## 2 Overview and Current Status for Urban Functional Zones
### The Basic Unit of Urban Functional Zone Identification  
The basic unit of urban functional zone identification mainly includes the grid, block, traffic analysis zone (TAZ), clustering unit, geoscene, and building unit (Figure 4). (1) Grid: the image is divided into a grid with square cells [43; 44]; (2) block: the research area is divided into blocks with irregular geometric size using urban road network data [45; 46]; (3) TAZ: the area is divided based on the urban traffic flow, considering the landform, road distribution, administrative division, and sampling convenience [2; 47]; (4) clustering unit: the unit, such as Voronoi cell, is formed by clustering location-based service data [4; 48]; (5) geoscene: the optimal boundary of the functional zone is obtained by multi-scale segmentation, resulting in the geoscene (image object) used as the basic unit [49; 50]; (6) building unit: taking buildings as the basic object of urban space cognition, the identification of functional zones can be completed by identifying the functions of buildings [51].  
Figure 3: The word cloud of the keywords from 102 articles.  
Figure 2: The statistics of the data sources of 102 papers.  
In addition, a few studies using administrative district or cadastral parcel as basic units on urban functional zone classification [52; 53].  
The statistics for the number of articles that used 6-unit types are shown in Figure 5. Among these articles, 25 articles used the grid, 30 adopted the block, 12 used the TAZ, 11 adopted the clustering unit, 13 used the geoscene, and 4 adopted the building unit. Besides, articles that take macro administrative district and others as the basic unit and use multi-unit for comparative analysis were not considered in the statistical analysis [54; 55].  
The grid unit is a straightforward basic unit. However, the shape and size of the units differ from the real boundaries of the functional zones, reducing the accuracy of urban functional zone identification [43; 56]. A block is a homogeneous unit, but under-segmentation may occur. A block may contain multiple functional zones, which cannot be determined [46; 57]. The TAZ is a unit based on geographic information or mobile data. Due to the limitation of data sources, the application scope of this unit is limited [41; 58].  
Figure 4: Different units of urban functional zone (a) grid, (b) block, (c) traffic analysis zone (TAZ), (d) clustering unit, (e) geoscene, (f) building unit.  
Figure 5: Statistics of the publications using the six units of the urban functional zone.  
The clustering unit is determined by the radiation range of location-based data. In underdeveloped areas, the density of location-based data is low, and there are only one or several clustering units in a large area of land, reducing the identification accuracy [48; 59]. The geoscene represents actual objects. It has the advantages of describing heterogeneous areas, automatic segmentation of functional areas, the extraction of functional zones at multiple scales, and providing highly accurate functional zone boundaries. However, this method requires the selection of multi-scale parameters, and the segmentation process is relatively complex [49; 60]. The building unit is a more detailed space unit. However, its outline cannot cover the whole urban area and the functional type of the building is easy to be misclassified or cannot be identified when the data support is weak [61].