This is the manual for the command-line scape_tool program that is distributed with DIRSIG and can be used to generate polygon meshes for terrains from a variety of input raster and point elevation datasets.

Background

This tool is primarily derived from code developed at Carnegie Mellon University (CMU) in the late 1990s. Mike Garland released the source code for a program named scape as a demonstration of the algorithms (and the implementation of those algorithms) described in the paper "Fast Polygonal Approximation of Terrains and Height Fields" written by himself and Paul Heckbert. CMU maintains the original website for all this work, including softcopy versions of the paper and the original scape code.

The original tool focused on producing a triangular irregular network (TIN), which is a polygon mesh of triangles with irregularly spaced vertexes. This irregularity allows the mesh vertex density to be denser where there is more structure that needs to be represented by closer points to capture stronger vertical gradients. The general algorithm for producing a TIN involves evaluating the current mesh and choosing a location to insert a new vertex that cuts an existing triangle into two or more new triangles. There are multiple algorithms for choosing where to insert a new vertex. The original tool included a standard Delaunay triangulation algorithm, but it also introduced a set of "data dependant" algorithms. These alternate algorithms would insert a new vertex in an attempt to minimize mesh fit metrics such as height or slope errors.

The scape_tool described here is simply an enhanced version of the original code written by Garland. These enhancements include the following additions:

  • The program originally supported only input raster datasets using the STM format.

    • The program now supports ENVI image files as input using any datatype (although the core algorithm and data structures operate on 16-bit data, so 32-bit and floating point elevation data is scaled into 16-bits).

    • The program also supports irregularly spaced ASCII/Text point data. This allows point data from ladar systems to be used to create terrain meshes.

  • The program originally supported only output polygon meshes using a simple, non-standard format.

Useful Supporting Tools

The following additional tools are useful when working on converting from point or raster elevation data to facetized terrains:

  • The Geospatial Data Abstraction Library (GDAL) toolbox. GDAL is not only a library of useful functions for geospatial data manipulation, but also a toolbox of useful command-line tools for manipulating and converting georeferenced raster and vector data in commonly used formats.

  • The Meshlab tool is a graphical tool for processing and editing of facetized object data. It has decent support for importing common 3D mesh data formats, it has an expansive library of tools to process (filter, convert, etc.) loaded data, and it can save to the Alias/Wavefront OBJ file format, which is the most commonly used format for importing 3D geometry into a DIRSIG scene.

  • The DIRSIG Object Tool is a command-line tool that can be used to perform an number of common operations on facetized object data.

Common Workflows

This section outlines workflows using a series of tools to convert common data formats into a terrain mesh.

Using a GeoTIFF or DTED input DEM

It is common for a DEM to stored in a GeoTIFF image container. The workflow for importing a GeoTIFF DEM is as follows:

Convert the GeoTIFF or DTED to Point Data

The first step is to convert the GeoTIFF or DTED raster data file to an ASCII/Text XYZ file using the gdal_translate tool from GDAL:

$ gdal_translate n40_w089_1arc_v2.tif -of XYZ n40_w089_1arc_v2.xyz

or

$ gdal_translate n40_w089_1arc_v2.dt2 -of XYZ n40_w089_1arc_v2.xyz

The output file consists of 3 columns: +W longitude (degrees), +N latitude (degrees) and altitude (meters).

Generate a TIN from the Point Data

The next step is to generate a TIN from the XYZ point data using the scape_tool included with DIRSIG:

$ scape_tool --xyz_filename=n40_w089_1arc_v2.xyz --xyz_frame=geodetic \
    --obj_filename=n40_w089_1arc_v2.obj --max_vertexes=100000

The --xyz_frame option tells the tool that the input XYZ data is in geodetic coodinates (latitude, longitude and altitude). The tool will then convert it from this absolute coordinate system to a scene ENU coordinate system. In addition to handling the horizontal conversion, it will correctly introduce the curvature of the earth as geoid relative altitudes are converted to the Cartesian altitudes of the Scene ENU coordinate system.

The --max_vertexes option specifies the maximum number of vertexes to insert into the mesh.

Post Process the TIN

The last step is some needed post-processing of the mesh in Meshlab.

  • Select FileImport Mesh to import the n40_w089_1arc_v2.obj file produced in the last step.

  • Run the FiltersCleaning and RepairingMerge Close Vertices tool

    • This will loselessly compress the vertex data produced by the scape_tool by finding duplicated vertexes (the same 3D point) in the vertex list.

  • Run the FiltersNormals, Curvature and OrientationCompute Vertex * Normals tool

    • This will add vertex normals for a smoother appearing terrain.

  • Select FileExport Mesh to export the mesh back to the OBJ file.