GRASS GIS manual: r.cost : 「r.cost - Creates a raster map showing the cumulative cost of moving between different geographic locations on an input raster map whose cell category values represent cost. 」
GRASS GIS manual: r.walk : 「r.walk - Creates a raster map showing the anisotropic cumulative cost of moving between different geographic locations on an input raster map whose cell category values represent cost. 」
Twitter kicks Android app users out for five hours due to 2015 date bug | Technology | The Guardian : "a single character G instead of Y took down twitter's api" : $ date -u "+%G" -> 2015 : $ date -u "+%Y" -> 2014 : Yの代わりにGを使ったのが原因
