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Track: Application Development Techniques
Graham S. Hayes
GIS Resource Group, Inc.
21 S Grove Street Suite 130
East Aurora, NY 14052
Telephone: 716-655-5541
Fax: 716-655-5540
E-mail: graham@gisrg.com
Eliminating Edge Effects when Clipping a TIN
A TIN representing surface elevation data is usually one of the largest datasets in a GIS. Large datasets have unique problems thar manifest themselves solely because of the size of the data volume. These problems include physical data storage limitations, longer access and display time, and memory allocation issues. An effective way to address these issues with large vector datasets in ArcInfo has been to utilize ArcInfo LIBRARIAN or ArcStorm. One of ArcInfo LIBRARIAN software's data management concepts is to break a large dataset into smaller, rectangular "tile size" chunks. Unfortunately, a TIN is not a vector coverage and, therefore, does not lend itself to direct inclusion and management by ArcInfo LIBRARIAN. However, one can use the ArcInfo LIBRARIAN directory structure if the TIN is broken, like the vector data, into tile size chunks. Upon first reading, the CLIP option in the CREATETIN command sounds like it will provide exactly what you need. Define a box as a CLIP window and extract
just the data required for each tile. The CLIP option, however, creates an uneven outside edge (hull) on the resulting TIN, leaving a gap between the clip box and the data points. The CLIP option does not interpolate elevations along the clip box, but merely ignores any data points outside the box. In order to break the TIN into pieces, elevation data must be maintained along the tile boundaries to ensure data continuity across the tiles. By creating cross-section profiles (one for each tile edge) with the ARCPLOT command SURFACEXSECTION, elevations are interpolated at regular intervals along the profile. These mass points are included as input to the CREATETIN command allowing the TIN to be clipped correctly. The purpose of the paper is to present a methodology using ArcInfo commands and AML procedures to split a TIN into multiple, rectangular pieces that retain the characteristics of the contours before the split.
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