by a ‘hard shell’ of tall buildings with commercial uses along the perimeter arterial roads, with a ‘soft yolk’ of low-rise residential use in the centre. The spatial structure of superblocks can also be analysed, per a taxonomy detailed by Barrie Shelton, through the classification of roads as ‘global’, being the arterial roads which provide for cross-city travel, ‘local’ roads, which provide local access to buildings within the superblock, and ‘glocal’ roads, which may cross the entire superblock, allowing through travel, and in many instances into neighbouring superblocks. Glocal roads differ from global roads however, in that they are narrow, have lower speed limits, and do not form part of the ‘supergrid’ structure. Shelton also describes the sidewalks of the global arterial roads as functioning as streets in themselves, or ‘sidewalk streets’, operating in a similar manner to the local streets. Sub-structure In a geoprocessing perspective there are two complementary ways of modeling city blocks: with sidewalks: using a direct geometric representation of the usual concept of city blocks. Not only sidewalks, but also inner alleys, common gardens, etc. Some street parts, such as a street greenway, isolated and with no related lot, can be also represented as a block without sidewalks. without sidewalks: represented by polygon obtained by the external border of the union of a set of touching land lots (illustration opposite). Always a block without sidewalks is within a block with sidewalks. The geometric subtraction of a block without sidewalks from block with sidewalks, contains the sidewalk, the alley, and any other non-lot sub-structure. Perimeter block A perimeter block is a type of city block which is built up on all sides surrounding a central space that is semi-private. They may contain a mixture of uses, with commercial or retail functions on the ground floor. Perimeter blocks are a key component of many European cities and are an urban form that allows very high urban densities to be achieved without high-rise buildings. Uses As an informal unit of distance In North American English and Australian English, the word "block" is used as an informal unit of distance. For example, someone giving directions might say, "It's three blocks from here". Online There have been online innovations and websites such as msnbc.com-owned EveryBlock, which uses geo-specific feeds from neighborhood blogs, Flickr, Yelp, Craigslist, YouTube, Twitter, Facebook and other aggregated data to give readers a picture of what is going