The scientific knowledge graph is a kind of graph reflecting the development process of scientific knowledge and the relationship between knowledge structure, which can enable researchers to understand specific research areas, correlations and new points of interest through means such as data mining, information processing, knowledge measurement and mapping. This paper adopts the bibliometric analysis method, uses COOC¹ software to process the samples, selects the amount of text, authors, keywords as the main discriminant indicators, and also uses VOSviewer software to visually analyze the main contents of the study.

First of all, descriptive and visual analysis of the time change law and author of the literature is carried out in order to clarify the development context of the three-line heritage; Secondly, the keyword co-occurrence and keyword clustering of the literature are analyzed, and the research hotspots of the third-line heritage are explored; finally, the key word evolution trend of the third-line heritage is drawn to explore its development trend.

Based on CNKI database, the “advanced search” mode is adopted to search by subject words to ensure the comprehensiveness and accuracy of literature analysis. The “three-line construction” and including “heritage” were set as the search subject, the matching method was “exact”, and the search time was from the beginning year to December 31, 2021. The important data in the document was exported in Refworks format as a sample for analysis, the retrieved literature was filtered (non-research literature such as announcements, essays, conference notices, etc.) was manually excluded, the COOC software was used for literature inspection and deduplication, and synonymous combination of keywords was obtained, and finally 286 literature were obtained as research samples (see Table 1).

After statistical analysis of the literature, it is found that the research on third-line heritage in China has shown a rapid upward trend. As shown in Figure 1, the volume of publication of three-tier heritage studies can be divided into three periods:

1) 2007-2011 is the embryonic period, this stage of the volume of publication is small, the average annual cumulative volume of nearly 3 articles (the earliest literature on the study of the third-line heritage is Mr. Chen Donglin’s “Three-line Construction, the Nearest Industrial Heritage” published in The National Geographic magazine of China in 2006, because the document is not included in CNKI, so the literature is from 2007), and the overall trend can be seen to be slowly rising.

2) 2012-2016 is the development period, because in 2013, in order to comprehensively promote the adjustment and transformation of the national old industrial base, the state formulated the “National Old Industrial Base Adjustment and Transformation Plan (2013-2022)”, which includes the industrial base during the

Source: The author sorts it out and self-makes it.

third-line construction period, which marks that the third-line heritage has received national attention, so the amount of documents issued is significantly more than the previous stage, the growth rate is faster, and the average annual cumulative number of documents is nearly 13.

3) 2017-2021 is the mature period, because China announced the first batch of national industrial heritage list for the first time in 2017, heavy steel section steel mills and other third-line construction industrial heritage was included in the list; In addition, in 2017, the State Administration of Cultural Heritage said in its reply to the “Proposal on Strengthening the Protection and Utilization of Heritage Resources in the Third-Line Construction” submitted by Jiang Gangjie, a member of the National Committee of the Chinese People’s Political Consultative Conference, that it would actively unite relevant departments and coordinate with the people’s governments of relevant “third-tier” cities to take effective measures to continuously improve the level of third-tier heritage protection [3]. Since then, the attention to this issue has become increasingly high, so the number of articles published at this stage has surged, with an average annual cumulative volume of nearly 41 articles. It is foreseeable from this trend that more scholars will conduct research on third-line heritage in the future, reflecting that there is still a large research space in the field of third-line heritage.

The subject area of a scholar’s research is closely related to the results of his academic papers, and when a scholar publishes a certain amount of papers in a certain field, the scholar can be regarded as belonging to the field [4]. The COOC software is used to analyze the volume of posts and cooperation of relevant authors, and its node size indicates the number of articles published by the scholar in the field of third-line construction, and the thickness of the connection between nodes indicates the strength of the cooperative relationship.

As can be seen from Table 2, the core authors in the field of third-line heritage research mainly include Xu Youwei (9 articles), Tan Gangyi (6 articles), Daniel Zhang (5 articles) and others. Scholars engaged in the study of third-line heritage have mainly formed two major author cooperation networks, specifically Xu Youwei, Tan Gangyi, Daniel Zhang, Xu Liquan, etc., and Chen Yurong, He Linjun, Ai Daiwen, Zhu Tingting, yu Minghong. The number of articles and the number of people published by these two major cooperation networks is relatively large, and the intensity of cooperation is relatively large. There are also some small cooperative networks. On the whole, the third-line heritage research has not formed a broad and close author cooperation network, and author cooperation needs to be strengthened.

In order to understand the research subject areas of third-line heritage researchers, the authors and keywords were analyzed in a binary clustering analysis. Specifically, by selecting all authors who have published 4 or more articles in the field of third-line heritage and have a theme frequency of 4 or more times as research objects, the author-theme binary cluster is constructed (see Figure 2).

It can be intuitively seen that “three-line construction” and “industrial heritage” are the keywords of basic research, and a large number of authors are gathered; there are also node themes that form strong correlations with specific scholars. For example, Xu Youwei and Daniel Zhang published high-quality results in the construction of the small third line and the research review; Lv Jianchang has done a lot of research on the protection and utilization of third-tier

Source: Authors organize their own work.

industrial heritage; Pu Peiyong takes ecological values and cultural landscapes as the starting point to study the transformation path of old industrial areas; Tan Gangyi and Xu Liquan explored the spatial layout of the third-line heritage based on multidisciplinary methods such as archaeology and typology; Wang Yi mainly focuses on the development and industrial layout of third-tier enterprises in Sichuan and Chongqing; Yu Minghong uses Mianyang City as a research site to conduct value assessment and conservation and utilization research from macro to micro aspects.

The co-lexical characteristics of keywords in the literature can reflect the current research hotspots. Researchers can directly analyze the topics in the field of study through the results of the common word. The co-occurrence frequency of keywords determines the intensity of hot spots, and the association relationship of keywords in the co-word network determines their centrality [5]. In the key word collinear, the size of the node is positively correlated with the frequency of the keyword, and the thickness of the line is positively correlated with the degree of co-occurrence. In order to understand the origin and development status of the third-line heritage research, cooc and VOSviewer software were used to visualize the keywords, and Figure 3 was a co-occurrence spectra of 22 high-frequency keywords with more than 5 occurrences.

In terms of the frequency of keywords, in addition to the two basic research hot keywords of “three-line construction” and “industrial heritage”, the main research hot keywords are “protection and reuse” (75 times), “value evaluation” (20 times), “cultural heritage” (19 times), and “small third line” (17 times). In addition, keywords such as “cultural and tourism integration”, “research review”, and “industrial architectural heritage” also show a certain degree of popularity (Table 3). This shows that the academic research on the third-line heritage mainly focuses on how to protect and reuse the heritage. The specific process is to first collect data and conduct field investigations of the third-line heritage, and then establish a value appraisal system, evaluate the value of the third-line

Source: The author sorts it out and self-makes it.

heritage, and then carry out multi-disciplinary and multi-dimensional transformation and design strategies, such as the integration of culture and tourism, the construction of museums, etc. The research objects range from macro to micro, including regional linkage (key areas of third-line construction, industrial heritage corridors, etc.), and focus on the architectural level (industrial architectural heritage, spatial form, etc.).

Research clustering is based on the hot spot network map through refining and summarizing the formation of research themes, visually displaying the main research areas. The principle is to generate knowledge clustering through the graph clustering algorithm, and extract the label subject words through the algorithm to characterize the corresponding research frontier areas [6]. The high degree of overlap between studies and the closest correlation between hot words show that co-citation between two clustering units is very common, while the correlation between other clustering units is poor.

Using COOC software to cluster high-frequency keywords, on the basis of research hotspot analysis, combined with the actual situation of third-line construction research, it can be summarized as follows: the theoretical architecture of third-line heritage (C0), the spatial layout and urbanization development of third-line construction (C1), the value assessment of third-line heritage (C2), and the protection and reuse of third-line heritage (C3).

1) The theoretical framework of the three-tier legacy

The third-tier heritage occupies an important position in China’s modern industrial heritage and has a far-reaching impact on the development of provinces and cities involved. High-frequency keywords such as “cultural heritage”, “industrial architectural heritage”, “cultural landscape” and “three-line spirit” appear frequently, indicating that a comprehensive understanding of the basic concept of third-line heritage is the premise for its protection and utilization.

Under the framework system of World Heritage and Industrial Heritage, Xu Songlin and Chen Donglin discussed the basic issues such as the concept, type and evaluation criteria of third-line heritage, which has far-reaching significance for the research of later scholars. In recent years, in addition to the detailed study of the material form of the third-line heritage, scholars have also elaborated in more detail on the intangible form of the third-line heritage. For example, Lv Jianchang et al. comprehensively explain the basic concepts through the analysis of the three dimensions of time, space and industry of the third-line industrial heritage and the specific description of the form, and the intangible form includes the macro strategic decision-making ideas and guidelines and policies of the third-line construction, the wisdom of the production organization and management of the third-line enterprises, the “three-line spirit”, the group memory of the third-line builders, and the technical and technological knowledge of the third-line enterprises. From the perspective of industrial archaeology [7], Tan Gangyi and others pointed out that the third-line heritage includes the cultural route of the three-line construction with industrial heritage as the main body, the core tangible cultural heritage such as the built environmental heritage, the industrial heritage, and the architectural landscape, as well as the cultural expressions such as related construction processes, technological processes, production skills, and intangible cultural heritage such as oral memory, collective form and the spirit of the times, which enrich the temporal and spatial types of the third-line heritage [8].

2) Spatial layout and urbanization development of third-line construction

The construction of the third line is the product of the special period of China’s “preparation for war” and “de-planning”, and it is also a special product of China’s exploration of industrialization and urbanization, and the planning layout presented by the third-line industrial heritage is quite different from the construction of ordinary industrial facilities. It is not only very directly related to countries and regions, for example, with geopolitics, military infrastructure, urban agglomerations, and railway distribution lines. And out of the consideration of combat readiness, the construction requirements of “large dispersion, small concentration, backing, and concealment” are generally followed.

Its hot keywords include “spatial form”, “third-tier enterprise”, “planning” and so on. The construction of the third line has had a vital impact on China’s economic and social development and has played an important role in the urbanization process of the whole country, especially in the western region. In 2021, China’s urbanization rate reached 64.72%, some cities to the outside “spread the pie” sprawl is not sustainable, China’s urban construction has gradually bid farewell to epitaxial expansion, turning to the stage of connotation development. In the case that the incremental space in the old city has been saturated, a new round of urban space reorganization and material renewal is urgently needed. Through the review and reflection on the planning and layout of old industrial zones in third-tier cities, it is conducive to exploring the path of protection and reuse.

At the macro level, Xu Liquan et al. started from the perspective of urban and rural regional planning and layout, and with the help of morphological typology methods, carried out comparative research from four aspects: regional planning layout, urban and rural (factory and mine) form, community organization model, and architectural type style [9]. From macro to micro analysis, summarize the laws and characteristics of the planning layout, and analyze the characteristics of the planning layout of the third-line construction area and the factory and mine planning and layout in northwest Hubei as an example, highlighting the “small and complete” independent unit layout, and re-examining the contemporary value significance of the third-line construction from the perspective of heritage value [10]; from the research perspective of historical geography and industrial geography, Wang Yi analyzed the layout and characteristics of enterprises in the construction period of the third line in Sichuan, and discussed the relationship between spatial layout and industrial development [11].

At the mesoscopic level, Liu Fengling started from the construction of the shipbuilding industry heritage corridor, and through value assessment, constructed the overall pattern of the three-line heritage corridor according to the basic elements of the corridor [12]; starting from the research perspective of third-line military enterprises, Wang Yi analyzed the development and layout of Chongqing’s military enterprises during the third-line construction period, with a view to promoting the development of regional economy while achieving national defense and security as much as possible [13].

At the micro level, Li Dengdian et al. took the 102 machine repair plant in Zaoyang City, Hubei Province as an example, used architectural methods to sort out the planning layout, and studied the site selection and layout of the plant and mine, the functional group and the causes of the planning layout [14]; Yuan Lei et al. divided the remains of the third-line construction into three categories of “public buildings, residential buildings and production buildings”, and selected two typical buildings of clubs and residences for research, which is conducive to the protection and reuse of buildings [15].

3) Valuation of third-tier heritage

In terms of the valuation of the third-line heritage, literature analysis and fieldwork are common research methods, which can be widely used in the field of third-line construction research. In addition, scholars have also drawn on the theories and methods of archaeology, anthropology, architecture, political economy and other disciplines to study the construction of the third line. For example, Zhou Jian et al. constructed a research system and research methods for Guizhou’s third-tier industrial heritage based on industrial archaeology [16]; Liu Hanxi took the relocation of the former third-line site in the former qianbei area of eastern Sichuan as an example, introduced the analytic hierarchy method to establish a value appraisal system, and combined quantitative and qualitative to make the research results more accurate [17]; from the perspective of international industrial heritage, Xu Songling analyzed the significance of third-tier industrial heritage based on the institutional value in political economy [18].

4) Protection and reuse of third-line heritage

How to protect and utilize is the purpose and significance of the third-line industrial heritage research. Many scholars have conducted relevant practical research on the protection and reuse of industrial heritage. Its hot keywords include “protection and reuse”, “small third line”, “cultural and tourism integration”, “industrial heritage corridor”, “museum” and so on.

From a macroscopic point of view, from the perspective of urban and rural location and functional use, Lv Jianchang first clarified the current situation of the protection and utilization of third-tier industrial heritage, and then proposed relevant countermeasures for building a database, solving financial difficulties, integrating tourism resources, exploring diversified third-tier museum construction, and strengthening basic theoretical research [19]. In view of the lack of integrity and regional status quo in the study of industrial heritage protection in China, some scholars have introduced the concept of heritage corridors. The conservation planning of the heritage corridor focuses on the integrity, starting from the overall spatial organization of the system. For example, Liu Fengling protects and utilizes the Chongqing section of the Yangtze River as a whole by constructing its heritage corridor, and then constructs a value assessment system at different levels [12]; Pei Ying can construct the third-tier riverside industrial heritage of the upper Reaches of the Yangtze River based on the holistic view, and conduct value assessment and conservation strategies [20].

From the perspective of microscopic cases, Gong Jiayi et al. [21] took the Southwest Yunshui Machinery Factory as an example and proposed a protection and reuse strategy from the characteristics of the cultural landscape; Wu Huirong et al. [22] took Pengzhou Guihua Town as the research object, analyzed the local third-line construction industrial cultural heritage resources, and proposed to build a global healthy cultural and creative tourism town; Yang Xiyu [23] Taking Panzhihua City as an example, he focuses on the use of research and study tourism, the launch of intangible cultural heritage tourism routes, and the integration with the construction of smart cities. In addition, Guizhou, Panzhihua and other provinces and cities have built third-line construction museums, preserving the memory of the times; Mianyang City, Sichuan Province, has made use of local red cultural resources to develop the Red Tourism Project of Liangdan City, which has been highly praised by General Secretary Xi Jinping [24].

At the same time, the investigation and research of the small third-line industrial heritage has also received the attention of scholars, Wang Weiliang and others [25] taking the Shandong “small third-line” industrial heritage, Xinhua Sand Turning Plant as a sample, proposing protection and renovation strategies; Xu Youwei et al. [26] Taking Huoshan mountain in Anhui Province as an example, through fieldwork, summarize the experience of rural small and third-line industrial heritage protection and development and coordinated development of rural economy, so as to help rural revitalization; Wang Wenzheng [27] conducted field research on the “small third line” in the Yimeng Mountainous Area in southern Luzhong based on the field survey, and finally established a value evaluation system to evaluate it and proposed corresponding protection measures.