Citizen science involves enlisting groups of untrained volunteers to participate and contribute to scientific research. Crowdsourcing and citizen science are often used interchangeably however there is a difference between the two. Crowdsourcing involves engaging a group of people within any project that utilises contributions from non-professionals, be it through collecting or analysing data. Citizen Science projects are the same, with the exception that they have a focused scientific outcome and engage non-professionals in some point of the scientific process [1, 2]. Citizen heritage science is the practice of engaging communities and individuals with scientific understanding of heritage. Examples of this are projects such as Micropasts  and Rekrei (formerly Project Mosul) , used crowdsourced images to create digital facsimiles of heritage artefacts, utilising citizen heritage scientists for both data collection and analysis. Using satellite imagery volunteers helped identify fossils across East Africa as well as pinpoint possible locations for the tomb of Gengis Khan across the Mongolian steppe [5, 6].
Citizen science can assist in heritage disaster management and is an effective methodology to understand community values of heritage [7, 8]. It is also a valuable tool used to transcribe, process and discover myriad types of metadata contained within collections, as can be seen in projects such as Transcribe Bentham, Old Weather, What’s on the Menu and Monitoring the ANZACs to name a few [9–11].
Citizen science projects that involve the collecting of data thrive in areas which are closely aligned to popular leisure activities. Participants can contribute to real scientific research whilst maintaining their current hobby. To date the vast majority of CS projects involving data collection are aligned with environmental conservation, such as e-Bird  and Budburst . Whilst projects such the Coastal and Intertidal Zone Archaeological Network, otherwise known as CitiZAN, and the Scottish equivalent Scottish Coastal Heritage at Risk have had great success in utilising volunteers to monitor remote heritage sites in danger, there are comparatively few citizen science projects involving collecting data for heritage .
Visiting heritage sites is a highly popular pastime in UK; in England 73% of the population visited a cultural heritage site in 2018  whilst in Scotland 75% of the population were actively engaged with cultural heritage . This represents considerable untapped potential for monitoring heritage sites: using mobile devices visitors can ‘act’ as sensors, recording one-off events or ongoing phenomena that would not otherwise be monitored without a regular presence at the site.
Smartphone cameras are capable of monitoring a variety of issues in heritage such as biological growth, erosion and one-off events such as vandalism [17–20]. The vast majority of scheduled monuments across the UK are small, unstaffed and not often visited by heritage professionals but are often destinations for tourists and popular walking sites for locals.
Similarly, when a heritage site is well known and frequently visited, it is likely there is a wealth of useful historical reference data stored in personal photographs. These can be used to qualitatively study the changing use of a site over time, as recently done by English Heritage in the exhibition ‘Your Stonehenge, 150 years of personal photographs’ .
Within heritage science, environmental and observational monitoring is key to ongoing care. However individual object monitoring cannot always be completely uniform; national heritage institutions often have a large portfolio of sites in their care spread over a large geographical area. Smaller and unstaffed sites such as cairns, standing stones and ruined churches often receive comparatively fewer visits from conservation and management teams meaning that long term trends, one off events and seasonal variants can go un-noticed or undocumented.
In this paper we present a methodology developed as part of the citizen heritage science project Monument Monitor as a viable way to collect digital conservation data. It uses photographs taken and provided by visitors to monitor specific aspects of heritage sites in Scotland. This utilizes the interested site attendee or ‘visitor-as-sensor’, to document patterns that may otherwise go unnoticed between visits from professionals.
In this paper we compare two different types of data collection: a ‘guided’ approach in which participants are directed to take and submit photographs that fit certain specifications through signage and an ‘open’ approach, in which participants respond to an open call for photographs through public channels. Unlike other Citizen Heritage Science projects, such as SCAPE and CiTIZAN, Monument Monitor does not actively recruit and training volunteers. Instead visitors to the site were prompted to submit digital conservation data (photographs) through signage or press outreach which are subsequently uploaded onto an online database. This interactive database was accessible to all key stakeholders in the project, creating an ongoing live feedback loop for site managers and heritage scientists alike.