城市地表开阔度是对地表形态开阔程度的定量描述,间接反映了通风、日照等城市物理性能。本文基于城市数字高程模型,以自然地形较为起伏的丘陵城市——南京为例,采用以光线追踪算法为核心的栅格计算模型,对南京老城区的地表开阔度进行了计算,并以典型样区分析和街区统计为方法,综合分析了建筑与自然地形对城市地表开阔度的影响,结果表明:南京老城区公共绿地、广场以及文物古迹用地的地表开阔度相对较高,均大于0.8,社区中心和居住用地的地表开阔度相对较低,均小于0.6,突出特点是居住用地地表开阔度小于金融商业用地。地表开阔度相对较低的街区主要临近主干道,形成较为明显的西北——东南方向分布的条带状特征,整体呈现出中心条带低四周高的宏观特征。

The sky view factor (SVF) has been commonly used to describe the shadow impaction of visualization or sunshine from neighborhood buildings or terrains in urban area. It reflects the influences of urban geometry on the microclimate in an urban area. Firstly, we provided a comprehensive review of SVF analysis both in natural terrain and urban area studies. Then we presented a simulation approach based on shadow casting algorithm to calculate continuous SVF values. And, using raster-based data model we analysed the effect of sun shadow shading by neighbor buildings, simulated solar radiation for many times by continuing translation of the solar altitude and azimuth and calculated whether cells were in shadow. The ratio of simulation times and the times of a cell in shadow is considered as an approximate value of SVF. An ArcGIS-embedded computer program is developed and a SVF map for entire area is generated by using high resolution urban digital elevation model of Nanjing old city. Some areas representing different land use types in Nanjing old city are selected for analysis of the small-scale spatial variation of the SVF. The result illustrates that SVF decreases as building height raises or building quantity increases. Mean, maximum, minimum and standard deviation of SVF for each land use category within Nanjing old city are calculated and they are significantly different in different areas. The mean SVF values occurring in parks, open space and areas of historical and cultural value are higher than those of other land use types, which are more than 0.8. The mean SVF values occurring in community centers and residential areas are lower than those of other land use types, which are lower than 0.6. One of the most noticeable results is the unexpectedly higher mean SVF value in commercial areas than that in residential area in Nanjing old city. Meanwhile, spatial distribution pattern of SVF is derived by regarding block as a basic statistical unit. The mean values in the city centre are high while that in urban fringe are usually low. The blocks with high values line up in a northwest to southeast alignment, which may have an effect on urban microclimate.