Esri TIN
The Esri TIN format is a popular yet proprietary geospatial vector data format for geographic information system (GIS) software for storing elevation data as a triangulated irregular network. It is developed and regulated by Esri, US. The Esri TIN format can spatially describe elevation information including breaking edge features. Each points and triangle can carry a tag information.
A TIN stored in this file format can have any shape, cover multiple regions and contain holes.
Source: Wikipedia
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