Old　buildings＇　surface　cracks　are　more　obvious　with　the　accumulation　of　its　disrepair,　which　will　impact　aesthetics,　durability　and　even　threaten　the　safety　of　persons　and　properties　if　feature　values　surpass　the　security　thresholds.　In　this　paper,　firstly,　the　algorithm　of　the　improved　Canny　operator　gray　threshold　edge　detection　and　location,　based　on　traditional　differential　operators　and　sub-pixel　positioning　algorithms,　achieve　coarse　location,　furthermore,　the　sub-pixel　location　algorithm　of　Gaussian　curve　fitting　based　on　neighbor　gradient　features　realize　sub-pixel　precision　positioning.　Finally,　crack　features　are　calculated　and　comparative　analysis　through　the　measurement　algorithm.　The　experiments　prove　the　algorithms　possess　high　accuracy　and　feasibility　with　important　application　values　of　assessment,　identification　and　maintenance.　©　(2013)　Trans　Tech　Publications,　Switzerland.