Objective  The Temple of Heaven is one of Beijing's seven UNESCO World Heritage Sites, and is also one of the heritage sites involved in the world heritage nomination of Beijing Central Axis. The sacrificial roads play a skeletal role in the historical pattern of both the Temple of Heaven and Beijing Central Axis, and have significant impacts on tree species along the roads and planting methods thereof. At present, the outer altar of the Temple of Heaven (hereinafter referred to as the "Outer Altar") is largely occupied. The restoration of sacrificial roads in the Outer Altar and the avenue tree planting will promote the realization of the commitment to the world heritage nomination of the Temple of Heaven in 1998. The restoration will make it possible to present and interpret the outstanding universal value (OUV) of the Temple of Heaven and Beijing Central Axis in an authentic and integrated way, and facilitate the inscription of Beijing Central Axis.

Methods  The research is based on the historical documents, drawings and archives about the Temple of Heaven and relevant heritage remains. By comparing different categories of drawings and archives in different periods and corroborating the historical documents and existing heritage remains, the research confirms the spatial location of the sacrificial roads in the Outer Altar in Ming and Qing dynasties, the evolution of these roads, and the scale of avenue tree planting, with the research scale being further corrected based on "step" (pronounced as "bu" in Chinese), a construction measurement unit prevailing in Ming and Qing dynasties.

Results  There are 9 sacrificial roads in the Outer Altar in Ming and Qing dynasties. The roads are respectively referred to as ① to ⑦, ⑩ and ⑪ in this research, which are all parallel or perpendicular to the altar wall. There are 4 roads, ②, ③, ⑥ and ⑪, in the north-south direction, and 5 roads in the east-west direction, namely ①, ④, ⑤, ⑦ and ⑩. The roads were formed mainly in Ming Dynasty. The emperors in Ming and Qing dynasties used road ① to enter and leave the Great Sacrificial Altar for the worship of heaven and earth. Before sacrifice, the emperors must inspect the sacrificial animals through roads ② and ⑩. Roads ③ − ⑦ allowed the emperors to enter and leave the Yuanqiu Altar to worship heaven. Road ⑪ is for the emperors to enter and leave the Chongyu Altar to pray for rain. The functions of the roads overlapped in terms of historical sacrifice. Today, three of the nine sacrificial roads' remains can still be found, while the others have all been occupied. The avenue trees of the sacrificial roads are planted in a "Yishu" (ceremonial) pattern, with two rows of acacia trees being planted on each side of road ①, one row of acacia trees being planted on each side of the rest sacrificial roads, and five rows of cypress trees being planted on the north side of the south outer altar wall. The above trees were initially planted in Ming Dynasty. There are 11 ancient acacia trees on existing sacrificial roads. The trees are mainly concentrated on the two sides of road ③, the south side of road ①, and the west side of road ②. The scale of the sacrificial roads and avenue tree planting are measured and positioned in "step", an ancient measurement unit. Among the sacrificial roads directly facing the altar door, road ⑥ is the widest with a width of 16.0 m; roads ① and ⑦ are the second widest with a width of 12.8 m; road ④ is narrower with a width of 11.2 m. As to the rest roads, roads ② and ⑤ share the same width of 11.2 m, with road ② being 27.2 m away from the west outer altar wall and road ⑤ 60.8 m away from the south side of Guangli Gate; road ③ is 8.0 m wide, and the distance from it to the west inner altar wall is 38.4 m. Compared with other roads, the roads directly facing the altar door have wider row spacing (28.8 m) for avenue tree planting, so as not to block the altar door, and the row spacing thereof is set as 28.8 m, except that the spacing for outer rows along road ① is 57.6 m. For the other roads, roads ② and ⑤ share the same row spacing of 16.0 m, and the row spacing of road ③ is 12.8 m. The spacing between avenue trees planted along the sacrificial roads is 8.0 m for road ①, and 9.6 m for the rest roads. The row spacing and tree spacing for the 5 rows of cypress trees in the south of the Outer Altar are both 4.8 m.

Conclusion  The sacrificial roads in the Outer Altar and the avenue trees planted along such roads jointly constitute the ritual space of the sacrificial route. The findings of this research may guide the formulation of the archaeological exploration plan for the occupied area in the Outer Altar, and the subsequent restoration of the pattern of sacrificial roads and avenue tree planting along them, while providing reference for improving the overall quality of the southern section of Beijing Central Axis and restoring the historical pattern and relevant sacrificial roads of the Altar of Agriculture. After clearing the encroachment of the sacrificial roads in the Outer Altar, it is suggested that a thorough archaeological excavation be carried out for roads ③, ⑥ and ⑦ with remains, and an exploration be conducted for the rest of the sacrificial roads without remains. The archaeological findings will be used to determine road size and construction methods. After that, the planting layout of the sacrificial roads in the Outer Altar should be restored according to the location of the existing ancient acacia trees, so as to restore the pattern and plant landscape of the Outer Altar in an authentic and integrated way, and accordingly restore the spatial system and order of royal rituals in Ming and Qing dynasties in the southern section of Beijing Central Axis.