Rabu, 23 April 2008

Triangulated Irregular Network (TIN) Formats and Terrain Processing

One issue in the mapping community is that there is no standard format for Triangulated Irregular Network (TIN) terrain files. Most geospatial applications use proprietary formats, which presents serious interchange problems when moving the data around (e.g. from clients to customers, or even within an organization). Unfortunately we are guilty of this as well in LPS, with our LTF TIN format. Many people get around the limitations of proprietary formats by using ASCII as a common interchange format, where the TIN mass points are listed in XYZ for each row (separated by commas, tabs, or whitespace). Some systems also support the notion of points codes for breaklines, which are a critical part of the TIN structure. The main problem with ASCII is that once you get over several million points, the file can be cumbersome to deal with - which means it may be necessary to divide up the data into tiles. Alternatively you can convert the TIN to a raster format, but this can be undesirable if you have dense mass or unevenly distributed points, or if you have breaklines in the TIN (since rasters don't support breaklines).

A case in point is the LIDAR data from the Washington State Geospatial Data Archive. While there are standards for LIDAR data, most commercial applications do not (yet) natively support it. Hence there is a need to make the data available in alternate formats. The data is available in ASCII and TIN format for each quarter quad, but the TIN format is in .e00 format, which is an old interchange format developed by ESRI. Hence, the site recommends Importing the .e00 files into ArcMap in order to use them. The only issue with that is that you need ArcMap... This is likely a reason for including XYZ ASCII as an option as well. Another option is a 3 meter DEM for the entire coverage area, which is possible to download from here. It is in Arc/Info binary grid format though...

So how do we handle all of this in LPS?

There are a few different options here, and the LPS and IMAGINE groups have been working on a solution. Prior to the LPS/IMAGINE 9.2 release, users had to either use the 3D Surface Tool in IMAGINE or the terrain Split and Merge tool in LPS Core. In 9.2, and moving forward in future releases, we are consolidating our efforts by extending the LPS Split and Merge tool and making it available in IMAGINE, which we have renamed to the "Terrain Prep Tool". It is available in Data Prep > Create Surface in IMAGINE, or from Tools > Terrain Prep Tool in the LPS Project Manager.
The Terrain Prep Tool adds new split/merge functionality inherited from LPS as well as resolving some longstanding limitations associated with the 3D Surface Tool (e.g. it dramatically increases the number of points that can be handled). For 9.2 we also added a few formats such as LAS and two flavors of ASCII (with and without point codes for breaklines). The 3D Surface Tool will likely remain available for a few more versions, until we have fully replaced it's functionality in the Terrain Prep Tool.

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