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**Flat Feature Processes from Triangulated Irregular Networks for Hydrological Modeling **

ZHU, Honglei (hzhu@clarku.edu) and SCHNEIDER, Kristin, Clark University, Clark Labs, 950 Main Street, Worcester, MA 01610

Key Words: triangulated irregular network, flat triangle, digital elevation model

Regular grid digital elevation models (DEM) and triangulated irregular networks (TIN) are widely used in a geographic information systems (GIS) to represent a digital terrain model. A GIS can be used to extract geomorphic information from a DEM or a TIN for hydrologic modeling. During processing, depressions and flat areas in such models prevent us from obtaining the information that hydrologic modeling requires. This paper proposes algorithms for removing flat features in a TIN model. Flat triangles are removed, then flat ridges and channels are processed.

1. Remove flat triangles in a TIN model. Flow directions cannot be calculated if a triangle is flat. A TIN model must be flat-triangle-free before it is used to extract features such as channel networks or watershed boundaries. Two cases are considered in this study, and algorithms are developed.

1) TIN model generated from a discrete point data set: Flat triangles are processed according to the status of flat triangles' neighboring triangles. Flat triangles without any flat triangle as neighbors are always given higher priority; therefore, stand-alone flat triangles are first processed in flat feature removal procedures. Flat triangles with three flat neighboring triangles are not considered until their neighbors are processed.

a) If a flat triangle is a stand-alone triangle, a centroid point at the center of a flat triangle is added, the point's elevation is interpolated, the flat triangle is split, and the TIN is adjusted.

b) If a flat triangle has one or two non-flat triangles, a vertex's elevation of the flat triangle is adjusted, or a point at the middle of a flat edge is added, or two points at the middle of two flat edges are added, and then, elevation values are interpolated, and the TIN is adjusted.

2) TIN model generated from a contour data set: Edges known as bridge and tunnel edges form flat triangles in a TIN when a TIN is generated from a contour data set. The contour data set is used to assist the process of flat triangle removal. By inserting a point at the middle of bridge or tunnel edges, flat triangles can be removed from a TIN model. The elevations of added points are interpolated, and the TIN is adjusted.

2. Flat Ridge and Channel Processing. Flat ridges and channels may prevent us from locating depression points and from obtaining the flow direction information based on a TIN model. Algorithms for processing such features are proposed. Neighboring triangles are used to assist the process.

Validation of the methodology is provided via a case study.