Grid indicators
The table grid_indicators contains the grid cell identifier (id_grid) and a certain number of indicators described below. When calculating grid indicators, you can calculate only a subset of these indicators. The whole list is the following: ["BUILDING_FRACTION", "BUILDING_HEIGHT", "BUILDING_POP", "BUILDING_TYPE_FRACTION", "WATER_FRACTION", "VEGETATION_FRACTION", "ROAD_FRACTION", "IMPERVIOUS_FRACTION", "FREE_EXTERNAL_FACADE_DENSITY", "BUILDING_HEIGHT_WEIGHTED", "BUILDING_SURFACE_DENSITY", "SEA_LAND_FRACTION", "ASPECT_RATIO", "SVF", "HEIGHT_OF_ROUGHNESS_ELEMENTS", "TERRAIN_ROUGHNESS_CLASS", "UTRF_AREA_FRACTION", "UTRF_FLOOR_AREA_FRACTION", "LCZ_PRIMARY", "URBAN_SPRAWL_AREAS", "URBAN_SPRAWL_DISTANCES", "URBAN_SPRAWL_COOL_DISTANCES", "BUILDING_HEIGHT_DISTRIBUTION", "STREET_WIDTH"].
Corresponding name in the table: BUILDING_FRACTION
Description: Total building fraction. If superimposed with other layers, it is not counted twice. Instead, the following priorities are used: "water", "building", "high_vegetation", "low_vegetation", "road", "impervious".
Method: SUM(Bu_Area after superimposition removal) / Cell_Area
Corresponding name in the table: HIGH_VEGETATION_FRACTION
Description: Total high vegetation fraction. If superimposed with other layers, it is not counted twice. Instead, the following priorities are used: "water", "building", "high_vegetation", "low_vegetation", "road", "impervious".
Method: SUM(High_veg_Area after superimposition removal) / Cell_Area
Corresponding name in the table: LOW_VEGETATION_FRACTION
Description: Total low vegetation fraction. If superimposed with other layers, it is not counted twice. Instead, the following priorities are used: "water", "building", "high_vegetation", "low_vegetation", "road", "impervious".
Method: SUM(Low_veg_Area after superimposition removal) / Cell_Area
Corresponding name in the table: ROAD_FRACTION
Description: Total road fraction. If superimposed with other layers, it is not counted twice. Instead, the following priorities are used: "water", "building", "high_vegetation", "low_vegetation", "road", "impervious".
Method: SUM(Road_Area after superimposition removal) / Cell_Area
Corresponding name in the table: IMPERVIOUS_FRACTION
Description: Total impervious fraction (other than roads). If superimposed with other layers, it is not counted twice. Instead, the following priorities are used: "water", "building", "high_vegetation", "low_vegetation", "road", "impervious".
Method: SUM(Impervious_Area after superimposition removal) / Cell_Area
Two indicators are calculated: average building height and standard deviation building height.
The first:
Corresponding name in the table: AVG_HEIGHT_ROOF_AREA_WEIGHTED
Description: Mean building’s roof height within the RSU (the building heights being weighted by the building areas)
Method: SUM(Bu_Wall_Height * Bu_Area) / SUM(Bu_Area)
The second:
Corresponding name in the table: STD_HEIGHT_ROOF_AREA_WEIGHTED
Description: Variability of the building’s roof height within the RSU (the building heights being weighted by the building areas)
Method: By default, the indicator of variability is the Standard Deviation (STD) defined as :
→ SUM(Bu_Area*(Bu_Wall_Height - AVG_HEIGHT_ROOF)^2)) / SUM (Bu_Area)
Two indicators are calculated: average building height and standard deviation building height.
The first:
Corresponding name in the table: AVG_HEIGHT_ROOF
Description: Mean building’s roof height within the grid cell
Method: SUM(Bu_Wall_Height) / SUM(Bu_Area)
The second:
Corresponding name in the table: STD_HEIGHT_ROOF
Description: Standard deviation building’s roof height within the grid cell
Method: SUM(Bu_Wall_Height) / SUM(Bu_Area)
Corresponding name in the table: BUILDING_SURFACE_DENSITY
Description: All building facades (free facades and roofs) included in a grid cell divided by the cell area
Method: building_fraction + free_external_facade_density
Corresponding name in the table: FREE_EXTERNAL_FACADE_DENSITY
Description: Sum of all building free facades (roofs are excluded) included in a grid cell, divided by the cell area.
Method: SUM(Bu_TotalFacadeLength * HEIGHT_WALL) - SUM(Bu_SharedFacadeLength * MIN(height_wall_Bu1, height_wall_Bu2)) / cell_Area
Two indicators are calculated: the sea and the land fractions.
The first:
Corresponding name in the table: LAND_FRACTION
Description: Sum all patches of land that are in a grid cell (land coming from the sea_land_mask layer) and divide by the cell area
Method: SUM(Land_Area) / Cell_Area
The second:
Corresponding name in the table: SEA_FRACTION
Description: Sum all patches of sea that are in a grid cell (sea coming from the sea_land_mask layer) and divide by the cell area
Method: SUM(Sea_Area) / Cell_Area
Corresponding name in the table: ASPECT_RATIO
Description: aspect ratio such as defined by Stewart et Oke (2012): mean height-to-width ratio of street canyons (LCZs 1-7), building spacing (LCZs 8-10), and tree spacing (LCZs A - G).
Method: A simple approach based on the street canyons assumption is used for the calculation. The sum of facade area within a given grid cell area is divided by the area of free surfaces of the given grid cell (not covered by buildings).
→ 0.5 * Cell_free_external_facade_density / (1 - Cell_building_fraction)
Corresponding name in the table: GROUND_SKY_VIEW_FACTOR
Description: Grid cell ground Sky View Factor such as defined by Stewart et Oke (2012): ratio of the amount of sky hemisphere visible from ground level to that of an unobstructed hemisphere. In our case, only buildings are considered as obstructing the atmosphere.
Method: The calculation is based on the ST_SVF function of H2GIS using only buildings as obstacles and with the following parameters: ray length = 100, number of directions = 60. Using a uniform grid mesh of 10 m resolution, the SVF obtained has a standard deviation of the estimate of 0.03 when compared with the most accurate method (according to Bernard et al. (2018)).
Using a grid of regular points, the density of points used for the calculation actually depends on building density (higher the building density, lower the density of points). To avoid this phenomenon and have the same density of points per free ground surface, we use an H2GIS function to distribute randomly points within free surfaces (ST_GeneratePoints). This density of points is set by default to 0.008, based on the median of Bernard et al. (2018) dataset.
References:
- Stewart, Ian D., and Tim R. Oke. "Local climate zones for urban temperature studies." Bulletin of the American Meteorological Society 93, no. 12 (2012): 1879-1900.
- Jérémy Bernard, Erwan Bocher, Gwendall Petit, Sylvain Palominos. Sky View Factor Calculation in Urban Context: Computational Performance and Accuracy Analysis of Two Open and Free GIS Tools. Climate, MDPI, 2018, Urban Overheating - Progress on Mitigation Science and Engineering Applications, 6 (3), pp.60.
Corresponding name in the table: EFFECTIVE_TERRAIN_ROUGHNESS_LENGTH
Description: Effective terrain roughness length (z0).
Method: The method for z0 calculation is based on the Hanna and Britter (2010) procedure (see equation (17) and examples of calculation p. 156 in the corresponding reference). The cell_frontal_area_index_distribution_Hx_y_Dw_z is used to calculate the mean projected facade density (considering all directions) and z0 is then obtained multiplying the resulting value by the cell_geometric_mean_height.
Warning: With the current method, if a building facade follows the line separating two grid cells, it will be counted for both grid cells (even though this situation is probably almost impossible with a regular rectangular grid).
References:
- Stewart, Ian D., and Tim R. Oke. "Local climate zones for urban temperature studies." Bulletin of the American Meteorological Society 93, no. 12 (2012): 1879-1900.
- Hanna, Steven R., and Rex E. Britter. Wind flow and vapor cloud dispersion at industrial and urban sites. Vol. 7. John Wiley & Sons, 2010.
Corresponding name in the table: EFFECTIVE_TERRAIN_ROUGHNESS_CLASS
Description: Effective terrain class from the effective terrain roughness length (z0). The classes are defined according to the Davenport lookup Table (cf Table 5 in Stewart and Oke, 2012)
Method: The Davenport definition defines a class for a unique z0 value (instead of a range). Then there is no definition of the z0 range corresponding to a certain class. We have arbitrarily defined the boundary between two classes as the arithmetic average between the z0 values of each class.
Warning: The choice for the interval boundaries has been made arbitrarily. A definition of the interval based on a log profile of class = f(z0) could lead to different results (especially for classes 3, 4 and 5).
References:
- Stewart, Ian D., and Tim R. Oke. "Local climate zones for urban temperature studies." Bulletin of the American Meteorological Society 93, no. 12 (2012): 1879-1900.
Several indicators result from this choice. They are identical as the ones described in the UTRF output table.
Several indicators result from this choice. They are identical as the ones described in the UTRF output table.
Several indicators result from this choice. The LCZ are calculated at the RSU scale and then aggregated to the most represented LCZ at grid cell scale. The LCZ fraction corresponding to each type is also an output, as well as the uniqueness. The resulting indicator is the following:
Corresponding name in the table: LCZ_UNIQUENESS_VALUE
Description: Indicates how unique is the LCZ type attributed to a given grid cell
Range of values: [0, 1] - the higher the value, the more unique is the LCZ type
Method: | Area_First_LCZ - Area_Second_LCZ | / (Area_Second_LCZ + Area_First_LCZ)
This does not lead to the calculation of a given indicator but to the creation of a new output file called (urban) sprawl_areas. It computes the urban sprawl areas layer from the grid cell geometries, each containing the fraction area of each LCZ type. A sprawl geometry is the union of grid cells having a majority of urban LCZs types (LCZ1 to LCZ10 plus impervious or bare rock soil type - LCZE or LCZ105). Note that:
- for being a sprawl area, at least two urban grid cells should be adjacent,
- any non urban grid cell located within a sprawl area will be considered within the sprawl area
Two columns are created when this key word is in the list of indicators:
First indicator:
Corresponding name in the table: SPRAWL_INDIST
Description: Compute the distance of each grid cell located inside the urban sprawl to the urban sprawl boundaries.
Range of values: [0, +inf]
Method: distance from cell centroid to sprawl boundaries
Second indicator:
Corresponding name in the table: SPRAWL_OUTDIST
Description: Compute the distance of each grid cell located outside the urban sprawl to the urban sprawl boundaries.
Range of values: [0, +inf]
Method: distance from cell centroid to sprawl boundaries
The method to determine a cool area is quite similar as the one used for determining urban areas. A cool geometry is the union of grid cells having a majority of non-urban LCZs types (LCZA - LCZ 101 - to LCZG - LCZ 107 - except impervious or bare rock soil type - LCZE or LCZ105). Note that for being a cool area:
- there should be at least 300 m of adjacent cool grid cells in all directions
- the cool grid cells should be located within a sprawl area
Corresponding name in the table: SPRAWL_COOL_INDIST
Description: Compute the distance of each grid cell located inside the urban sprawl to a cool area.
Range of values: [0, +inf]
Method: distance from cell centroid to cool area boundaries
Corresponding name in the table: ROOF_FRACTION_DISTRIBUTION_Hx_y
Description: Compute the fraction of building of a cell belonging to the height interval [x, y[ meters.
Range of values: [0, 1]
Method: SUM(Bu_Area_Hx_y) / SUM(Bu_Area)
Corresponding name in the table: STREET_WIDTH
Description: average street width needed by models such as TARGET
Method: A simple approach based on the street canyons assumption is used for the calculation. The area weighted mean building height is divided by the aspect ratio (defined as the sum of facade area within a given RSU area divided by the area of free surfaces of the given RSU (not covered by buildings).
→ Cell_avg_height_roof_area_weighted / Cell_aspect_ratio
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