AspMap takes advantage of the presence of a spatial index associated with a Layer. When a spatial index is present, your application can realize significant performance improvements in zoom, pan operations, the SearchShape, SearchBuffer, SearchNearest and SearchByDistance methods.
There is a file that constitutes the spatial index of an shapefile or TAB file. This file has the same name as the .shp or .tab file, and has .stx extension for shapefiles and .ttx extension for TAB files. It should be present in the same folder as the .shp or .tab file.
This file should be created by the Spatial Index Builder utility (INDEXBLD.EXE) or BuildIndex method.
NOTE: if you have changed the contents of a shapefile of TAB file you have to rebuild the associated index file.
Spatial databases support spatial indexing of geometry fields. Each spatial database engine supports its own set of commands and tools for spatial indexing. Additional information is available below.
SQL Server: http://technet.microsoft.com/en-us/library/bb964712(v=sql.105).aspx
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