In urban settlements, tertiary and quaternary economic activities are the most common. These include trade, commerce and services such as tourism and education. Urban function can be conceptualized as a function of the city in relation to society, the hinterland or other human settlements; as activities that take place in cities; or as a relationship between urban (social) needs and urban (spatial) forms. Urban functions are generators that shape us. The study of urban functions makes it possible to characterize urban space and study its dynamics. Thus, the industrial function, which in the nineteenth century was located in the suburbs and even in large city centers, was closely linked to the residential function, diverted from the 1960s to huge specialized industrial areas, located on the outskirts of cities, near major road junctions or multimodal platforms. How do changing physical factors affect urban functions? Rural functions refer to the use of rural areas. These are mainly residential and primary activities such as agriculture. Ministry of Infrastructure and Transport (2011) Creating places for people: An urban design protocol for Australian cities. Retrieved 10 December 2016. The term “urban form” is used to describe the physical characteristics of a city. It refers to the size, shape and configuration of an urban area or parts thereof. How it is understood, structured or analyzed depends on the scale.

The characteristics of the city`s form range from highly localized features such as building materials, facades and windows to housing type, street type and their spatial or larger-scale layout. The concept of urban form also includes non-physical aspects such as density. Kostof S (1991) The Shaped City: Urban Patterns and Meanings Through History. Thames and Hudson, London As technology advances, OD driving data[1], POI data[2], mobile phone data[7], social media data[7] and vehicle trajectory data[32] and other big data[33-37] are being used to identify urban functions. Although scientists can quickly and accurately identify types of urban functions, it is difficult to identify the improvement and weakening of urban functions. This research shows that urban land use conversion is an effective method for quantitatively identifying changes in urban functions. Our research identifies not only the types of urban functions, but also the improvement and weakening of urban functions that compensate for vacancy in this area. With the diversification of research methods, urban land use data are becoming increasingly accessible. Therefore, the research method described in this article can be widely extended to other cities, especially to study the rapid changes in urban functions in less developed cities. We mainly used urban land cover maps and digital topographic maps of the city centre at a scale of 1:10,000, covering the years 2003, 2007, 2011 and 2015. The basic data sources for our study are topographic maps drawn by professional surveyors. In addition, planners created a detailed map of urban use based on topographic maps through field visits and made annual updates.

According to the Classification Code of Urban Land Use Standards and Planning of Land to be Developed (GB 50137-2011), officially published in 2012, urban development land in downtown Changchun can be divided into nine categories: residential land (RL); commercial land (CL); public lands (PL); industrial land (IL); Logistics and storage areas (fiber optics); road and transportation lands (RTL); urban supply areas (LMs); green spaces and squares (GSL); and Sonderland (SL). Urban land use maps contain very detailed land use classifications and reliable data; Therefore, they form an important basis for our study. Residential, commercial, utility, and industrial land are the four main types of urban land, accounting for nearly 70% of all urban development areas. For this reason, we have focused this study on changes in these four types of urban development lands. Based on urban land use maps for 2003, 2007, 2011 and 2015, a database of urban land use plans for Changchun Central City was created. In 2003, 2007, 2011 and 2015 the total area was 66.03 km2, 75.93 km2, 97.38 km2, 108.21 km2. The public service area is 27.25 km2, 36.80 km2, 32.27 km2, 36.33 km2. The total area of industrial land is 8.81 km2, 12.56 km2, 13.78 km2, 18.48 km2. The area of industrial areas is 54.64 km2, 64.34 km2, 71.92 km2, 75.10 km2. As mentioned in sections 2.3.1 and 2.3.2 above, the area of urban land use may be greater than or equal to zero, and the area of urban land regeneration may be larger, less than or equal to zero. Thus, the change in the area of conversion of urban land use through urban land use development and regeneration may also be uncertain.

According to the relationship between urban land use planning and rehabilitation illustrated in Table 1, the state of change in urban function can theoretically be divided into six categories: low improvement; strong improvement; low weakening; strong weakening; Stabilization; and counterweight stabilization. Since the counterweight stabilization status did not appear in our empirical research, it was not further discussed. The area with a stable residential function has been constantly expanded. The number of stable residential research units increased from 2 in 2003/2007 to 9 in 2011/2015 (Figure 2). Research units with stable residential function showed clear spatial characteristics distributed inside the first ring and outside the fourth device. In the first ring road (NE-1, E-1, S-1, SW-1, W-1 and NW-1), urban construction has become increasingly mature and stable, and the development and rehabilitation of urban land has been completed in stages. As a result, the life function has become increasingly stable. In the northwest and north directions outside the fourth ring road, residential function remained stable from 2003 to 2015. This is related to the government`s urban planning and the city`s development strategy. The Changchun municipal government did not orient urban spatial development along the northwest and north directions, which is also reflected in subsequent research on commercial, public and industrial functions.

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