks), other methods of access are described, as well as how to work with changesets. The chapter ends with OAuth, a method to allow OSM authentication through third party applications keeping the necessary user information. Chapter 23 continues with XAPI, which is a different API that, although offers only read requests and its data may be a few minutes old, it allows more complex queries, returns more data than the standard API (e.g. historic versions) and allows RSS feeds from selected objects. Next, the Name Finder and Nominatim search engines for gazetteer purposes are covered. Lastly, GeoNames is mentioned, which, although not an OSM relative, can be used in combination with other OSM tools.
Chapter 24 presents Osmosis, a tool to filter and convert OSM data. Apart from enabling read and write of XML files, this tool is also able to access PostgreSQL and MySql databases for read and write purposes. It also describes how to create and process change files in order to continually update a local dataset or database from the OSM server. Chapter 25 moves deeper into more advanced editing, presenting the basics of large-scale or other automated changes. As such changes can affect a lot of people and their contributions, the chapter begins with ‘a note of caution’, discussing that, although power editing is available to everyone, a contact and discussion with those whose data is to be changed should be made.
Chapter 26 focuses on imports and exports including some of the programs that are used for specific data types. The final chapter presents a rather more detailed overview of how to run an OSM as well as a tile server, covering the requirements and installation. There is also a presentation of the API schema, and alternatives to the OSM API are also mentioned.
The book ends with the appendix, consisting of two parts, covering geodesy basics, and specifically geographic coordinates, datum definition and projections; and information on local OSM communities for a few selected countries.
Overall, the book is accessible and comprehensive.
Chapters 1 and 2 give a general description of the OSM project and correspond to the first three chapters of Ramm et al. The history of OSM is more detailed here. The main OSM web page description does not include related websites but, on the other hand, it does describe how to use the slippy map as well as how to interact with data. The chapters also focus on the social aspect of the project, briefly presenting more details on a user’s account (e.g. personalisation of the user’s profile by adding a user photo, home location to enable communication with other users in the area or notification of local events).
Chapter 3 corresponds to chapters 4 and 5 of the first book. There is a more detailed description of how GPS works, as well as of how to configure the receiver; however, the other ways of mapping are less detailed. A typical mapping example and a more comprehensive description of the types of GPS devices suitable for OSM contribution, which are provided in Ramm et al., are missing.
Chapter 4 corresponds to chapters 6, 7 and 8 of the first book. Some less than important aspects are missing, such as the data model history. However, Ramm et al. is much more detailed on how to map objects, classifying them according to their importance and providing practical examples of how to do it, while in this chapter a brief description of tags is provided. Both books succeed in communicating the significance of following the wiki suggestions when it comes to tagging, despite the ‘any tags you like’ freedom. An interesting point, which is missing from the first book, is the importance of avoiding tagging for the renderer, explained here with the use of a comprehensive example.
Chapter 5 describes the editors Potlatch, JOSM and Merkaartor, corresponding with chapters 9, 10, and 11 of Ramm et al. Having the three editors in one chapter allows for a comparison table between them, giving a much quicker insight. A practical example with a GPS trace file helps in understanding the basics operation with these editors. More attention is given to Potlatch, while the other two editors are described only briefly. No other editors are described or mentioned.
Chapter 6 provides a practical example of using the three editors and shows how to map objects, which was covered in chapters 6, 7 and 8 in the first book. While the first book is more detailed and includes a wider range of mapping cases, here the reader becomes more familiar with the editors and learns how to provide the corresponding information. In addition to the material in the first book, here we have an example of finding undocumented tags and using OSMdoc.
Chapter 7 corresponds to chapter 12 of the first book, with a detailed description of the four basic tools to check OSM data for errors. However, Ramm et al. offers a broader view by mentioning or briefly describing seven other error-checking tools.
Chapter 8 deals with map production, similar to chapters 2, 16 and 18 of Ramm et al. The Osmarender tool is described in detail in both books. Kosmos renderer, however, is described in much more detail here, although it is no longer developed. The chapter’s summary here is very useful, as it presents briefly the 3 rendering tools and compares them. What is missing from this book, however, is a description of Mapnik (chapter 17 of Ramm et al.) and also the use of tiling in web mapping.
Chapter 9 corresponds to chapters 15, 22 and 23 of Ramm et al. Regarding planet files, Bennett provides a description of a way to check the planet file’s integrity, which can be useful for automating data integration processes. Moving on to OSM’s API, this book is confined to describing ways of retrieving data from OSM, unlike the first book that also includes operations to create, update or delete data. XAPI, however, is more detailed in this book, including how to filter data. In this chapter’s summary a brief description and comparison of the ways to access data is helpful. On the other hand, Ramm et al. briefly describes additional APIs and web services that are not covered here.
Chapter 10 matches chapter 24 of the first book. In both cases Osmosis is described in detail, with examples of how to filter data. The first book includes a more complete description of command line options, classified according to the data streams (entity or change). This book, on the other hand, is more explanatory on how to access data based on a predefined polygon, and further explaining how to create and use a customised one. The first book mentions additional tasks, such as ‘log progress’, ‘report integrity’, ‘buffer’, ‘sort’, while here only the latter is used during an example. An advantage of Bennett’s book, however, is that the use of Osmosis with a PostgreSQL database, as well as how to update data and how to automate a database update procedure, is explained more comprehensively and extensively.
The last chapter talks about future aspects of OSM. The OSM licence and its future development is explained in a comprehensive way, corresponding to the end of chapter 20 of the first book, with the use of some good examples to show where the present OSM licence is problematic. However, throughout Bennett’s book, licence issues are not covered as well as in Ramm et al. (chapters 13, 20), and the reader needs to reach the end of the book to understand what is allowed and what is not with the OSM data. Moving on, MapCSS, a common stylesheet language for OSM, is explained in detail, while in the first book it is simply mentioned at the end of chapter 9 during a discussion of Potlatch 2. The book ends with Mapzen POI collector for iPhone, covered in chapter 11 of the first book.
When compared to the first book, what is missing here is the use of OSM for navigation in mobile devices (chapter 19), large-scale editing (chapter 25), writing or finding software for OSM (chapter 21) and how to run an OSM server (chapter 27). Another drawback is the lack of coloured images; in some cases (e.g. chapter 7 – the NoName layer) it is difficult to understand them.
So which book is for me?
Both the books more or less deal with the same information, as shown by the chapters’ comparison and sequence.
Although there are areas where the two books are complementary, in most cases Ramm et al. provides a better understanding of the matters discussed, using a broader and more extensive view. It addresses a wide range of readers, from those unfamiliar with OSM to the advanced programmers who want to utilise it elsewhere, and is written with a progressive build-up of knowledge, which helps in the learning process. It also benefits from the dedicated website where updates are provided. Bennett’s book, on the other hand, would be comparably more difficult to read for someone who has not heard of OSM, as well as for those in need of using it but who are not programming experts. There is a hidden assumption that the reader is fairly technically literate. It suffers somewhat from not being introductory enough, while at the same time not being in-depth and detailed.
As the two books are sold at a similar price point, we liked the Ramm et al. book much more and would recommend it to our students.
Following successful funding for the European Union FP7 EveryAware and the EPSRC Extreme Citizen Science activities, the department of Civil, Environmental and Geomatic Engineering at UCL is inviting applications for a postdoctoral position and 3 PhD studentships. Please note that these positions are open to students from any EU country.
These positions are in the ‘Extreme Citizen Science’ (ExCiteS) research group. The group’s activities focus on the theory, methodologies, techniques and tools that are needed to allow any community to start its own bottom-up citizen science activity, regardless of the level of literacy of the users. Importantly, Citizen Science is understood in the widest sense, including perceptions and views – so participatory mapping and participatory geographic information are integral parts of the activities.
The research themes that the group explores include Citizen Science and Citizen Cyberscience; Community and participatory mapping/GIS; Volunteered Geographic Information (OpenStreetMap, Green Mapping, Participatory GeoWeb); Usability of geographic information and geographic information technology, especially with non-expert users; GeoWeb and mobile GeoWeb technologies that facilitate Extreme Citizen Science; and identifying scientific models and visualisations that are suitable for Citizen Science.
Research Associate in Extreme Citizen Science – a 2-year, postdoctoral research associate position commencing 1 May 2011.
The research associate will lead the development of an ‘Intelligent Map’ that allows non-literate users to upload data securely; and the system should allow the users to visualise their information with data from other users. Permissions need to be developed in accordance with cultural sensitivities. As uploaded data from multiple users sharing the same system increase over time, repeating patterns will begin to emerge that indicate particular environmental trends.
The role will also include some general project-management duties, guiding the PhD students who are working on the project. Travel to Cameroon to the forest communities that we are working with is necessary.
Complete details about this post and application procedure are available on the UCL jobs website.
PhD Studentship – understanding citizen scientists’ motivations, incentives and group organisation – a 3.5-year fully funded studentship. We are looking for applicants with a good honours degree (1st Class or 2:1 minimum), and an MA or MSc in anthropology, geography, sociology, psychology or related discipline. The applicant needs to be familiar with quantitative and qualitative research methods, and be able to work with a team that will include programmers and human-computer interaction experts who will design systems to be used in citizen science projects. Travel will be required as part of the project. A willingness to live for short periods in remote forest locations in simple lodgings, eating local food, will be necessary. French language skills are desirable.
The research itself will focus on motivations, incentives and understanding of the needs and wishes of participants in citizen science projects. We will specifically focus on engagement of non-literate people in such projects and need to understand how the process – from data collection to analysis – can be made meaningful and useful for their everyday life. The research will involve using quantitative methods to analyse large-scale patterns of engagement in existing projects, as well as ethnographic and qualitative study of participants. The project will include working with non-literate forest communities in Cameroon as well as marginalised communities in London.
Complete details about this post and application procedure are available on the UCL jobs website.
PhD Studentship in geographic visualisation for non-literate citizen scientists - a 3.5-year fully funded studentship. The applicant should possess a good honours degree (1st Class or 2:1 minimum), and an MSc in computer science, human-computer interaction, electronic engineering or related discipline. In addition, they need to be familiar with geographic information and software development, and be able to work with a team that will include anthropologists and human-computer interaction experts who will design systems to be used in citizen science projects. Travel will be required as part of the project. A willingness to live for short periods in remote forest locations in simple lodgings, eating local food, will be necessary. French language skills are desirable.
Complete details about this post and application procedure are available on the UCL jobs website.
In addition, we offer a PhD Studentship on How interaction design and mobile mapping influences participation in Citizen Science, which is part of the EveryAware project and is also open to any EU citizen.
How Many Volunteers Does It Take To Map An Area Well? The validity of Linus’ law to Volunteered Geographic Information
10 January, 2011
The paper “How Many Volunteers Does It Take To Map An Area Well? The validity of Linus’ law to “ has appeared in The Cartographic Journal. The proper citation for the paper is:
Haklay, M and Basiouka, S and Antoniou, V and Ather, A (2010) How Many Volunteers Does It Take To Map An Area Well? The validity of Linus’ law to Volunteered Geographic Information. The Cartographic Journal , 47 (4) , 315 – 322.
The abstract of the paper is as follows:
In the area of volunteered geographical information (VGI), the issue of spatial data quality is a clear challenge. The data that are contributed to VGI projects do not comply with standard spatial data quality assurance procedures, and the contributors operate without central coordination and strict data collection frameworks. However, similar to the area of open source software development, it is suggested that the data hold an intrinsic quality assurance measure through the analysis of the number of contributors who have worked on a given spatial unit. The assumption that as the number of contributors increases so does the quality is known as `Linus’ Law’ within the open source community. This paper describes three studies that were carried out to evaluate this hypothesis for VGI using the OpenStreetMap dataset, showing that this rule indeed applies in the case of positional accuracy.
To access the paper on the journal’s website, you can follow the link: 10.1179/000870410X12911304958827. However, if you don’t hold a subscription to the journal, a postprint of the paper is available at the UCL Discovery repository. If you would like to get hold of the printed version, email me.
7 January, 2011
EveryAware is a three-year research project, funded under the European Union Seventh Framework Programme (FP7).
The project’s focus is on the development of Citizen Science techniques to allow people to find out about their local environmental conditions, and then to see if the provision of this information leads to behaviour change.
The abstract of the project highlights the core topics that will be covered:
‘The enforcement of novel policies may be triggered by a grassroots approach, with a key contribution from information and communication technology (ICT). Current low-cost sensing technologies allow the citizens to directly assess the state of the environment; social networking tools allow effective data and opinion collection and real-time information-spreading processes. Moreover theoretical and modelling tools developed by physicists, computer scientists and sociologists allow citizens to analyse, interpret and visualise complex data sets.
‘The proposed project intends to integrate all crucial phases (environmental monitoring, awareness enhancement, behavioural change) in the management of the environment in a unified framework, by creating a new technological platform combining sensing technologies, networking applications and data-processing
tools; the Internet and the existing mobile communication networks will provide the infrastructure hosting this platform, allowing its replication in different times and places. Case studies concerning different numbers of participants will test the scalability of the platform, aiming to involve as many citizens as possible thanks to
low cost and high usability. The integration of participatory sensing with the monitoring of subjective opinions is novel and crucial, as it exposes the mechanisms by which the local perception of an environmental issue, corroborated by quantitative data, evolves into socially-shared opinions, and how the latter, eventually, drives behavioural changes. Enabling this level of transparency critically allows an effective communication of desirable environmental strategies to the general public and to institutional agencies.’
The project will be coordinated by Fondazione ISI (Institute for Scientific Interchange) and the Physics department at Sapienza Università di Roma. Other participants include the L3S Research Center at the Gottfried Wilhelm Leibniz Universität, Hannover, and finally the Environmental Risk and Health unit at the Flemish Institute of Technological Research (VITO).
At UCL, I will run the project together with Dr Claire Ellul. We will focus on Citizen Science, the interaction with mobile phones for data collection and understanding behaviour change. We are looking for a PhD student to work on this project so, if this type of activity is of interest, get it touch.
29 November, 2010
The website GPS Business News published an interview with me in which I covered several aspects of OpenStreetMap and crowdsourced geographical information, including aspects of spatial data quality, patterns of data collection, inequality in coverage and the implications of these patterns to the wider area of Volunteered geographical Information.
The interview is available here .
21 October, 2010
One issue that remained open in the studies on the relevance of Linus’ Law for OpenStreetMap was that the previous studies looked at areas with more than 5 contributors, and the link between the number of users and the quality was not conclusive – although the quality was above 70% for this number of contributors and above it.
Now, as part of writing up the GISRUK 2010 paper for journal publication, we had an opportunity to fill this gap, to some extent. Vyron Antoniou has developed a method to evaluate the positional accuracy on a larger scale than we have done so far. The methodology uses the geometric position of the Ordnance Survey (OS) Meridian 2 road intersections to evaluate positional accuracy. Although Meridian 2 is created by applying a 20-metre generalisation filter to the centrelines of the OS Roads Database, this generalisation process does not affect the positional accuracy of node points and thus their accuracy is the best available. An algorithm was developed for the identification of the correct nodes between the Meridian 2 and OSM, and the average positional error was calculated for each square kilometre in England. With this data, which provides an estimated positional accuracy for an area of over 43,000 square kilometres, it was possible to estimate the contribution that additional users make to the quality of the data.
As can be seen in the chart below, positional accuracy remains fairly level when the number of users is 13 or more – as we have seen in previous studies. On the other hand, up to 13 users, each additional contributor considerably improves the dataset’s quality. In grey you can see the maximum and minimum values, so the area represents the possible range of positional accuracy results. Interestingly, as the number of users increases, positional accuracy seems to settle close to 5m, which is somewhat expected when considering the source of the information – GPS receivers and aerial imagery. However, this is an aspect of the analysis that clearly requires further testing of the algorithm and the datasets.
It is encouraging to see that the results of the analysis are significantly correlated. For the full dataset the correlation is weak (-0.143) but significant at the 0.01 level (2-tailed). However, the average values for each number of contributors (blue line in the graph), the correlation is strong (-0.844) and significant at the 0.01 level (2-talled).
An important caveat is that the number of tiles with more than 10 contributors is fairly small, so that is another aspect that requires further exploration. Moreover, spatial data quality is not just positional accuracy, but also attribute accuracy, completeness, update and other properties. We can expect that they will also exhibit similar behaviour to positional accuracy, but this requires further studies – as always.
However, as this is a large-scale analysis that adds to the evidence from the small-scale studies, it is becoming highly likely that Linus’ Law is affecting the quality of OSM data and possibly of other so-called Volunteered Geographical Information (VGI) sources and there is a decreased gain in terms of positional accuracy when the number of contributors passes about 10 or so.