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Calculations and assumptions

Please note that numerical information generated by CityCAD should only be used for broad feasibility analysis, and is not intended for use at the detailed design stage.

This articles describes the calculations used for:

Perimeter Block Terraced Housing

This diagram shows how the number of terraced houses in a block is estimated. The total perimeter of the block is calculated, and divided by the average house width to obtain the total number of dwellings. Please note that this estimate may be higher than the actual number which is achievable on the site. You may want to reduce the number of dwellings, or increase the average width in order to account for reduced numbers at the corners of the block.

Detached/Semi-Detached Housing

The diagram illustrates the principle of block area, plot size and average unit area. Please note that visualizations of individual houses (using the Block Appearance panel) may not correspond exactly with the numerical value attached to the block. The average unit area is the average total floorspace of a house. The plot size is the area of the block belonging to one house, and the block area is the total area of the block. Plot boundary lines are assumed to have no thickness.

No. of Dwellings = (Block Area) x (Density)

Average Plot Size = (Block Area) / (No. of Dwellings)

Please note that the Average Unit Floor Area is calculated independently of the density. For example, you could type in a very large number for the Average Unit Floor Area and the Plot Size would not change. In practice, this might mean unrealistically tall houses, but the designer needs to check that data is appropriate for their project.

Total GFA = (No. of Dwellings) x (Average Unit Floor Area)

Floor Area Ratio = (Total GFA) / (Block Area)

GFA, NFA, Space for Habitable Rooms

GFA (Gross Floor Area) is calculated as the total area of all floors of buildings in blocks. Please note that walls have no thickness in CityCAD.

NFA can be entered as a percentage of the GFA or an absolute number which must be less than or equal to the GFA.

Space for Habitable Rooms is entered as a percentage of the NFA, which is assumed to contain habitable rooms. Again, no allowance is made for wall thicknesses, and so depending on your project you may want to account for this when choosing a percentage.

Setbacks and Block Area

Setbacks are created in the Route Properties. The area of a Setback is still assumed to be part of the Block Area. Increasing the setback will not change the Block Area, although it may reduce the GFA (Floor Area) of the buildings on a block.

Average distance to shops, green spaces, etc

In the Liveability tab, the average distance from a dwelling to shops, green spaces and other land uses is calculated by the distance from the average centre of a block containing the dwelling to the closest point on the edge of a block containing the shop, green space or other land use.

Shading according to distances

When shading according to distances from a specific land use, if a block or city item actually contains that land use then it is assumed to be 0m away from it.

Average no. of parking spaces

The exception to this is the Average Number of Parking Spaces within 5 mins' walk from a dwelling. In this calculation, if a route has parking spaces, only those parking spaces within 5 mins' walk from a dwelling are counted.

CityCAD assumes that 400m represents 5 mins' walk.

Average widths of streets, bus lanes, cycle lanes etc

This calculation in the extended Movement Tab is a weighted average. If more than one streetscape element type, eg. a bus lane, is present in a route, then each one is treated as a separate one in calculating averages.

Entrances

In the Liveability tab, the average entrance spacing is an arithmetic average. Only blocks which have entrances are included in the calculation.

Average Entrance Spacing = (Total Perimeter of Blocks with entrances) / (Total Number of Entrances).

In the Liveability tab, the average population per entrance is weighted according to the population of the block. Only blocks which have entrances are included in the calculation.

Route Gradient

The area of junctions at nodes is flat, and therefore the geometry of the node area will affect the gradient of adjacent routes. Note that changing the level of individual Streetscape Elements (eg. pavements) will not affect the gradient of the route.

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